WO1987006420A1

WO1987006420A1 - Method and apparatus for adjusting video record and reproduce systems

Info

Publication number: WO1987006420A1
Application number: PCT/US1987/000823
Authority: WO
Inventors: Allen J. Trost
Original assignee: Ampex Corporation
Priority date: 1986-04-11
Filing date: 1987-04-10
Publication date: 1987-10-22
Also published as: DE3790203T; GB2198907B; JPS63503111A; DE3790203C2; GB8726935D0; GB2198907A; JPH0554760B2

Abstract

To improve video image quality in communication channels wherein video signal information is reproduced multiple times, a system is provided to selectively recirculate a video signal through a videotape recording and playback machine (306) and to identify changes in signal parameters after multiple reproductions of the video signal.

Description

METHOD AND APPARATUS FOR ADJUSTING VIDEO RECORD AND REPRODUCE SYSTEMS

The present invention relates to video recording and playback systems and, more particularly, to a method and apparatus for adjusting such systems to minimize deterioration of video signals recorded on and reproduced from a magnetic record medium.

Video record and reproduce equipment is often utilized in communication channels in which television or other video signals are recorded on and reproduced from a magnetic record medium. Each sequence of recording and playing back of a video signal results in the reproduction of a copy (i.e., a copy of a copy) that is a generation removed from the original video signal received by the communication channel. In practice, some deterioration of video signals occurs during the record and reproduce processes because of imperfections in the record and reproduce equipment. This deterioration is manifested in the reproduced video signals as signal level, amplitude, frequency and/or phase errors. The extent of deterioration generally depends upon factors such as the characteristics of equipment utilized to record and playback the video signals, as well as the character- istics of equipment utilized to transmit the signals. Furthermore, the extent of deterioration increases markedly as the number of record and playback sequences experienced by the video signal increases. The deterioration is cumulative in that the deterioration resulting from each record/playback sequence through imperfect or isadjusted video record and reproduce equipment additively combines with deteriorations resulting from all previous and following sequences through the equipment. Even a few record/playback sequences of an analog video signal through such

SUBSTITUTE SH equipment, such as occurs during electronic editing processes, produce severe deterioration of the final generation of the video signal obtained.

In conventional analog machines for recording and reproducing video signal information on flexible magnetic tape (herein referred to as VTR machines) , it is known that multi-generation deterioration of the resulting video signal can be partially alleviated by precise adjustment of VTR machine controls determina¬ tive of various properties of the video signals. For instance, the properties of color television signals recorded and reproduced by VTR machines are influenced by VTR machine parameters such as system gain and phase, black signal level, system differential gain, system differential phase, and system frequency and transient responses. In practice, precise adjustment of controls influencing these parameters are made during the manufacture of VTR machines, and control mechanisms are provided on the machines so that users of the machines can re-adjust some of the parameters. However, such adjustments and re-adjustments can be difficult and time consuming, especially because misadjustments may not be evident until video signals have been recorded and reproduced several times. Also, adjustment of a VTR machine for one parameter usually affects the proper setting of the controls for other parameters. Consequently, several trial and error adjustments of each of several VTR machine controls is typically necessary before a VTR machine is properly adjusted for the desired quality recording and repro¬ ducing of video signals.

Video record and reproduce equipment used to provide color television signals for teleproduction and broadcasting applications commonly include accessory devices, such as time base correctors (TBC) , synchron¬ izers and other video signal processing devices, which function to ensure television signals provided for such applications are stable, conform to signal-defining standards and are free of objectionable noise. For example, TBC's and synchronizers are in the nature of adjustable time delay devices that accept video signals from unstable or unsynchronized sources and so delay them that the video signals provided at their outputs remain entirely stable relative to an established timing reference. Such devices typically function to remove timing differences in received video signals by directing the signals through the adjustable time delay device and altering the delay in accordance with the timing difference between the received video signal and the stable timing reference so that the video signal is output by the device synchronously with the stable timing reference.

TBC's and synchronizers currently used in tele¬ production and broadcasting applications are digital in design, and include a memory in the form of an array of digital data storage elements serving as the adjustable time delay device. Inasmuch as television signals are commonly in analog form, analog to digital converters (ADC) , digital to analog converters (DAC) , digital sig¬ nal processing circuitry and analog video signal pro- cessing circuitry are provided to interface the storage elements to the analog video signal source and to the utilization device that is to receive the stabilized analog video signal. The size of the array of data storage elements is selected to provide a storage ca- pacity sufficient to enable the video signal to be de¬ layed for an interval that permits removal of the max¬ imum timing difference that occurs in the video signal with respect to the timing reference. Since new blank¬ ing and television synchronizing signals are commonly inserted in the video signal after it is retrieved from the memory, it is the practice not to store in the memory most of each horizontal blanking interval and

SHEET the vertical sync interval included in the received video signal. Therefore, the capacity of the memory is usually somewhat smaller than that which would be necessary to store the entirety of an interval of the video signal corresponding to the interval of maximum time delay to be provided by the memory.

Synchronizers serve to synchronize video signals from different, unsynchronized sources. Most commonly, they are arranged to accept video signals supplied by remote sources and delay them so that they are provided synchronously with the timing reference of local sources. Some synchronizers are designed to provide a single television field or a single pair of interlaced television fields for synchronous combination with video signals provided by local sources. Field and frame synchronizers are examples of these. Other synchronizers are capable of synchronizing a continuous stream of video signals to the timing reference of local video sources. Such devices typically include a data storage element array of a size sufficient to store data from at least one and frequently two television fields. In synchronizers having the capacity to store data from two television fields, received video data is alternately routed for storage to different portions of the array of data storage elements on a field by field basis. Similarly, data is retrieved alternately from the different array portions on a field by field basis, and as data is retrieved from one of the array portions, received video data is stored in the other different array portion.

TBC's serve to remove timing instabilities or errors that commonly occur in video signals that, for example, are recorded on and reproduced from a magnetic medium. Generally, both continuously changing timing errors and step timing errors occur in such signals.

SUBSTITUTE SHEET In some cases, these errors are of a limited range and a memory of small storage capacity is able to provide the required delay for removing the errors. However, rotary head helical scan VTR machines of current design are able to reproduce video signals at different speeds and directions of transport of the magnetic tape to create stop, slow, fast and reverse motion effects. This alters the relative transducer to tape speed, creating a constant frequency error depending on the speed and direction of tape transport. In addition, the reproduction of television fields recorded on the tape will be skipped or repeated intermittently, which introduces an instantaneous discontinuity or step change in signal timing of a number of horizontal lines and an alteration of field sequence in the reproduced video signal relative to that of the timing reference. These are large timing errors and, as a- consequence, such machines are provided with TBC's having memories with large storage capacities, frequently of a size sufficient to store data from at least one television field. Also, some TBC's with large storage capacities are arranged to perform also the functions of a synchronizer, and when doing so, operate in the manner generally described hereinbefore.

TBC devices, particularly those used in teleproduction and broadcasting applications, often perform additional functions in addition to ordinary time-base correction. The most common additional functions are: dropout compensation, system phasing control, picture enhancement, high speed tape shuttle picture composing, and advanced timing of output video signals. Broadly speaking, the performance of each of these functions affects the video signal and, there- fore, adjustment of the functions can enhance ultimate video signal quality obtained. It is also known to use TBC and synchronizer devices in conjunction with systems in which informa¬ tion is stored on video discs as well as in systems in which video information is stored on flexible magnetic tape. Video disc systems using one transducing head per disc recording surface having TBC and synchronizer devices have been adjusted by storing video information in a digital memory and then recirculating the stored information through the system on a non-real time basis. In other words, in such video disc systems, information retrieved from the video disc machine is recirculated after an interruption in time and used for the performance of adjustments of TBC and synchronizer devices. Such systems have been limited to non-real time recirculation of the video information because of the one transducing head per disc recording surface architecture.

As further background to the present invention, it is useful to describe a typical television signal. For teleproduction and broadcast applications such television signal is a composite of several different signal components, generally, falling within one of two classes of signals, namely, a video information signal component and several synchronizing signal components. The common television signals are formed of lines of horizontally distributed video information separated by intervals of horizontal line related synchronizing signals defining the beginning of each line. The horizontal lines are further organized into rasters of vertically distributed lines defining fields of lines separated by vertical field related synchronizing signals. In turn, the fields are organized into frames, each composed of two spatially interlaced fields of horizontal lines, with lines of each field having a different raster position upon display. The various synchronizing signals included in television signals serve to synchronize the processing of the television signals and the operation of the processing and other television signal utilization devices.

In color television signals organized according to some standards, such as the NTSC standard, the synchronizing signals include vertical and horizontal blanking intervals, each formed of a composite of sev¬ eral synchronizing signals. The vertical blanking in¬ terval includes a vertical blanking level extending be¬ tween leading and trailing signal transition edges that determine the duration of the vertical blanking inter¬ val. On to this blanking level is impressed a number of horizontal blanking interval pulses, a number of equalization pulses, a serrated pulse interval defining a vertical sync pulse, and a burst (typically 9 to 11 cycles) of the sinusoidal chrominance subcarrier signal (color burst) following each horizontal sync pulse dur¬ ing about the latter one half of the vertical interval. Each horizontal blanking interval during the latter one half of the vertical blanking interval and the entire field of lines between consecutive vertical blanking intervals includes a horizontal blanking level extend¬ ing between leading and trailing signal transition edges. These edges determine the duration of the horizontal blanking interval. Impressed on each horizontal blanking level is a horizontal sync pulse followed by a color burst. One horizontal sync pulse and one color burst are provided for each horizontal line of the television signal and serve to keep the horizontal scanning and color generation synchronized. The vertical sync pulse is provided for each field of the television signal to keep vertical scanning syn¬ chronized. The serrations of the vertical sync pulse prevent loss of horizontal scanning synchronization. Equalization pulses are provided to insure proper scanning motion synchronization with the required spatial interlacing of the two fields that compose a television frame. The horizontal and vertical blanking

SUBSTITUTE SHEET levels serve to blank the display during horizontal and vertical retraces, with the associated transition edges effecting a smooth signal change between the video information signal intervals and the blanking inter- vals.

NTSC color television signals are organized into frames of 525 horizontal lines occurring at a frame rate of 30 Hertz. Since each frame includes two fields, each consisting of 262-1/2 horizontal lines, the field rate is 60 Hertz. The horizontal line rate is approximately 15,750 Hertz and the color subcarrier frequency is about 3.58 Megahertz.

In the case of the NTSC standard color television signal, the frequency of the color subcarrier component is such that each horizontal line contains exactly 227-1/2 cycles of a continuous color subcarrier wave. As a result, there is a half cycle phase difference between the chrominance component on adjacent horizontal lines. Because of the line-to-line phase alternation in the relationship between the color subcarrier component and the horizontal synchronizing component, horizontal lines cannot be readily inter¬ changed with each other without the exercise of care to maintain phase coherence in respect of the chrominance component of the color video signal. Such care also has to be exercised when using one field of a video signal in place of another.

In summary of the general format in which video signal information is provided for broadcasting under the NTSC standard, it may be said that television signals are composites of video information signals and several synchronizing signals. More particularly, television signals are formed of lines of video infor¬ mation separated by intervals of horizontal and vertical synchronizing signals. In terms of raster

SUBSTITUTE SHEET displays, the synchronizing signals organize the video information into separate horizontal lines that are uniformly distributed vertically to define fields. More particularly, the vertical sync pulses define the beginning of each field at the top of the raster, and horizontal sync pulses define the beginning of each horizontal line in the raster. The odd-numbered lines in the interlaced raster define one field, and the even-numbered lines define another field. Thus, it can be said that each frame is composed of odd and even fields of interlaced horizontal lines. In color television signals, the synchronizing signals also include color subcarrier signals. A burst signal of such subcarrier is provided for each horizontal scanning line and serves as a phase reference for color generation, or hue, in the line.

In the prior art, it is known to provide repeated reσirculation of single fields of video information in order to provide still pictures. However, when a single field of NTSC color video information is successively repeated, the phase of the chrominance component in the successively repeated fields will not follow the required line by line color subcarrier reference phase sequence. This results in signal disturbances unless preventative measures are taken. Typically, the preventative measures include separation of the chrominance and luminance signal components from a composite video signal, followed by manipulations of the chrominance signals which shift their phase by 180', followed by recombining the chrominance and luminance signals. However, such differential processing of chrominance and luminance signals is not entirely satisfactory because it can cause degradation in video signal resolution. Moreover, such processing is not performed when a continuous color video signal is passed through the

SUBSTITUTE SHEET video signal path formed by the apparatus. Consequent¬ ly, any adjustments of the apparatus made with the use of a video signal including such a processed chrominance component is likely to be inaccurate because of the presence of the degradations introduced into the video signal by the chrominance signal processing.

In view of the preceding discussion, it can be appreciated that the task of identifying and cor- recting factors that cause signal deterioration in systems involving multiple generations of video signals is complex. Furthermore present methods and systems for accomplishing the task are not always completely successful, especially in systems involving VTR machines. Thus, there exists a substantial need for improved and convenient methods of, and means for, adjusting the video signal processing elements of VTR machines, associated TBC and other operatively associated devices that affect the properties of video signals as they pass therethrough so that video signals are not degraded when passing through such elements. The object of the present invention is to meet this need, especially for video record and playback machines of the analog type used in conjunction with time base corrector devices that employ digital signal processing techniques.

The method and apparatus of the present invention enables such adjustments to be conveniently made rapidly and accurately by sequentially recording and reproducing an interval of a color video signal that is passed through a selected delay between each consecutive reproduction and following recording. The delay is selected to maintain a synchronous relation¬ ship between the chrominance component of the video signal and an independently generated stable color subcarrier reference timing signal, while permitting the interval of the color video signal to be so recorded and reproduced a selected number of times repetitively and continuously without interruption in time. One embodiment of the method includes the following steps: Providing a video test signal to a VTR machine; reproducing and recirculating the video test signal information through a selected delay corresponding to an even number of horizontal line intervals to the input of the VTR machine; operating the VTR machine to record the recirculated signals; repeating the storing, reproducing recirculation, and recording steps for a selected multiplicity of cycles; and adjusting video controls of the VTR machine based upon changes in signal properties detected by comparing different generations of the recirculated signals.

A system according to the present invention, in one embodiment, includes a VTR machine of the analog type having time base correction circuitry including a memory of sufficient capacity to provide a selected delay of a selected interval of a color video signal. Means are coupled to the memory for receiving the selected interval of the video signal after the delay and for recirculating it for recording and reproducing on the tape of the VTR machine. A control means is coupled to the VTR for causing the selected interval of the video signal to be recorded, reproduced and recir¬ culated a selected number of times. Means are coupled for receiving the recirculated video signal and respon- sively adjusting controls of the VTR machine based upon changes in recirculated signals detected by comparing different generations of the recirculated signals. The delay provided by the memory is selected to maintain a synchronous relationship between the chrominance component of the video signal and an independently generated stable color subcarrier reference timing signal, while permitting the interval of the color video signal to be so recorded and reproduced the selected number of times repetitively and continuously without interruption in time.

The foregoing and other aspects of the present invention will become apparent to those of ordinary skill in the art upon consideration of the following detailed description of the preferred embodi¬ ments of the present invention in connection with the accompanying drawings, in which:

FIGURE 1 is a pictorial view of a tape guide drum assembly for a videotape recording and playback machine of the rotary head helical scan type.

FIGURE 2 is a schematic diagram of a section of videotape having information magnetically recorded thereon by, for example, the assembly of FIGURE 1;

FIGURE 3 is a functional block diagram of a videotape recording and playback apparatus in a system according to the present invention; and

FIGURE 4 is a functional block diagram of a videotape recording and playback apparatus incorpora- ting a system according to the present invention.

The environment of the present invention may be understood by referring to FIGURE 1, which generally shows a cylindrical tape guide drum assembly 11 such as used in VTR machines that employ rotating heads to record and reproduce video information from flexible magnetic tape as the tape is transported in a helical path about the drum. More particularly, drum assembly 11 includes an upper drum 13 that is rotatable about a central shaft 17 and a stationary lower drum 15 that is axially aligned with the upper drum. Λ first guide member 21 is mounted to guide magnetic tape 19 onto stationary lower drum 15, and a second guide member 23 is mounted to guide the tape off upper rotary drum 13. At least two electromagnetic transducing heads 27 and 29 are mounted at circumferentially spaced locations on rotary drum 13. For present purposes, the trans¬ ducing head 27 can be understood to operate to record video information on tape 19, and transducing head 29 can be understood to operate to playback information from the tape. VTR's constructed to record and playback video information according to the Type C tape format standard typically include such transducing heads, and such heads are commonly operated to record and playback information simultaneously.

In operation of the drum assembly 11 in FIGURE 1, tape 19 is introduced onto drum 15 from the lower right by guide 21, is trained about the drums in a helix-like path indicated by arrows 19A, and is removed from the upper rotary drum 13 by guide member 23. To record and playback video signal information on tape 19 with transducing heads 27 and 29, rotary drum 13 is rotated in the direction indicated by arrow 13A in synchronization with transport of tape 19.

By employing drum guide assembly 11 as described above, tracks 31 of video information such as shown in FIGURE 2 can be magnetically recorded on tape 19 by transducing head 27. More particularly, the information tracks 31 are discrete and parallel, and are positioned at a sufficiently small angle relative to the longitudinal centerline of tape 19 that the length of each track 31 substantially exceeds the width of tape 19. The information recorded along tracks 31 typically is a composite television signal including video information and synchronizing information used in the control of the operation of television signal processing devices upon playback of the recorded information from the tape 19. During playback, however, the previously discussed distortions and other instabilities can appear in the reproduced video signals.

FIGURE 3 generally shows a video system comprising a VTR machine 106 of the analog type and a TBC device 107 connected to correct time displacement errors in the video signals at the output of the VTR machine. Although a TBC device is usually provided in a stand-alone chassis, a VTR machine and a TBC device can be considered functionally integral as shown in the system in FIGURE 3. The TBC device 107 should be understood to include conventional analog- to-digital conversion circuitry at its input, and conventional digital-to-analog conversion circuitry at its output. In the preferred embodiment, TBC device 107 includes a memory of sufficient capacity to store, in digital form, at least one field of the video information portion of a composite television signal. As will be explained in further detail hereinbelow, the memory of TBC 107 can act to delay the video information played back by the VTR for an interval corresponding to the interval of video information that is repetitively recorded on and played back from tape 19. This delay provided by the memory cooperates with the simultaneous record and playback operations performed by transducing heads 27 and 29 to enable repetitive recording and playing back of the interval of composite video information continuously without interruption (i.e., in real time) for a selected number of times to obtain a desired number of generations of the interval of composite video information.

The system in FIGURE 3 further includes one or more controls 108 for selectively adjusting elements that materially affect the signal processing circuits within VTR machine 106 and TBC device 107. For example, controls 108 can include variable capacitances and resistances to control bandpass widths of filters in the signal processing circuitry. Also, controls 108 usually provide adjustment of parameters such as system gain and phase, black level, system differential gain, system differential phase, and system frequency and transient responses. In addition, controls 108 enable adjustment of the delay interval provided by TBC 107. In digital TBCs, such adjustment is achieved by controlling the memory address generators operatively associated with the memory of the TBC. Through control of the memory address generators, the delay interval provided by the TBC can be adjusted in increments corresponding to a selected multiple of an interval of one horizontal line of the composite video signal. In versatile digital TBCs constructed for use with VTR's used in teleproduction and broadcast applications, the TBCs are controllable to adjust the delay in increments corresponding to fractions, e.g., one half, of one cycle of the color subcarrier signal component included in composite color video signals. The manner in which such adjustments are made i the delay provided by the TBC 107 in accordance with the present invention is described in further detail hereinafter.

The system in FIGURE 3 further includes a video signal generator 113 connected to a video switch 123 via a line 115. Video signal generator 113 can be, for example, a video camera, another VTR machine, or a computer that is programmed to provide video information. Preferably, signal generator 113 includes means to provide standard video test signals such as color-bar test patterns. Signal generator 113 also provides indicating βignals to a counter 131 that, in the illustrated embodiment, is shown as integral to video switch 123. In practice, the indicating signals are derived from horizontal sync pulses present in the video test signals. Counter 131 is a conventional device that sequentially counts the indicating signals and provides an output to actuate video switch 123 whenever a preselected count number is reached. As further shown in the system in FIGURE 3, video switch 123 has two operating positions. In its first position, switch 123 connects video signal generator 113 to VTR machine 106 via line 125. In its second position, video switch 123 provides recirculation of video signals through VTR machine 107 by connecting line 125 to line 133 at the output of TBC device 107. The return routing can be provided through a direct cable, as indicated by line 133, or through a conventional routing switcher.

Further in the system of FIGURE 3, a signal monitor 139 is connected to receive output signals from TBC device 107. The purpose of monitor 139 is to detect changes in parameters of video signal information caused by misadjustment of the system through which the video signal passes as the signal is repetitively recorded on and reproduced from the tape associated with VTR 108. Monitor 139 can be, for example, an oscilloscope or a voltage comparator that indicates changes in signal level caused by misadjust ents in system gain.

Operation of the complete system of FIGURE 3 will now be described. It should be assumed that initially video switch 123 is in the first position, and that video signal generator 113 provides pulses to counter 131 and a selected interval of a video test signal to VTR machine 106. Upon receiving the video test signal, VTR machine 106 records and nearly simultaneously plays back the signal. This is accom- plished by placing VTR machine 106 in the edit mode and controlling the record and playback transducers, such as transducer heads 27 and 29 on drum guide assembly 11 of FIGURE 1, to record and reproduce the video test signal respectively, along tape 19. In practice, such reproduction of the video test signal occurs about one-third field interval after the signal is recorded. This interval results in establishing a part of the signal delay through the system and is selected to achieve the desired repetitive recording and playing back of the selected interval of video test signal. The balance of the delay is provided by the normal signal propagation delay through the system and the selected memory delay in TBC 107. After the selected interval video test signal is initially transmitted to VTR machine 106, counter 131 responds to the pulses from signal generator 113 and actuates switch 123 to the second position. In the second position, switch 123 permits recirculation of signals from the output of TBC device 107 to the input of VTR machine 106. Switch 123 remains in the second position for a period commensurate with the desired number of cycles, or generations, of the recirculated video test signals.

The system in FIGURE 3 further operates such that, after the video test signal information is played back, the signal information is transmitted to TBC device 107 for digitizing and storage in the device's memory. After a predetermined delay period in memory equal to the desired total delay less the combined delays resulting from the normal signal propagation delay through the system and circumferential separation of the record transducing head 27 and playback transducing head 29 (FIGURE 1) , the stored information is recalled from memory. the input of VTR machine 106. Upon receiving the recirculated test signal information, VTR machine 106 again records, reproduces, and re-transmits the signal information to TBC device 107. In turn, TBC device 107 once again digitizes and stores the transmitted signal, then recalls and recirculates the signal information after the selected delay. The steps involved in recirculating, recording, reproducing, and storing the video signal information can be continued for any desired number of cycles or generations. Preferably, upon each cycle or after a preselected number of cycles, the video signal from TBC device 107 is transmitted to monitor 139 to display changes in the properties of the test signal information caused by the repeated recirculations.

In practice, the system of FIGURE 3 can be operated in two stages. In the first stage, video test signal is routed directly to TBC device 107 without recording and playing back by VTR machine 106. The test signal is then stored in digital form in the TBC device's memory and then is recalled and recirculated through TBC device 107 after the selected interval of delay. The steps of storing and recirculating the video test signal are repeated a selected number of cycles. As a consequence, changes in signal values can be detected by comparing different generations of the signals, and adjustments can be determined and made via controls 108 for TBC device 107 to minimize signal deterioration after multiple generations of recirculation. In practice, such adjustments usually include adjustments to the analog-to-digital and digital-to-analog conversion circuits. After such

SUBSTITUTE SHEET adjustments to TBC device 107 are made, the second stage is executed.

In the second stage of operation of the system of FIGURE 3, video test signal information is routed through both VTR machine 106 and TBC device 107 and then is recirculated as previously described. During this second stage, adjustments are made to VTR machine 106 via controls 108 based upon changes detected by comparing different generations of the recirculated test signal information. Again, the adjustments are normally made to minimize signal deterioration after multiple generations of recirculation. Such adjustments are normally made by a VTR machine manufacturer prior to shipment of his product, but the adjustments can be made by a user following the same techniques. In practice, user adjustments are usually made to fine tune the VTR's system gain and phase, system frequency and transient responses, system differential phase, and system parameters determining black, white and chrominance signal levels.

It should be appreciated that the above- described system permits changes in properties of recirculated video signals to be detected relatively easily because the changes in signal properties are usually multiplied with each recirculation genera¬ tion. For example, if a signal property is distorted one tenth of a percent with each recirculation, then after twenty recirculation generations, the signal property is changed by about two percent. Such relatively large changes or distortions are, of course, easier to detect and correct than the smaller changes

SUBSTITUTE SHEET that occur after only a single pass of signal information through VTR machine 106 and TBC device 107.

In the preferred embodiment of the invention, an interval of video information corresponding to an even number of consecutive horizontal lines taken from a composite video signal is recirculated for the selected number of times. To facilitate the adjustment procedure for NTSC type television signals, the interval is selected to be either 262 or 264 horizontal lines in duration. By selecting such an interval, the phase of the chrominance component included in the recirculated composite video signal will remain matched throughout the duration of the recirculations to the phase of the color subcarrier reference to which operations of VTR's, TBCs and other television signal processing devices are synchronized. Moreover, this phase matching is maintained without the need to separate the chrominance component from the composite video signal after each playback of the signal and, therefore, without the need to process the separated chrominance component to adjust its phase to that of the color subcarrier reference before recirculating and recording the video signal again. Such processing of the chrominance component would be necessary if an interval of video information corresponding to one television field (262-1/2 horizontal lines in NTSC type television signals) , or an odd number of horizontal lines was selected for recirculation. As described hereinabove, such processing is to be avoided because each such processing degrades the video signal and because successive such processings, as would be necessary because of the several recirculations of the video signal, would multiply the degradation. Moreover, adjustments of VTR 106 and TBC 107 using such a degraded signal would normally be erroneous, because the degradations are not a product of the normal operation of the VTR and TBC that is intended to be corrected by the adjustment procedure of the present invention.

In one preferred embodiment of the invention, an interval of video information corresponding to 262 consecutive horizontal lines taken from a composite NTSC type video signal is selected for recirculation the selected number of times. For such an interval of video information, the delay period between successive recordings of the video signal information corresponds to one field interval in time (i.e., the duration of 262-1/2 lines) . As will be described in further detail hereinafter, output signal processing devices of TBCs organize the consecutive lines of video information received from the TBC's memory into consecutively properly interlaced fields by inserting the required ^• synchronizing components in the video information at the proper times determined by the applicable television signal standard. As described hereinabove, about one-third of the delay is of a mechanical nature due to spacing of playback head 29 from record head 27 on the drum guide assembly of FIGURE 1. The remainder of the delay is due to TBC device 107 and signal propagation delays of the other circuits in the recirculation path.

It should also be appreciated that the

SUBSTITUTE SHEET system of FIGURE 3 can be operated in a mode where, during signal recirculation, VTR 106 is caused to record recirculated video information. The result of the recording will be a videotape that carries the multiple generations of the recirculated field, with each generation being recorded separately on the videotape. The generations can be individually evaluated by repetitively playing back a single recorded track from the tape to obtain continuously a selected generation of the video test signal. Based upon evaluation of the information continuously played back from the tape, desired adjustments can be made manually to VTR 106 and TBC 107.

FIGURE 4 shows a system for automatically adjusting a VTR machine 306. In this embodiment, VTR machine 306 should be understood to include an integral TBC device of the type previously described having a memory of a capacity sufficient to store an interval of video information corresponding to the amount of delay required to be provided by the TBC. Existing TBCs having a memory capable of storing the video information portion of a composite video signal of at least one television field have sufficient capacity for this purpose. In the system of FIGURE 4, lines 351a and 351b carry recirculated signals from VTR machine 306 to a sample-and-hold circuit 355. The sample-and-hold circuit 355 includes first and second switches 359 and 361, respectively, that are connected for control by a digital logic circuit 365. Together, the sample-and-hold circuit 355 and the logic circuit 365 comprise means for automatically adjusting parameters and levels of video signals that are reproduced by VTR machine 306. Particulars of digital logic circuit 365 and sample-and-hold circuit 355 of FIGURE 4 will now be described; however, it should be understood that those circuits are shown by way of example only and that alternative circuits can be provided to accomplish substantially the same functions. Thus, in FIGURE 4, a first AND gate 367 in logic circuit 365 is connected to control first switch 359 in sample-and-hold circuit 355 and, similarly, an AND gate 371 in logic circuit 365 is connected to control second switch 361 in sample-and- hold circuit 355. Both AND gates 367 and 371 receive video drive signals, on line 375, that indicate occurrence of an input of video test signal information. The video drive signals can originate, for example, from a conventional signal generator. In addition to the video drive signal, first AND gate 367 receives, via line 377, signals indicative of the occurrence of a selected recirculation cycle of video test signal information through VTR machine 306. Similarly, second AND gate 371 receives signals on line 381 that indicate occurrence of a later selected recirculation of test signal information. In practice, the signal on line 377 preferably indicates the first generation of test signal information and the signal on line 381 indicates a preselected later generation of test signal information.

The components of sample and hold circuit 355 in FIGURE 4 will now be described. In that circuit, first switch 359 is connected to a capacitor 383 in parallel with a first input of a differential

SUBSTITUTE SHEET amplifier 387. Likewise, second switch 361 is connected to a capacitor 389 in parallel with the second input of differential amplifier 387. Capaci¬ tors 383 and 389 and differential amplifier 387 are conventional components. When connected as shown in FIGURE 4, the output signals from differential ampli¬ fier 387, indicate the difference in voltage of capacitors 383 and 389. The output of differential amplifier 387 is carried on line 391 to VTR machine 306.

Before proceeding with a description of the operation of the complete system of FIGURE 4, operation of digital logic circuit 365 in conjunction with sample-and-hold circuit 355 will be described. Initially, it is worthwhile to again mention that digital logic circuit 365 and the sample-and-hold circuit 355 are described by way of example only and that other equivalent circuits can be provided. In operation of digital logic circuit 365, AND gate 367 provides a logical "1" output signal if, and only if, its inputs on lines 375 and 377 are both logical "l's". Likewise, AND gate 371 provides a logical "1" output if, and only if, its inputs on lines 375 and 381 are both logical "l's". In practice, the input on line 377 is preferably predetermined such that lines 375 and 377 both carry logical ^M1^M signals only during the period of the first recirculation of video signal information; accordingly, in such a situation, the output of AND gate 367 will be a logical "1" only during the first recirculation. Similarly in practice, the input on line 381 is predetermined so that lines 375 and 381 both carry a logical "l" only during the selected "nth" signal recirculation; accordingly, only after the "nth" recirculation generation, will the output of AND gate 371 be a logical "1". With the preceding in mind, it can be understood that sample-and-hold circuit 355 operates such that occurrence of a logical "1" on line 369 causes closure of switch 359 and, similarly, occurr¬ ence of a logical "1" on line 373 causes closure of switch 361.

Operation of the complete control system of FIGURE 4 will now be described. Initially, the VTR machine 306 is placed in the standard edit mode and a tape is played back which has one field of video test signal information recorded on a track extending along the tape. The VTR machine 306 is caused to reproduce the recorded field of video test signal information and then to recirculate the signal for the desired number of recirculation cycles. In other words, the system is controlled to first commence the reproduction of the field of recorded video test signal with transducing head 29 and, after an interval that is less than one television field interval by an amount determined by the separation of the transducing heads 27 and 29 on the drum assembly 11 of FIGURE 1, i.e., after an interval of about two- thirds of a television field interval, to commence recording again in an adjacent track a field of video signals containing the selected even number of horizontal lines (262 in the preferred embodiment) taken from the original field of video test signal, using transducing head 27. Thereafter, the two transducing heads continue to record and playback the selected even number of horizontal lines simultaneously, with the recording and playing back of a given portion of a given generation of the signal separated in time by the delay selected for the recirculation path (a delay corresponding to 262 horizontal lines in the preferred embodiment) and occurring in adjacent tracks on the tape. As previously described, this continuous and simultaneous recording and playing back of the video test signal lasts for a selected number of generations of video test signals.

Upon initiation of signal recirculation in the system of FIGURE 4, a pulse is provided on line 375 with the duration of the pulse being sufficient to encompass the selected number of recirculation cycles; for example, the pulse on line 375 could persist for twenty recirculation cycles. Concurrent with initiation of the first recirculation cycle, a pulse is provided on line 377. With pulses on both lines 375 and 377, the AND gate 367 provides a logical "1" output signal which, in turn, maintains closure of switch 359 within sample-and-hold circuit 355. With switch 359 closed, the video output signals from VTR 306 appear across capacitor 383 and provides a voltage charge representative of the voltage level of the initially recirculated video signal information. After the recirculation process proceeds to the nth recirculation generation, a pulse is provided on line 381. That pulse causes the AND gate 371 to provide a logical "1" output on line 373 and, thereby, causing closure of switch 361 in sample-and-hold circuit 355. With switch 361 closed, capacitor 389 is charged to a voltage representative of the voltage level of the nth generation of the recirculated video signals. Thus, with capacitors 383 and 389 both charged as described above, dif¬ ferential amplifier 387 provides an output signal equal to the difference in voltage at its inputs. In the case where the voltages on capacitors 383 and 389 are proportional to system gain, for example, the output of differential amplifier 387 represents the difference in video system gain that occurred over a series of "n" recirculations through VTR machine 306. Alternatively, capacitor 383 can be charged with a reference voltage other than the voltage representa¬ tive of the first recirculatior of a video test signal information; in such an instance, the output of differential amplifier 387 would represent the difference between a property of the nth recircula- tion signal and the selected reference voltage.

The sample-and-hold circuit 355 can be operated to evaluate the long term average of a signal parameter or to evaluate the signal parameter at a particular instant during the video test signal interval. If a long term average evaluation, for example, over the entire interval of the video test signal is of interest, each of the switches 359 and 361 are operated to remain closed for the entire playback interval of the video test signal during the particular generations from which samples are taken by the sample-and-hold circuit 355. Such operation is achieved, for example, by providing logical "1" pulses at the appropriate times on a lines 377 and 381 of a duration corresponding to the entire playback interval of the selected generation of the video test signal. If, however, an evaluation of the signal parameter at a particular instant during the video test signal is desired, the duration and time of occurrence of the logical "1" pulses are selected so that switches 359 and 361 are closed during the playback of the selected generations of the video test signal at the desired instant and for the desired duration to be evaluated. It will be appreciated that the duration and time of the sampling of the two generations of the video test signal will be selected according to the nature of the signal parameter evaluation desired.

The output of differential amplifier 387 of

FIGURE 4 can be used to automatically adjust controls such as system gain controls in VTR machine 306 or in an associated time base corrector device. In practice, the output signals from amplifier 387 normally provide negative feedback to minimize changes in video signals as a result of several generations of recording and reproducing the signals through VTR machine 306.

At this juncture it should be understood that comparisons of video signal information values from different recirculation generations should be derived from the same video field. For example, if conditions were such that two interlaced video fields Fl and F2 forming a single raster frame of video information pass through VTR machine 306 during each recirculation cycle, it is preferred to compare the value of a property of a signal from the first generation of field Fl with a value of a property of the same signal from a later generation of field Fl, not field F2.

To facilitate comparison of the recirculated video signal information from generation to genera¬ tion, it is preferred that the selected delay period be such that recirculated video signal information is repetitively displayed at nearly the same raster position upon successive recirculations. For this reason, the preferred delay period equals an even number of horizontal line intervals that is most nearly equal to the duration of one video field. Further to obtain meaningful comparisons between recirculated generations of video test signals, especially in the case of color video signals, it is necessary that recirculation be properly delayed after each generation to achieve proper phase synchronization with the color subcarrier reference signals.

For purposes of explanation of the required delay period for synchronization and stability of the chrominance component relative to the color subcarrier reference, it is instructive to consider the case where a field of color video information according to the NTSC standard are to be recirculated through the system of FIGURES 2 or 3 with a delay period through recirculated signal path equal to the duration of one television field. With this delay, the chrominance component of each horizontal line of the video signal information retrieved from the TBC memory would have the same phase relationship relative to the horizontal sync pulse defining the beginning of the line as the original, first generation of the video signal information. As discussed previously, NTSC color video signals have a chrominance component whose phase alternates line to line by 180^* relative to the occurrence of the horizontal sync pulse. Also, because the horizontal lines of consecutive fields are timed to be spatially interlaced upon display at different vertically displaced raster positions, the chrominance component of horizontal lines consecutively displayed at a given raster position alternates in phase by 180^*. Thus, a sequence of four consecutive television fields occurs between displays at a given raster position of horizontal lines having a chrominance component of the same phase. Consequently, the recirculation of the video signal information through a delay of one television field interval would ^'introduce an undesirable phase discontinuity in the chrominance component relative to the color subcarrier reference. Such discontinuity would prevent proper adjustment of the VTR and TBC in accordance with the present invention.

To overcome the adverse effects of such phase discontinuity, the correct chrominance phase during the recirculations of the video test signal, the delay period through the recirculation path is made equal to an even number of horizontal lines. This assures that the chrominance phase of the lines retrieved from the TBC memory will be matched to the required phase defined by the phase of the color subcarrier reference. As previously described, this is accomplished in the preferred embodiment of the invention by providing a delay from the TBC memory so that a total delay is provided through the recirculation path corresponding to an interval of 262 lines. As a result of this delay period, which is less than the delay of a complete field under the NTSC standard, one horizontal line interval of the original video test signal will be discarded by operation of the TBC memory and not recirculated. As a result, the remaining horizontal lines of the field of video test signal retrieved from the TBC memory are displaced in time by an interval corresponding to the discarded line. While the re ainng horizontal lines are so displaced in time, the video test signal is provided by the TBC in a horizontally and vertically synchronized relation to the horizontal and vertical reference signals with respect to which the operations of the VTR, TBC and operatively associated devices are commonly synchronized and controlled. This synchronizing provision of the remaining horizontal lines of the video test signal is a result of the inherent operation of TBC's in providing horizontal lines of video signal synchronously relative to a stable horizontal reference timing signal, and in providing a number of horizontal lines corresponding to one television field interval synchronously relative to a stable vertical synchronizing reference timing signal. The aforementioned time displacement of the horizontal lines retrieved from the TBC memory coupled with the maintenance of horizontal and vertical signal timing stability results in the recirculated video test signal that gradually moves vertically through the display provided by a monitor. This movement occurs at the rate of one horizontal line per television field interval. Such movement does not interfere with the ability to adjust VTRs, TBCs and operatively associated devices in accordance with the present invention, because the chrominance, horizontal and vertical timing of the recirculated video test signal is maintained synchronous relative to the

SUBSTITUTE SHEET corresponding stable reference timing signals.

The manner in which the TBC operates to maintain the recirculated video test signal synchronized relative to the stable reference timing signals will be better understood from the following description of the operation of a typical digital TBC device. Common digital TBCs include a memory arranged to store digital representations of samples taken of analog video signals received by the TBC. The memory is operatively associated with a memory address generator that controls the times when and storage locations at which the digital representations are stored and retrieved in the memory. The memory address generator usually is organized in two parts, one for controlling the storage of digital representations in the memory and the other for controlling the retrieval of the representations from the memory. Ordinarily, the operation of the memory address generator governing storage in the memory is controlled by timing signals derived from the color burst, horizontal sync and vertical sync components included in and extracted from the received video signal. Thus, the received video signal is stored in the TBC memory at times determined by the timing of the received video signal itself. In retrieving the stored video signal from memory, however, the memory address generator is controlled by stable timing reference signals derived from the stable color subcarrier, horizontal sync and vertical sync reference signals that are commonly used to synchronize and control the operations of VTRs, TBCs and other operatively associated devices. The memory address generator is arranged in such TBCs to supply storage and retrieval memory address signals so that the retrieval of a video signal representation at a particular storage location normally occurs an interval following its storage corresponding to one-half the maximum delay the memory is capable of providing. This normal interval between storage and retrieval at a particular storage location occurs when the received video signal is properly timed relative to the stable reference signals.

A difference from the proper timing relationship is reflected as a corresponding difference from the proper timing relationship between the timing signals derived from the received video signal and the stable timing reference signals. This difference results in a change in the interval between the times a storage location within the TBC's memory is selected by the memory address generator for storage of a particular video signal representation and for subsequent retrieval. Such change in the interval between the storage and retrieval of the video signal representations results in a change in the delay of the transmission of the video signal through the TBC that compensates for the difference in the timing of the received video signal. For example, if the video signal is received by the TBC earlier than the proper time defined by the stable timing reference signals, the storage locations within the TBC memory are accessed for storage earlier by the memory address generator. Since the times of retrieval of video signal representations from storage is fixed by the stable timing references signals, the earlier storage of the video signal representations results in a longer interval of storage of the video signal representations in the TBC memory. The longer interval of storage in the memory differs from the normal interval of such storage by the same amount as the earlier time of receipt of the video signal differs from the proper time of receipt. Thus, the longer storage interval within the memory compensates for the change in the time of receipt of the video signal. It will be appreciated that the TBC also functions in a comparable manner to shorten the storage interval within the memory to compensate for the receipt of video signals later than the proper time defined by the stable timing reference signals.

Most digital TBCs are arranged to store digital representations of the intervals of the composite video signal defined by consecutive horizontal line blanking intervals and often provide a short interval of line identifying data for each one or a plurality of the horizontal line intervals. The color burst, blanking, horizontal sync and vertical sync (together with the pre and post equalization intervals) synchronizing signal intervals contained in the received composite video signal are discarded in favor of new corresponding signals inserted in the video signal retrieved from the TBC memory. These new synchronizing signals are inserted in the retrieved video signal by output signal processing devices that are in operative association with TBCs. Such devices function to insert the new synchronizing signals into the video signals so that the sequence of horizontal lines retrieved from the TBC memory are organized into fields of video signals of the proper number of horizontal line intervals (for NTSC television signals, 262-1/2 line intervals), with the various new synchronizing signals located in the video signal at the proper times. Such signal processing devices function to insert the synchronizing signals according to the number of consecutive horizontal lines retrieved from the TBC memory, without regard to the particular raster line location in which a particular retrieved horizontal line appeared in the composite video signal received by the TBC. Consequently, the stored line intervals of digital representations can be retrieved in any order and any number of the stored intervals can be retrieved from the memory, and organized into field intervals of a composite video signal through the operation of output signal processing devices commonly included in digital TBCs.

This characteristic of digital TBCs is used to advantage in the method and apparatus of the present invention to enable repetitive recording and playing back of an interval of the original video test signal corresponding to an even number of horizontal line intervals synchronously with the color subcarrier, horizontal sync and vertical sync related timing reference signals. More specifically, as previously described with the embodiments of the present invention illustrated in FIGURES 1-4, the video test signal played back by transducing head 29 and received at the input of, for example, TBC 107 is delayed from the time it was recorded by the circumferentially displaced transducing head 27 by an interval of about one-third of a television field interval. This delay in the playback of the video signal is seen by the TBC as a timing difference relative to the proper timing relationship between the video signal received by the TBC and the timing reference signal. TBC responds to this timing difference as previously described to introduce an compensating delay between the storage and retrieval times of the stored video signal digital representations. In accordance with the present invention, control 108 (FIGURE 3) associated with the memory address generator of TBC 107 is adjusted to set the cycle of storage location addresses generated for the retrieval of stored video signal digital representations so that the retrieval of the representations is changed to introduce a compensating delay that provides a total delay for the video test signal through the recirculation path corresponding to an even number, preferably, of 262 horizontal line intervals. In this manner, the phase of the chrominance component in the recirculated composite video signal remains synchronized to that of the color subcarrier timing reference signal. Such adjustment is accomplished by discarding one horizontal line interval of the original video test signal. As previously discussed, this results in a gradual movement of the video test signal vertically upon the display. However, since the chrominance, horizontal and vertical timing of the recirculated video test signal is maintained synchronous relative to the corresponding stable reference signal by virtue of the cooperative action of the TBC memory and operatively associated output signal processing device, no interference in the adjustment of the VTR, TBC and any other operatively associated devices in

SUBSTITUTE SHEET the video signal path occurs.

It can now be understood that such discarding of one horizontal line interval in the above-described manner will, upon display of the remaining recirculated field of video test signal, cause shifting of the displayed image information in the vertical direction upward by one line per each recirculation. Thus, if the test signal information is a color-bar test pattern, the pattern will appear to roll upward by one line with each generation. This will cause apparent vertical rolling of the raster display. However, vertical sync stability is maintained by virtue of the operation of the output signal processing device associated with the TBC. As described previously, such device operates to insert the vertical synchronizing signal interval and associated pre and post equalization synchronizing signal intervals in the sequence of horizontal line intervals of the video signal retrieved from the TBC memory at the proper times relative to the stable timing reference signal. Thus, the vertical movement is an artifact that ordinarily does not substantially affect detection of changes in signal values or adjustments of video signal parameters. In other words, adjustment of video signal parameters can be readily made as long as the recirculated video signal information is properly timed with respect to horizontal synchronizing and color subcarrier synchronizing signals during recirculation. In fact, if such synchronization can be adequately maintained during recirculation through a VTR without passage through an operatively associated time-base corrector, the time base corrector would be unnecessary and could be replaced by fixed delay of an appropriate length.

At this juncture, it can be appreciated that an alternative way of providing a satisfactory delay period (i.e., a period having an even number of lines of delay) is to adjust the memory address generation to cause the TBC to introduce a total delay in the recirculation signal path corresponding to the duration of 264 horizontal lines. This is achieved by setting the cycle of storage location addresses generated for the retrieval of stored video signal digital representations so that the retrieval of the representations is delayed an additional horizontal line interval. The additional delay results in maintaining the phase of the chrominance component in the recirculated composite video signal synchronized to that of the color subcarrier timing reference signal. However, this is at the expense of introducing a gradual movement of the video test signal vertically downward upon its display on a monitor. As in the embodiment arranged to provide a total delay in the recirculating signal path corresponding to 262 horizontal line intervals, the recirculated video information contained in the test signal is properly timed with respect to the horizontal synchronizing and color subcarrier synchronizing signals, and the video test signal is synchronized to the color subcarrier, horizontal and vertical synchronizing reference signals. Thus, the vertical shifting artifact does not ordinarily interfere with comparisons of signal values from generation to generation. Embodiments of the present invention can be arranged to adjust VTR's, TBC's and other operatively associated devices designed for PAL, SECAM and other television signal standards as well. However, television signals organized according to such other television signal standards have signal level, frequency, phase, timings and other well known properties that are different from television signals organized according to the NTSC standard. In constructing embodiments of the present invention to adjust devices designed for such other television signal standards, it will be necessary to provide a delay in the video signal recirculation path having an interval selected according to the standard that will maintain the recirculated video signal synchronous relative to the chrominance, horizontal and vertical synchronizing reference timing signals. The required delay interval is readily determined from the properties of the television signal standard for which the VTR, TBC and other operatively associated devices are designed, and therefore need not be described in detail.

The electronic edit control systems commonly associated with VTR's for teleproduction and broadcast applications can be operated to enable the continuous generation of any selected plurality of sequences of a number of regenerations of a video signal, with each sequence consisting of regenerations of a different video signal. This has advantages of making use of the existing edit control system to execute the steps of the recirculation technique carried out, for example, by the apparatus shown in FIGURES 3 and 4, and of making use of a convenient technique for performing the apparatus adjustment method of the present invention.

In embodiments of the present invention using electronic edit control systems for executing the steps

SUBSTITUTESHEET of the video signal recirculation and apparatus adjustment method, an interval of a continuous color video test signal is first recorded by the VTR along a length of tape. This can be achieved, for example, by the use of the signal generator 113 of FIGURE 3. For this purpose, the switch 123 is positioned permanently to couple line 115 to the input line 125 to the VTR 106 for the duration required to record the interval of the continuous color video test signal. During this recording of the test signal, other typical control signals, namely, a control track signal and a time code signal are also recorded along the tape synchronously with the video signals. The time code identifies each frame of video signals formed by two interlaced television fields by a unique address signal in units of hours, minutes, seconds and frames. For NTSC color television signals, the time code is in the form of the time and control code standard adopted by the Society of Motion Pictures and Television Engineers. The interval of continuous video test signal recorded on the tape can be of any length desired. Ordinarily, the length of the interval will be selected to provide continuously and without interruption in time a plurality of sequences of a selected number, typically, 20, regenerations each of the video test signal that will enable sufficient time to perform adjustments of the apparatus in accordance with present invention without interruption in time.

After the selected length of continuous color video test signal is recorded on the tape, a list of time code signals identifying series of unique edit entry and edit exit frames of the recorded video test signal is selected and input to the edit control system via operator controls provided for that purpose. Each selected edit entry code specifies a particular frame at which a recirculated field is to be first recorded after its initial reproduction from the tape, and each selected edit exit code specifies a particular frame at which such recirculated field is to be last recorded. Thus, each pair of edit entry and exit codes define the selected number of recirculations of the video test signal, hence, regenerations of each sequence of the plurality sequences.

Present electronic edit control systems are controllable to select either of the two interlaced fields of a frame identified by a time code signal for commencing or terminating an edit. In accordance with the present invention, the edit control system is operated to record successive sequences of recirculated video signals at locations along the tape separated by a tape length corresponding to that required to record one field. Since the aforedescribed helical scan VTR's record one field in each track along the tape, the series of edit entry and exit time codes are selected and the edit control system is controlled to respond to the edit exit code of one sequence and the edit entry code of the following sequence to interrupt the record operation of the VTR for an interval of time required by the VTR to transport the tape a distance separating adjacent recorded tracks on the tape.

Following the input of the series of edit entry and exit time code signals, the edit control system is placed in an insert edit mode of operation. In this mode, the edit control system controls the operatively associated VTR to replace an interval of previously recorded information with a new recording of information that is phase or time coherent with the remaining previously recorded information. Upon entering this mode, the edit control system first controls the VTR to synchronize the transport of the tape to the desired normal record/reproduce speed and position the tape relative to the record transducing head at which the new recording is to be commenced. During this interval, the aforedescribed reproduce transducing head (29 in FIGURE 1) is operated to reproduce the previously recorded video test signal. While each field is reproduced and recirculated as the tape is transported to position the track identified for commencing the new recording at the location of the rotating transducing head, the record transducing head (27 in FIGURE 1) is disabled by the edit control system during this period. As the identified track reaches the location of the rotating transducing heads, the recorded time code signal identifying the track is reproduced by the time code transducing head of the VTR and is communicated to the edit control system. In response to the receipt of this reproduced time code signal, the edit control system enables the record transducing head to record the recirculated video test signal obtained from the last field of the video test signal reproduced by the reproduce transducing head before the identified track reaches the location of the rotating transducing heads. The record transducing head remains enabled by the edit control system until the track on the tape identified by the edit exit time code signal paired with the associated edit entry time code signal that commenced such insert edit operation reaches the location of the rotating record transducing head. During this interval, the recirculated video test signal is repetitively recorded, reproduced and recirculated through the signal path defined, for example, by the VTR 106, TBC 107 and any other operatively associated devices in the recirculation path 133 (see FIGURE 3) as previously described. This results in the generation at output line 135 of the number of generations of the video test signal corresponding to the interval defined by the pair of edit entry and exit time codes.

When the track defined by the edit exit time code arrives at the location of the rotating transducing heads, the corresponding time code signal recorded on the tape is reproduced by the time code transducing head of the VTR. This time code signal is communicated to the edit control system which responds by disabling the record transducing head, thereby, terminating the recording of the recirculated video test signal.

As previously described, the next edit entry time code of the selected series of edit time codes identifies a track recorded on the tape two track locations from the track along which was recorded the last recirculation of the previous sequence of recirculated video test signal. Therefore, the record transducing head remains disabled during the time one recorded track is scanned by the record transducing head. As will be appreciated, this corresponds to the interval of one television field. However, during this interval, the reproduce transducing head is operable to reproduce the video test signal recorded in the track and couple same to the operatively associated TBC and any other operatively associated devices for presentation, for example, to a monitor 139 (FIGURE 3) .

When the track defined by the next selected edit entry time code arrives at the location of the rotating transducing heads, the corresponding time code signal is reproduced by the time code transducing head of the VTR. As previously described, the edit control system responds to the reproduced edit entry time code signal to again enable the record transducing head. As a result, the transducing head is enabled to record the recirculated video test signal obtained from the last field of the video test signal reproduced by the reproduce transducing head before the identified track reaches the location of the rotating transducing heads. As will be appreciated from the foregoing, the last field is the field of previously recorded original video test signal located in the track separating the tracks identified by the edit exit time code of the immediately previous completed recorded sequence of recirculated video test signal and by the edit entry time code of the next sequence of recirculated video test signal to be recorded. During the following interval terminating with the reproduction of the time code signal from the tape corresponding to the next edit exit time code of the series of selected time codes, the recirculated video test signal is repetitively recorded, reproduced and recirculated through the signal path defined by the VTR and operatively associated devices coupled to the rotating record and reproduce transducing heads. As during the previous sequence of recirculations of the video test signal, this results in the generation at the output of the VTR the number of repetitive generations of the video test signal corresponding to the interval defined by the second pair of edit entry and exit time codes included in the series of selected edit time codes.

The edit control system continues to control the VTR in aforedescribed manner in response to the selected series of edit codes it receives from the operator via an input included in controls 108 (FIGURE 3) , until the last of each of the sequences of recirculated video test signal defined associated pairs of edit entry and edit exit time codes is recorded on the tape. As will be appreciated, this results in the continuous and uninterrupted provision at the output of the VTR of the several sequences of multiple generations each of the video test signal. Furthermore, by appropriate selection of the edit exit and entry time codes defining each sequence and the delay interval provided by the TBC memory, the recirculated video signal will remain synchronized to the chrominance, horizontal and vertical timing reference signals. This is conveniently achieved by selecting pairs of edit exit and entry time codes separated by an amount corresponding to a multiple of the number of fields defining a color frame (which is four in color television signals organized according to the NTSC standard) and operating the TBC memory to provide a total video signal delay in the video signal recirculation path corresponding to 262 horizontal line intervals. It should be appreciated, however, that the lengths of all sequences of recorded recirculated video test signal do not have to be same. Hence, the interval separating the edit entry and exit time codes defining each sequence can change from sequence to sequence. However, maintaining the lengths of the sequences uniform does facilitate accurate adjustment of the video signal processing devices.

From the foregoing description of the embodi¬ ment of the method of practicing the present invention through the use of existing edit control systems designed for use with teleproduction and broadcast type VTR's, it will be appreciated that such embodiment of the method is similar to that practiced by the apparatus illustrated in FIGURE 3. In this regard, the video switch 123 of such apparatus performs the equivalent function performed by the edit control system in enabling and disabling the record transducing head 127 carried by drum assembly 11 of FIGURE 1. When the switch 123 is in the position that couples line 115 to the input line 125 to the VTR 106, it disconnects the VTR 106 from the recirculation path 133. The same result is obtained when the edit control system disables the record transducing head, since this also terminates the recirculation and recording of the video test signal. Placing the switch 123 in the condition that couples the input line 125 to VTR 106 to the recirculation path 133 allows the recirculated video test signal to be repetitively recorded on the tape by the VTR 106. This also occurs when the edit control system enables the record transducing head.

The embodiments of the method and apparatus of the present invention previously described to provide repetitive recording and playing back of intervals of composite video information continuously without interruption, i.e., in real time, to obtain a desired number of generations of intervals of a composite video signal rely upon delaying each interval an amount corresponding to an even number of horizontal line intervals as it is repetitively recirculated for such recording and playing back to maintain a synchronous relationship between the chrominance component of the video signal interval that is recirculated and the color subcarrier reference timing signal. However, such synchronous relationship can be achieved and maintained by lengthening or shortening each horizontal line contained in the recirculated interval of video information by an amount corresponding to one-half, or odd multiple thereof, cycle of the color subcarrier signal component of the video signal. In embodiments such as illustrated in FIGURES 3 and 4, this is achieved by adjusting controls 108 associated with the memory address generator of the TBC to retrieve each horizontal line interval of the stored video test signal at a time that is changed relative to the horizontal reference timing signal by the aforesaid fraction of the color subcarrier signal cycle. As a result of such adjustment, the recirculated video signal moves gradually in the horizontal direction when display on a monitor. However, the recirculated video signal remains synchronous to the chrominance, horizontal and vertical reference timing signals throughout the duration of recirculation. This movement occurs at the field rate of the video test signal (60 Hertz in NTSC television signals) and is an amount corresponding to the number of multiple one-half cycles of color subcarrier signal that each horizontal line interval is adjusted.

Regardless of the manner in which the delay is achieved by the TBC memory for the recirculation path, the VTR's, and any other operatively associated devices are adjusted by comparing two or more generations of the selected video signal interval obtained through recording and reproducing the selected interval by the VTR. By comparing such two or more generations, changes in the properties of the video signal can be accurately determined, since such changes can only result from operations performed on the video signal by the devices during the passages of the signal through the devices between the compared generations.

Although the present invention has been described with particular reference to the illustrated embodiments and although various modifications and alternatives have been discussed, such disclosure is not to be interpreted as limiting. Indeed, various other modifications and alternative embodiments will likely become apparent to workers skilled in the art after having read the disclosure. For example, although the embodiments have been described in detail with respect to color television signals organized according to the NTSC standard, the video signal adjustment technique of the present invention is adaptable to television signals organized according to other standards. Therefore, the appended claims should be interpreted as covering all of the various alternative embodiments as fall within the true spirit and scope of the present invention.

Claims

WHAT IS CLAIMED IS t

1. A method of adjusting video signal values to improve video image quality provided by a videotape record and playback machine comprising the steps of: recording at least one video field interval including video test signal information; reproducing and then storing said video test signal information included in the said at least one video field interval; after a delay period equal to the duration of an even number of horizontal lines and approxi¬ mating one video field interval, retrieving the stored said video test signal information for recording again; repeating the steps of recording, reproduc¬ ing, storing and retrieving said video test signal information for a selected number of times to form a generation of said video test signal upon each reproduction of it; and adjusting a video signal processing machine to change video signal values in response to changes detected between different generations of the video test signal information.

2. A method for improving video image quality provided by a video record and playback machine of the analog type comprising the steps of:

(a) delaying video test signal information in a memory operatively associated with the video record and playback machine;

(b) operating the record and playback machine to record the test signal information from the memory, to reproduce the recorded information, and then to store the reproduced information in the memory;

(c) repeating step (b) above to provide multiple generations of the test signal information with a selected delay between consecutive recordings such that each generation is recorded and temporarily stored in the memory for a delay period equal to the duration of an even number of horizontal lines and approximately one video field interval; and

(d) adjusting the record and playback machine to decrease errors in the test signal in accordance with changes in the reproduced test signal produced in the multiple generations of the test signal.

3. A method according to Claim 2 wherein the recirculated signals are recorded and displayed from a single field of video information.

4. A method according to Claim 2 wherein the test signal information is recorded by a first video transducing head and reproduced by a second video transducing head such that the time between recording and reproduction is less than the time required to record a single video field.

5. A method according to Claim 2 wherein the test signal information is recorded by a first video transducing head and reproduced by a second video transducing head such that the time between recording and reproduction is the selected delay.

6. A method for adjusting video signal parameters in a video record and playback machine of the analog type including digital circuitry having operatively associated memory to provide a selected delay, said method comprising the steps of:

(a) providing video test signal information to the video record and playback machine;

(b) operating the video record and playback machine to record said test signal information and thereafter reproduce and transmit the recorded information to the memory;

(c) after a delay selected to equal an even number of horizontal lines of video information, recirculating signal information from the memory to the input of the video record and playback machine;

(d) operating the video record and playback machine to record said recirculated test signal information;

(e) operating the video record and playback machine to reproduce the most recently recorded signal information and transmit the same to the memory;

(f) repeating steps (c) through (e) for a selected multiplicity of cycles; (g) determining changes in signal values after the selected number of recirculation cycles; and

(h) adjusting video signal parameter controls of the record and playback machine to decrease errors in the test signal in accordance with changes in the reproduced test signal produced in the multiplicity of recirculation cycles.

7. A method as defined in Claim 6 further including the steps of counting the number of recirculation cycles and operating a video switch to terminate recirculation of video signals after said predetermined number of recirculation cycles.

8. A method as defined in Claim 6 wherein test signal information is recorded by a first video transducing head and played back by a second video transducing head such that the time between record and playback is less than the delay of the recirculation path.

9. A system for providing adjustment of video signal parameters to improve image quality provided by analog videotape recording equipment comprising: (a) an analog videotape recording machine including operatively associated digital delay circuitry that provides memory having a selectable delay;

(b) coupling means selectively connecting the output of the recording machine to its input to recirculate video information from the digital delay circuitry and to replace in the memory, after each recirculation cycle, stored information with recircu¬ lated information; and (c) comparator means connected to provide a comparison of signal values received from the memory with selected values.

10. A system as defined in Claim 9 wherein the comparator means is connected to receive values representative of different generations of a selected video parameter and is operative to provide an output proportional to the difference between the values.

11. A system as defined in Claim 10 wherein the output from said comparator is connected to the adjustment means and provides an output proportional to the differences between said values.

12. A system as defined in Claim 9 wherein the coupling means includes a video switch.

13. A system as defined in Claim 9 wherein the coupling means is a routing switcher.

14. A system as defined in claim 13 further including adjustment means for adjusting video signal values in response to detected changes in the test signal information from one generation to another.

15. A system as defined in claim 14 wherein said adjustment means comprises an electronic edit control system.

16. A system as defined in claim 15 wherein said electronic edit control system provides means to define the selected number of recirculations of the video test signal information.

17. A method of adjusting video signal values to improve video image quality provided by a videotape record and playback machine comprising the steps of: recording at least one video field interval including video test signal information having chrominance components; reproducing and then storing said video test signal information included in the said at least one video field interval; after a delay period selected to maintain a synchronous relationship between chrominance components of the video test signal information and a color subcarrier reference timing signal that is independently generated, retrieving the stored said video test signal information for recording again; repeating the steps of recording, reproduc¬ ing, storing and retrieving said video test signal information for a selected number of times to form a generation of said video test signal upon each reproduction of it; and adjusting a video signal processing machine to change video signal values in response to changes detected between different generations of the video test signal information.

18. A method according to claim 17 wherein the delay period is selected to be equal to the duration of an integrated odd multiple of one-half cycle of the color subcarrier timing signal.