US3810166A

US3810166A - Electronic character generating systems

Info

Publication number: US3810166A
Application number: US00357142A
Authority: US
Inventors: L Atkin
Original assignee: Aston Electronic Developments Ltd
Current assignee: Aston Electronic Developments Ltd
Priority date: 1972-05-05
Filing date: 1973-05-04
Publication date: 1974-05-07
Anticipated expiration: 1991-05-07
Also published as: GB1374206A

Abstract

An electronic system for generating characters for television display by employing selected combinations of analogue waveforms. Ramp waveforms, usually of sawtooth form, are generated at line and field periods and are used to generate in turn triangular and parabolic waveforms. A memory contains data representing the shape of individual elements of characters for each quadrant of a display area and is actuated by a digital store. The memory serves to gate selected combinations of all the waveforms, together with blanking waveforms derived from the triangular waveforms, to synthesize a video character waveform. Means is provided for producing from the video character waveform a further video waveform, defining an area to outline the characters, which acts on a conventional video signal to hold the latter at the black level so that the video character waveform can be inserted thereon.

Description

Unlted States Patent 1 [111 3,810,166 Atkin [451 May 7, 1974 ELECTRONIC CHARACTER GENERATING 3,696,388 10 1972 Eichelber ger 340/324 A SYST S 3,711,849 l/l973 Hasenbalg 340/324 A [75] Inventor: gzufgrnge Walter Atkm, Richmond, Prima'y Examiner David L. Trafton g Attorney, Agent, or FirmSughrue, Rothwell, Mion, [73] Assignee: Aston Electronic Developments Zinn & Macpeak Limited, Deepcut, Camberley Surrey, England [22] Filed: May 41 1973 An electronic system for generating characters for [21] AppL NOJ 357,142 television display by employing selected combinations of analogue waveforms. Ramp waveforms, usually of sawtooth form, are generated at line and field periods Foreign Appllcatlon y Data and are used to generate in turn triangular and para- May 5, 1972 Great Britain 20983/72 bolic waveforms.

, A memory contains data representing the shape of [52] 340/324 178/7'5 340/324 A individual elements of characters for'each quadrant of [5 i] it. a di p y area and is actuated a digital Store The [58] of searchmf 340/324 324 memory serves to gate selected combinations of all the 178/75 D waveforms, together with blanking waveforms derived from the triangular waveforms, to synthesize a video [56] References cued character waveform, Means is provided for producing NIT STATES PATENTS from the video character waveform a further video 3,335,416 8/1967 Hughes 340/324 A 1 wav f rm, fining a r to in the h r 3,440,639 4/1969 Sander et al 340/324 A which acts on a conventional video signal to hold the 3,594,756 /1 71 G un g t /3 latter at the black level so that the video character 3,631,456 12/1971 Yokoyama .1 340/324 A waveform can be inserted thereon 3,668,687 6/1972 Hale 340/324 AD 3,696,387 lO/l972 Nussbaum 340/324 AD 22 Claims, 13 Drawing Figures LEMEN TS.

I V W IX I Y Z I HORIZONTAL VERTICAL MOD/FIERS. MOD/F IE RS.

PATENTEDMAY 1 1914 3810.166

sum 1 [1F 9 LEMENTS.

K I OUADRANT l IDENTIFICATION iz I V [W IX lY Zl F/g.7. W W

HORIZONTAL VERTICAL MOD/F IE RS MOD/F IE RS.

Fig. 2.

Fig.4

QUA 0 I{EL EMENT L MODIFIER x QUAD, EL EMEN 7 0 NO MOD/F/ER ELEMEN T e MOD/FIER w QuA02{ EL EMENT( MODlF/ER x QUAD2( EL EMENT 0 N0 MOD/F/ER QUA D 3(ELEMENTI7Z MODIF/ER x QUADJ ELEMENT 0 NO MOD/F/ER ELEMENT MOD/F/ER w ELEMENT c MODIF/ER Y ELEMEN Tm MOD/F/ER x QUAD {ZLEMENT 0 NO MOD/PIER QUAD {ELEMENTC MOD/F/ERZ LEMENTC MOD/HER Y :ATENTEUHAY 7 I974 3,810,166

SHEET 3 OF 9 TO RAMP PROCESSOR UNIT M /RE F E RENCE VOLTAGE 6v r-o 11,4760 ML 10 MIXER u/v/r SIZE CONTROL 220 FROM TERMINAL 5 POSITION 7412] 3 .$YNC.

*0 7 0 3 (FROM ev TERMINAL 9 OR 73 02 047 11 10 047 39K -avh H=680P- 0 l V=0'WF CHARACTER TIMEBASE u/v/r. Fig. 5.

asa-sza I }F ROM MOD/FIE R UNITS. AA-O BIAS FROM MIXER UNIT.

ELEM-m [GENERATOR PATENTED HAY 7 E974 SHEET 5 [IF 9 ELECTRONIC CHARACTER GENERATING SYSTEMS BACKGROUND TO THE INVENTION The present invention relates to an electronic system for generating video signals representing characters for television display.

It is known to superimpose characters such as captions etc. on a television picture display by directly placing caption cards 'or the like in front ofa television camera. It is however, for many purposes more convenient to automate this process so that the characters are generated electronically. Electronically generated characters for displays of this type may be built up from a matrix of dots; however, such characters tend to show stepped edges to curved and sloping strokes. Where smooth, well formed outlines are desired it is generally I necessary to use analogue waveform generation techniques and this invention relates especially to the latter aspect.

Hitherto analogue characterrgenerating systems have had a number of disadvantages. The characters are generally restricted to a fixed size, since for instance the parabolic waveforms used to form circular or eliptical character-strokes are produced by frequencysensitive circuits. Further, while it is often considered desirable for maximum clarity when white characters, for instance, are displayed against light-toned backgrounds, that they should have thin black outlines, the apparatus generally required to provide such outlines is relatively complex and costly.

It is a general object of the invention to provide an improved electronic system of the above-mentioned kind.

SUMMARY OF THE INVENTION According to the invention there is provided an electronic system for generating video signals representing characters for television display; said system comprising means for producing ramp waveforms conforming with vertical and horizontal character rates, respectively, means for producing from each of said ramp waveforms further waveforms including at least substantially parabolic waveforms, means for producing directly from selected combinations of all the waveforms outputs representing character elements, and means for producing said video signal fromv said character element-representing outputs.

The ramp waveform producing means would usually have separate inputs connectible to line and field sync pulse sources and preferably also a common input for receiving a character size control signal.

The further waveform producing means preferably provide, for vertical and horizontal senses, as well as parabolic waveforms, additional ramp waveforms such that there are provided in total waveforms both of sawtooth and of triangular form, (the latter being substantially isosceles) and a substantially square waveform one edge of which corresponds to the vertex of the isosceles-triangular waveform. The vertices of the triangular waveforms in their preferred form define the vertical and horizontal character centre-lines respectively, and the character elements are thereby generated symmetrically about the centre-lines in each case.

It will be appreciated that, at least over the range of frequencies corresponding to the range of character sizes desired, all the further waveform producing means, with special reference to the parabolic waveform producing means, should function substantially independently of waveform frequency. Thus, asall waveforms substantially retain their shape and amplitude as the ramp waveform frequency is adjusted the size of the displayed characters may be altered without substantially changing their form. It is a novel and preferred feature of the invention that parabola generating means are employed require (except as may be required for the compensation of high-frequency response) no reactive components, and which are thus frequency independent, enabling the size-of circular or eliptical character elements to be changed in a straightforward and convenient manner.

In order that the characters may be positioned as required on a television display, a digital store may be provided, controlled by a keyboard or other means, and addressed or interrogated by means of the outputs of horizontal and vertical counter units eachreceiving for instance appropriate square waveforms from the further waveform producing means. The coded information output from the digital store (which may be in the form of some standard character code, e.g., ASCII Code) at any instant, must be translated into information specifying the nature, i.e., the shape, of the corresponding character elements, and this may be achieved bya fixed memory means performing the function of a look-up table; The character elements are conveniently and preferably specified within the character shape memory, for each quadrant of the character display area.

The character element producing means may comprise a number of element generator units, and typically fifteen may be required to generate an adequate repertoir of character elements for the synthesis ofa full alpha-numeric character font together with a useful range of punctuation signs and symbols.

Each element generator unit receives a selected combination of waveforms from the waveform producing means, and preferably has characteristics such that the rate of transition of its output waveform is substantially dependent upon and is controlled by the rate of change of that selected combination. Thus the profile of the eventual video character waveform has a rate of rise and fall, in directions both parallel'to and perpendicular to the scanning lines, which is also controlled thereby and substantially dependent thereon.

The outputs of the element generator units maybe gated to a common output, as demanded by the character shape memory unit, in order to synthesise the required sequence of characters. Further gating or blanking signals may also be applied at the outputs of the element generator units inorder that partial or shortened elements may be generated where necessary; a means for generating vertical and horizontal modifiers to drive this further gating may be provided and its function will be clarified in the following description.

Preferably there is provided a means for producing a definitive outline to the characters, and the invention provides a novel method of generating a waveform defining an area outlining the characters by operating upon the character waveform. Since this waveform in its preferred form consists of signal excursions, representative of the profile of the characters, having distinct and controlled rise and fall periods, new video waveforms may be derived therefrom, the first of which is defined by the narrow peaks thereof to delineate the area of the, displayed characters and the second of which is defined by the broader troughs thereof to delineate an area outlining the displayed characters.

BRIEF DESCRIPTION OF DRAWINGS An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 depicts the quadrant identification used in connection with the generation of characters;

FIG. 2 identifies and depicts the elements and blanking modifiers used in the generation of characters;

FIGS. 3 and 4 depict typical characters together with the elements and modifiers applicable thereto;

FIG. 5 is a block schematic diagram of a charactergenerating system made in accordance with the inventron;

FIG. 6 is a circuit diagram of one of the character timebase units used in the system shown in FIG. 5;

FIG. 7 is a circuit diagram of one of the ramp processor units used in the system shown in FIG. 5;

FIG. 8 is a circuit diagram of one of the parabola generator units used in the system shown in FIG. '5; I

FIG. 9 is a circuit diagram of one of the element generator units used in the system shown in FIG. 5;

FIG. 10 is a circuit diagram of the mixer unit used in the system shown in FIG. 5;

FIG. 11 is a' circuit diagram of the outline generator unit used in the system shown in FIG. 5;

FIG. 12 is a circuit diagram of the video processor unit used in the system shown in FIG. 5; and

FIG. 13 is a circuit diagram of the modifier unit used in the system shown in FIG. 5.

DESCRIPTION OF PREFERRED EMBODIMENT Referring initially to FIG. I, the square area K on which a character is to be displayed, usually on a cathode ray tube, is notionally divided into quadrants denoted l-4. FIG. 2 depicts graphically the various rectilinear and curvilinear elements of equal thickness and denoted a which are used in the production of characters superimposed on the area K. FIG. 2 also depicts graphically the various horizontal and vertical blanking modifiers denoted v 2 which are again used in the production of the characters. These modifiers v z serve,.

when coincidently generated with the elements a 0 to blank or mask out the portions of the elements a 0 which fall in the shaded areas.

As described hereinafter, the system employs circuits for generating waveforms representing the elements a 0 and modifiers v z and all alphanumeric characters can be produced by an appropriate combination of these waveforms. Thus, for example, FIGS. 3 and 4 show the letters G and A together with the elements and modifiers applicable to each quadrant l-4 of the area K.

FIG. depicts a character-generation system and as shown the system has ahorizontal character timebase unit and a vertical character timebase unit 11. The

units

10, 11 have common inputs connected to a terminal 12 which receives a signal which controls the size of the characters. The size of the characters may be selected from one ofa number of pre-set sizes or may be varied continuously by means ofa variable voltage. The unit 10 has an input which is connected to a terminal 9 which receives line sync pulses from an associated T.V. system. Similarly the unit 11 has an input connected to a terminal 13 which receives frame or field sync pulses from the associated T.V. system. In certain applications these sync pulses may be generated within the system itself. The units l0, I] produce outputs 10a, 11a respectively which are in the form of ramp waveforms, more particularly sawtooth waveforms as depicted, representing the horizontal and vertical character rates. The units l0, 11 each employ a capacitor charged from a current source controlled by the input 12 to the

units

10, 11. These capacitors are each repeatedly charged and discharged between upper and lower voltage levels. The values of the capacitors and the charge currents are selected to provide the appropriate vertical and horizontal character rates. The current sources in the

units

10, 11 may be adjustable together so as to vary the frequency of the sawtooth waveforms without altering the ratio of the frequencies of the waveforms.

The output 10a from the unit 10 is fed to the input of a ramp processor unit 14 and similarly the output 11a from the unit 11 is fed to the input of a ramp processor unit 15. The unit 14 provides four outputs designated 14a to 14a which have waveforms as illustrated i.e. the output 14a is the same shape as the input 10a, the output 15b is the inverse 'of the input 14a, the output is a triangular wave derived from the input 10a and the output 14a is a square wave also derived from the input 10a. The unit 15 also provides four outputs designated l5a-15d which correspond to the outputs 14a-14d respectively. The output 14d from the unit 14 is connected to the input of a horizontal counter unit 19 which also receives the line sync pulses which provide a reset facility. Similarly. the output 15d from the unit 15 is connected to the input of a vertical counter unit 20 receiving the field sync pulses again providing a reset facility. The output 14b from the unit 14 is connected to the input of a parabolic generator unit 16 which provides an output 16a having a waveshape as illustrated. The outputs 15a, 15b from the unit 15 are connected to the inputs of

parabola generator units

17, 18 respectively. The

units

17, 18 have

outputs

17a, 18a with a waveshape as illustrated. The shape and amplitude of the

outputs

16a, 17a, 18a are substantially independent of the frequency of the basic sawtooth waveforms. The outputs 14a-14c, 1511-150, 16a, 17a, 18a are used to produce the waveforms corresponding to the elements a 0 depicted in FIG. 2 by appropriate combination of parts of the sawtooth, triangular and parabolic waveforms and are connected to busses generally designated 22. Fifteen element generator units 23-38 which produce these element-waveforms each has three inputs two of which may be connected to selected ones of the busses 22, i.e. to selected ones of the outputs 14a-l4c, Isa-15c, 16a, 17a and 18a, via potentiometers represented in FIG. 5 as variable resistors. The third input to each unit 23-38 receives a bias voltage v via a potentiometer also represented as a variable resistor. As described, each element generator unit 23-38 may be in the form of a differential amplifier constituting a level detector. The position and angle (and size in the case of the curved elements) of the display elements represented by the resultant waveform from each of the units 23-38 is dependent on the bias level and on the shape and amplitude of the inputs thereto. The output from each of the units 23-38 is connected to agate 39-54 which is triggered via an enable input 39a-54a provided by an output collectively designated 101, of a character shape memory unit 78. The output from each gate 39-54 is connected to the base ofa P-N-P transistor 55-70. The emitters of these transistors 55-70 are connected together via a common output 71a so as to synthesize by combination the particular character to be displayed at any instant. The output 71a is connected in turn to an input of a mixer unit 79. A horizontal modifier generator unit 86 and a vertical modifier generator unit 87 have inputs connected to the outputs 14c 150 of

units

14, 15, respectively via the busses 22 and via potentiometers represented as variable resistors in FIG. 5. The units 86-87 also receive bias voltages v via potentiometers also represented as variable resistors. The

units

86,87 86, 87 have a plurality of outputs, denoted 104, 105 respectively. These

units

86, 87 serve to provide the modifiers v-x and y-z respectively shown in FIG. 2. The outputs from the

units

86, 87 are selectively connected to the bases of the transistors 55-70 via diodes so that, when required, the transistors 55-70 may provide outputs with waveforms dependent on both the element and the modifier waveforms applied thereto.

The

units

86, 87 are also controlled by outputs, collectively designated 102, from the memory unit 78. This unit 78 has two sets of inputs, namely a quadrant address 103 and a'shape address 106, which specify digitally the appropriate selection of the units 23-38 and the outputs from the units 86-87. The inputs 105 to the unit 78 are taken from the outputs 14d, d of the

units

14, 15 and the inputs 106 are taken from the output ofa digital character storage unit 21 which provides encoded information defining a sequence of character. The unit 78 then converts this encoded information into a corresponding sequence of gating signals. The unit 21 has two sets of inputs 108, 109 which collectively represent the character address. The inputs 108 are taken from the outputs of the horizontal counter unit 19 and the inputs 109 are taken from the outputs of the vertical counter unit 20. The unit 21 is controlled by a control unit 96 which has provision to enable a desired sequence of characters to be. stored and edited. Unit 96 may be actuated by a manual keyboard as indicated in FIG. 5 or alternatively, for instance, connected to an external computer or other data source. The mixer unit 79 has blanking inputs as well as the main input 71a and provides two outputs 79a and 79b, one the inverse of the other, which represent the video character waveform. The blanking inputs serve inter alia to inhibit the character signals during the inter-character periods both vertically and horizontally. A typical input waveform 71a and typical outputs 79a, 79b are illustrated in FIG. 5. The outputs 79a, 79b are connected to an outline generator unit 95 which produces an output 950 representing the character for display and a complementary output 95b used to provide a definitive outline for the character. As described hereinafter the outline generator unit 95 has two limiting amplifiers. The first amplifier is arranged to transmit only the narrower peaks of the video character waveform, so as to produce an output forming the waveform defining the displayed character area. The second amplifier transmits only the broader troughs of the video character waveform, so as to produce an output forming a keying signal by means of which the output of the first limiting amplifier may be inlaid into the television display. The

outputs

95a, 95b which may have typical waveforms as illustrated are connected to a video processor unit 93 which has an input connected to a terminal 94 which usually receives a clamped video signal from the associated T.V. system. In certain applications such as, for example, where an alphanumeric display only is required the video input would not be used. The output from the unit 93 is taken through an amplifier 98 which delivers a system output 98a, having a typical waveform as shown, corresponding to the incoming video signal at terminal 94 with the selected character inset therein.

FIG. 6 depicts the basic circuit employed for the

character timebase units

10, 11. In FIG. 6, the variation in components for the horizontal and vertical timebase units is designated by the letters W and V and similar designations-are used in the case of the other circuits. As shown in FIG. 6, the circuit employs a capacitor C which is charged by a current source VT The output sawtooth waveform developed across the :capacitor is applied to a terminal 5 of an integrated circuit uA760 acting as'a comparator to compare the rising voltage of the ramp of the sawtooth with a fixed reference voltage applied to a terminal 4. The output from the integrated circuit ,u.A760, which changes state whenever the rising voltage of the sawtooth exceeds the reference voltage, is taken to the mixer unit 79 as one of the blanking inputs and is fed back to the switch on the transistor VT, to thereby discharge the capacitor C. In this way the basic sawtooth is generated. The size control signal serves to vary the bias on the base of the transistor VT The sync pulses are applied to an integrated circuit 74121, acting as a one-shot multivibrator, whichserves to discharge the capacitor and to inhibit charging thereof during the sync period (see 10a FIG. 5). The circuit has a further control signal (not shown in FIG. 5) which is denoted 5 position. This signal serves to shift the resultant character in the display by increments by taking the initial discharge voltage (sync period, 10a FIG. 5) on the capacitor C below zero. As shown, this control signal is applied to the emitter of the transistor VT In this circuit the' NPN transistor may be of type BC109 and the PNP transistors may be of type B C179. The diodes may be of type IN4148.

FIG. 7 depicts the basic circuit employed for the

ramp processor units

14, 15. The circuit employs a differential amplifier VT VT which receives the input waveform from the associated

unit

10, 11 and a fixed voltage developed across the emitter load of transistor VT The amplifier VT VT produces the sawtooth voltages across its collector loads and these voltages are fed via emitter followers VT VT to form the outputs 14a, 15a, 14b, 15b. Transistors VT.,, VT have their bases driven by the amplifier VT VT and have a common emitter load which produces the triangular output 14c, 15c by following the most positive voltage at the collectors of the transistors VT VT The triangular waveform developed across the emitter load of the transistors VT VT is also applied to an input of an integrated circuit 1.1.A760 acting as a comparator. The output pulses from the comparator uA760, integrated and stored on capacitor C1, drive a field-effect transistor FET. This circuit acts as a peak detector and controls a transistor VT This transistor VT, defines the current in the differential amplifier VT VT and serves to precisely clamp the negative-going peaks of the triangular waveforms to zero, at the same time precisely defining the-d.c. level of the sawtooth waveforms. The sawtooth waveform developed across the collector load of the transistor VT is also applied to an integrated circuit ,uA760 acting as a switch to produce the

square wave outputs

14d, 15d and the inverse thereof. In this circuit the transistors VT, VT may in fact be combined in integrated circuits of the type SG382I thus ensuring that the relationship between the triangular and sawtooth waveform is closely maintained despite temperature and other changes. The diodes may be of the type IN4148.

FIG. 8 depicts the basic form of the parabola generator units. The circuit employs an integrated circuit $63402 acting as a double-balanced modulator or multiplier to generate the

parabolic waveforms

16a, 17a, 18a. The circuit SG3402 is adapted to multiply together voltages supplied to

terminals

10 and 12, and since in this case these voltages are substantially the same linear waveform the square function thereof, i.e. parabolic waveform, is produced at terminal 4 and its inverse at terminal '11. The potentiometer shown between these terminals acts as an amplitude control. The

potentiometer feeding terminals

7 and 3 of the circuit SG3402 serves to set the required d.c.'bias conditions.

In a similar manner to the circuit shown in FIG. 7, the parabolic waveform produced by the integrated circuit 803402 has its negative peaks precisely clamped to zero by a feedback network composed of a comparator uA760, a storage capacitor C1 and a field effect transistor F.E.T. forming a peak detector and controlling current source VT VT The NPN transistors may be of type BC109 and the PNP transistor maybe of type BCI 79. The diodes may again be of type IN4148. It will be appreciated that the double-balanced modular $03402 carries out the squaring function on the sawtooth or triangular waves in a manner effectively independent of waveform frequency; other circuit techniques having similar properties could equally employed.

FIG. 9 depicts one of the element generator units 23-38 which also incorporates the transistor 55-70 (FIG. 5) with its associated gate and diodes. In this preferred form, in contradistinction to the diagrammatic representation of FIG. 5, the transistor 55-70 and its associated gate 39-54 are not separate entities but the transistor 55-70 itself serves also as the gate 39-54. As shown in FIG. 9, the circuit employs links whereby the appropriate selection of the busses 22 can be made. The inputs from the busses 22 are connected to the bases of transistors VT VT forming a differential amplifier. The resistors Rw, Rx, Ry, R2 are selected according to the identity of the element to be generated. The bias voltage v, shown separately in FIG. 5, is in fact produced by a network within the circuit as shown. The differential amplifier VT,, VT drives the bases of transistors VT VT the emitters of which are coupled together and drive the base of the output transistor 55-70. The current in the differential amplifier VT VT is controlled by a transistor VT which receives its bias voltage from the mixer unit 79. This transistor VT has a variable resistor as its emitter load which serves to set the dc level of the output waveform. The gate input 39a-54a and the inputs from the modifying units 86-87 are applied to thebase of the transistor -70, and serve to turn off the transistor 55-70 when present.

The PNP transistors may be of type BC179 and the diodes may again be of type IN4148. The NPN transistor may be combined as. an integrated circuit of type SG382I, again with the benefit of relative immunity from thermal drifts.

The function of the element generator unit. FIG. 9, will now be described, taking the simple case of Element a shown'in FIG. 2. Since Element 0 lies parallel to one of the scanning axes, only one sawtooth waveform, in this case the horizontal sawtooth 14a for example, is required for its generation. The sawtooth 14a is linked from the appropriate bus 22 onto resistor Rw, the ratio between resistors Rw and Ry being chosen to give the required waveform amplitude at the base of the transistor VT Rx is omitted in this case, while R2 has a nominal value to feed bias voltage v to the base of the transistor VT Consider now the situation where the, sawtooth 14a begins its sweep. The transistor VT will be cut off and the transistor VT fully conducting. As sawtooth 14a rises in voltage the transistor VR commences to conduct, its collector voltage falls and the output voltage from the unit 71a, i.e. on output also falls. (This assumes, of course, that the element is gated ON by the gate input (39a-54a) being low and the modifier input diodes being non-conducting). However, once the voltage at the collector of the transistor VT exceeds that of the transistor VT the transistor VT will conduct, the transistor VT will turn off, and the output voltage on 71a will again rise. The input sawtooth 14a has thus been converted into an output triangular wave similar to waveform 71a shown in FIG. 5. The negative peak of this waveform corresponds to the centre of the element 0 shown in FIG. 2, and this centre may be moved to right or left by adjustment of bias voltage v. The width of element a is defined by the width of the output waveform 71a at a critical voltage (set in the Mixer Unit 79) and may be adjusted by means of the variable resistor in the emitter of the transistor VT It will be appreciated that if the bias voltage is now modulated by adding to it a suitable amplitude of vertical sawtooth waveform via the resistor RX, the position of the peak of the output triangular waveform 71a will be similarly modulated to produce diagonal elements such asfand g in FIG. 2. If either of the input waveforms is a triangular rather than-a sawtooth then the output triangular waveform 71a will have two negative peaks (corresponding to the rising and falling slopes of the input) resulting in the double elements, such as c, d, e in the case of a single input waveforms to the Element Generator, and such as h, i etc. in the case of two (vertical and horizontal) input waveforms. The elliptical elements 1 and m are of course the result of the combination of the horizontal parabola 16a with the appropriate one of

vertical parabolas

17a, 18a in one of the element generators.

FIG. 10 depicts the circuit used for the mixer unit. The circuit employs a limiting differential amplifier VT VT VT, The transistor VT receives the common output 71a from the element generator units 23-38, and the transistor VT receives an additive blanking signal developed across the collector load of an amplifier VT, fed from an array of transistors VT,-VT receiving the individual blanking inputs. The

transistors VT VT may be combined in integrated circuits of type SG382l.

FIG. 11 depicts the circuit for the outline generator unit. As shown the circuit has additional control inputs denoted character off, key off" and outline off.

The circuit employs a differential amplifier VT VT whose anti-phase outputs are further amplified and inverted by transistors VT and VT respectively. However, the amplitude of the voltage swings at the collectors of the transistors VT and VT is set (as defined by the biasing network of transistor VT in relation to the base-emitter junction voltages of transistors VT VT., and VT VT such that the transistor VT turns on only after the transistor VT is fully turned off and vice versa. The resulting clipping action, illustrated by

waveforms

95a and 95b in FIG. 5, ensures that the final character waveform lies entirely within its corresponding outline or key waveform.

The NPN transistors may be of type BCIO9 and the PNP transistors may be of type BCI79.

FIG. 12 depicts the circuit of the video processor unit.

A video signal into which the character waveforms are to be inserted is applied to the base of an emitterfollower VT the video waveform being charged or DC restored such that its black-level is at zero volts. Transistor VT is normally non-conducting; however, when a character is to be inserted the negative-going outline waveform 95b, developed across resistor R1, and applied to the base of the transistor VT via transistors VT and VT inhibits any positive-going excursion of the emitter of the transistor VT holding the video signal at its black level and thus reserving a hole into which the character waveform may be inserted. The video waveform is now fed to a second emitter-follower VT Transistor VT is normally non-conducting except when the positive-going character waveform 95a, developed across resistor R2 in series with the baseemitter junction of VT drives the base of the transistor VT positive, causing the video waveform to rise to its white level. The final waveform is buffered by an amplifier VT VT A subsidiary video input, marked Cursor in FIG. 12, is provided for use where composing aids, such as a cursor. are required to be added to the video signal. Subsidiary outputs, marked Key Out and Characters Out, are also provided for use where it is more convenient for the insertion of the characters into a video signal to be accomplished in equipment located remotely from the character generator.

The PNP transistors may be of type BC179 and the NPN transistors of type BClO9.

FIG. 13 depicts the circuit of the

modifier units

86, 87 for clarity showing only typical output switches. The appropriate triangular waveform 14c, 15c is applied to one input of a differential amplifier VT VT The balance of the amplifier may be set by adjusting the voltage level at the second input. Transistors VT,,, VT,, for instance, comprise a gate such that, when the gate input 102 is high one output of the differential amplifier is available at the emitter of the transistor VT and this output 104 may be connected to one or more selected modifier inputs of the element generator units 23-38, to inhibit the corresponding element waveform in the manner of modifiers W or Y in FIG. 2. Similarly the modifier waveform 104 available typically at the emitter of the transistor VT, may be connected to in- 10 hibit element waveforms in the manner of modifiers X or Z in FIG. 2, and the modifier waveform I04 available at the emitter of the transistor VT may be connected to inhibit in the manner of modifier V in FIG. 2.

I claim:

I. An electronic system for generating video signals representing characters for television display; said system comprising:

a. means for producing ramp waveforms conforming with vertical and horizontal character rates, respectively;

b. means for producing from each of said ramp waveforms further waveforms including at least parabolic waveforms;

c. means for directly producing from selected combinations of said waveforms outputs representing character elements; and

' d. means for producing said video signals from said character element representing outputs.

2. A system according to claim 1, wherein said ramp waveform producing means has separate inputs connectible to line and field sync pulse sources.

3. A system according to claim 1, wherein the ramp waveform producing means has means for controlling the frequency of the ramp waveforms to thereby control the size of the characters.

4. A system according to claim 1, wherein said waveforms include triangular waveforms.

5. A system according to claim I, wherein said waveforms include sawtooth waveforms.

6. A system according to claim 1, wherein said further waveform producing means also produces square waveforms .having transistions substantially corresponding to the horizontal and vertical character centre-lines respectively. I

7. A system according to claim 1, wherein said further waveform producing means comprises parabola generator means each receiving a ramp waveform and producing a parabolic waveform therefrom, the parabolic waveform generating means operating 'substantially independently of the ramp waveform frequency.

8. A system according to claim 1, wherein the character element producing means essentially processes the selected combinations of waveforms in a linear manner.

9. A system according to claim 1, wherein the rate of transition of the outputs of the character element producing means is substantially dependent upon and controlled by the rate of change of the selectedcombination of input waveforms. 7

10. A system according to claim 1, wherein there is further provided a memory u'nit serving to selectively actuate the character element producing means according to the character to be synthesised, the memory unit serving to actuate said character-element producing means to separately specify the shape of the characters in quadrants of a display area.

11. A system according to claim 10, wherein there is further provided a digital storage unit adapted to provide to the said memory unit a sequence of character codes specifying a sequence of characters to be displayed.

12. A system according to claim 11, wherein means are provided to enable the recording, editing and accessing of data in the said digital storage unit, representative of a sequence of characters to be displayed.

13. A system according to claim 11, wherein there are further provided horizontal and vertical counter units effectively clocked from the horizontal and vertical ramp waveforms respectively, the counter units having reset inputs respectively connectible to line and field synchronising pulse sources and outputs collectively specifying a character address and feeding said storage unit.

14. A system according to claim 1, wherein the character-element producing means comprises a plurality of substantially identical element generator units having inputs connected to a selected combination of the waveforms.

15. A system according to claim 10, wherein the character-element producing means comprises a plurality of substantially identical element generator units having inputs connected to a selected combination of the waveforms, the output from each element generator unit being gated by signals from said memory unit.

16. A system according to claim 1, further comprising means for selectively modifying the outputs from the said character-element producing means, said modifying means serving to generate waveforms which blank out selected parts of the outputs from the said character element producing means in relation to the quadrants of the character display areas.

17. A system according to claim 16, wherein the output from each element generator unit is gated by signals from the modifying means.

18. A system according to claim 16, wherein said modifying means comprises a horizontal modifier gen erator unit and a vertical modifier generator unit which generate signals which blank out selected horizontal and vertical parts of the character display.

19. A system according to claim 10, wherein there is further provided means for selectively modifying the outputs from the said character-element producing means, said modifying means being controlled by signals from the memory unit to. generate waveforms which blank out selected parts of the outputs from the said character element producing means in relation to the quadrants of the character display areas.

20. A system according to claim 1, wherein a video processing means is provided having a first input for receiving a background video signal, a second input for receiving a video signal derived from said outputs of the element producing means representing the synthesised characters and an output reproducing the background video signal having superimposed thereon a character representing component.

21. A system according to claim 1, wherein means are provided to derive from said outputs from the element producing means or from the said video signal representing the synthesised characters, a further video signal representing a definitive outline for the synthesised characters.

22. A system according to claim 20, wherein means is provided for producing a further video signal representing a definitive outline for the synthesised characters and acting as a key whereby the said video signal representing the synthesised characters may be inlaid into the said background video signal.

UNITED STATES PATENT OFFICE @ETIIQTE 9F CECTIGN Patent 3,810,166 Dated May 4, 973

Inventor(s) Laurence Walter ATKIN.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE SPECIFICATION:

Col. 5'Iine5 5 change "character" to --characters--.

cfilumli 5 line 68 delete "only the broader troughs of",

Colwnn 6, line 4 after "display. insert -If the second amplifier is allowed to transmit only the broader troughs of the video character waveform then this keying signal forms an outline completely surrounding the character.

Column 6, line 19, Change to COlumn delete "71a" after "unit" and insert 713-- after "output".

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RUTH C. MASON C. MARSHALL DANN Arresting ()jfizer ('nnmlissiuncr uj'luu'nls and Trademarks

Claims

1. An electronic system for generating video signals representing characters for television display; said system comprising: a. means for producing ramp waveforms conforming with vertical and horizontal character rates, respectively; b. means for producing from each of said ramp waveforms further waveforms including at least parabolic waveforms; c. means for directly producing from selected combinations of said waveforms outputs representing character elements; and d. means for producing said video signals from said character element representing outputs.

5. A system according to claim 1, wherein said waveforms include sawtooth waveforms.

6. A system accOrding to claim 1, wherein said further waveform producing means also produces square waveforms having transistions substantially corresponding to the horizontal and vertical character centre-lines respectively.

7. A system according to claim 1, wherein said further waveform producing means comprises parabola generator means each receiving a ramp waveform and producing a parabolic waveform therefrom, the parabolic waveform generating means operating substantially independently of the ramp waveform frequency.

9. A system according to claim 1, wherein the rate of transition of the outputs of the character element producing means is substantially dependent upon and controlled by the rate of change of the selected combination of input waveforms.

10. A system according to claim 1, wherein there is further provided a memory unit serving to selectively actuate the character element producing means according to the character to be synthesised, the memory unit serving to actuate said character-element producing means to separately specify the shape of the characters in quadrants of a display area.

18. A system according to claim 16, wherein said modifying means comprises a horizontal modifier generator unit and a vertical modifier generator unit which generate signals which blank out selected horizontal and vertical parts of the character display.

19. A system according to claim 10, wherein there is further provided means for selectively modifying the outputs from the said character-element producing means, said modifying means being controlled by signals from the memory unit to generate waveforms which blank out selected parts of the outputs from the said character element producing means in relation to the quadrants of the character display areas.