CA2331111A1 - Anti-copying methods and devices for digital information signals - Google Patents

Abstract

In order to protect audio signals against copying, it is proposed to mix the audio signal with at least one non-audible disturbance signal. A first disturbance signal may be a low-frequency signal which is added to the audio signal, while a second disturbance signal may be a high-frequency disturbanc e signal which is multiplied with the audio signal. Additionally, the second disturbance signal may be modulated by a modulating signal comprising either the original or inverted orginal signal, or spoken messages indicative of an illegal copy. Additionally, or alternatively, digital information may be cop y protected by briefly interrupting the writing of the information to an information carrier. The latter measure is also effective for copy protectin g software.

Description

WO 99/57723 PCT/EP99/0323t Anti-copying methods and devices for digital information signals.
BACKGROUND OF THE =CNVENTION
The present invention relates to anti-copying methods and devices for audio and/or video signals, in particular digital audio signals, and for software information.
Present day digital techniques allow perfect copies of audio signals to be made. For example, the digital audio information of a compact disc (CD) carp be digitally registe-red on tape or can even be used to physically reproduce the CD by means of a so-called CD burner (recorder for compact discs) . The possibility of digital cop~~ring, producing perfect copies of the original, has resulted in many illegal copies of CDs or other information carriers being made. This in turn leads to a substantial loss in copyright royalties. For this reason, there is a need for protection against copy:ing of audio signals.
The Prior Art offers several copy protection methods which all have drawbacks.
European Patent Application EP 0 298 691, for example, proposes to modulate an audio signal by an additional signal.
The degree of modu:Lation is such that the modulation is not audible . Instead, a. detector is used t~o detect the modulating signal. As in this known method the copy protection sia~nal is not audible in the copy unless special equipment is used, its uselessness is severely limited.
European Patent: Application EP 0 348 570 discloses a copy protection method in which an audible "spoiler signal" is added to the original audio signal. This has the disadvantage that the resulting audio signal is always distorted, even when played back by an authorized party.
SUMMARY OF THE INVENTION
Tt is an object of the present invention to provide a method for protecting audio signala against unauthorized

2 copying which does not suffer from the drawbacks of the Prior Art.
It is a further object of the present invention to provide a method for protecting audio signals which does not disturb the replay of those signals bay authorized per:~ons.
It is an additional obj ect of thae present invention to provide a method for protecting audio signals which does not require special equipment on the part of the user.
It is a yet further object of the present invention to provide devices for protectedly recording data carriers, as well as copy protected information carriers.
Accordingly, the present invention provides a method of protecting an audio signal against: copying, the. method comprising the step of mixing the audio signal with at. least one non-audible disturbance signal wh_Lch becomes audible upon copying the audio signal.
By mixing the audio signal with a non-audible disturbance signal,. that is an additional signal having a frequency lying outside the norma3. audible range, it is possible to obtain a combined audio signal which is, wren replayed as it is, effectively undisturbed but which shows audible disturbances when copied.
Advantageously, a first disturbance signal is a low freguency signal which is added to th.e audio signal. When the original audio signal and the low-frequency first disturbance signal have comparable amplitudes, the resulting mixed output signal will be highly resistant against copying to an analogue tape recorder. This aspect of the present invention is based on the insight that slowly varying signals cause the magnetic heads of a tape recorder too effectively erase the recorded signal, or at least to suppress the recording of the signal.
Preferably, the low-frequency disturbance signal has a frequency of approximately 2 Hz, advantageously between 0.5

3 5 and 3 Hz , and has a substant ial ly sinusoidal shape . These low frequencies are not audible, nor can they be reproduced by regular audio sets, yet they have the above-mentioned erasing effect .

WO 99/57723 PCT/EP!~9/03231 Advantageously, a second disturbance signal is a high-frequency signal which is multiplied with the audio signal.
This may effectively result in the original audio signal being substantially interrupted ("chopped") at a high interruption frequency. Although this .regular interruption is normally not audible due to the high (non-audible) frequency involved, copying of the signal will z-esult in an incomplete demodulation of the combined :signal due to the characteristics of the copying devices.
1p Preferably, the high-frequency disturbance signal has a frequency of approximately 20 kHz, and has a substantially sinusoidal shape, although other shapes such as block pulses are also possible.
Preferably, bath the first, low-frequency disturbance signal and the second, high-frequency disturbance signal are mixed with the original audio signal to produce a combined, copy-protected audio signal.
Advantageously, the audio signal is a digital signal representation involving a sampling frequency, and the second disturbance signal has a frequency which varies in time, preferably from approximately half to approximately three quarters of the sampling frequency. 'l~'his results in a second disturbance signal having both a varying (preferably increasing) frequency and, due to the aliasing effects involved, a varying (preferably decreasing) amplitude. This results in a clearly audible amplitude modulation in the copied signal.
The second disturbance signal may also be fx°equency and/or amplitude modulated by auxiliary modulating :signals, e.g. derived from the original audio signal.
The invention. further provides a device for protecting audio signals against copying, comprising signal generation means for generating at least one non-audible disturbance signal, mixing means for mixing at least one disturbance signal with the audio signal, and oui~put means for outputting the resulting mixed audio signal.
In accordance with a further aspect, the present invention provides a method of producing a copy-protected digital information carrier, compri~~ing the step of recording

4 digital information on the carrier, characterized by interrupting the recording during short intervals.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be further explained with reference to the accompanying drawings, in which:
Fig. 1 schematically shows possible applications of the method of the present invention.
Fig. 2 schematically shows a first disturbance signal as used in the method. of the present invention.
Fig. 3 schematically shows a cambination of the: first disturbance signal of Fig. 2 and an audio signal.
Fig. 4 schematically shows the signal of Fig. 3 as recorded on a tape: recorder.
Fig. 5 schematically shows the low-frequency disturbance signal as in Fig. 2.
Fig. 6 schematically shows the low-frequency disturbance signal of Fig. 5, multiplied with a cc>ntinuous high-frequency disturbance signal.
Fig. 7 schematically shows the :l. ow-frequency signal of Figs. 5, to which a continuous high-frequency signal is added.
Fig. 8 schematically shows a low-frequency disturbance signal combined with high-frequency :bursts.
Fig. 9 schematically shows an alternative embodiment of the signal of Fig.. 8.
Fig. 10 schematically shows a disturbance signal which is frequency and/or amplitude modulated.
Fig. 11 schematically shows the effect of applying the disturbance signal of Fig. 10 to an audio signal.
Fig. 12 schematically shows an original audio signal which is to be copy protected.
Fig. 13 schematically shows the signal of F:i.g. 12, multiplied with a high-frequency disturbance signal.
Fig. 14 schematically shows the signal of Fig. 1.3 after gain adjustment.
Fig. 15 schematically shows the signal of Fig. 14, compensated for gain adjustment.

Fig. 16 schematically shows a sy~>tem for carrying out the method of the present invention.
Fig. 17 schematically shows a device for writing digital data to an information carrier in a copy-protected manner.

5 DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
As shown in Figs. la and lb, t:he copying of (digital) audio signals may be considered to involve two categc>ries or groups of copying operations:
Group 1: from digital to ana7_og signals, and Group 2: from digital to dig~_tal signals.
Although some of the techniques described in this dlocument may also apply t.o copying from analog to analog signals (signal representations) , the following discussion will focus on the copying of digital audio aignals, that i:a audio signals registered in a digital representation (sampled audio signals represented digitally by ones and zeros).
Especially group 2 (Fig. lb) poses problems with respect to copyright, as the CD-burners and. DAT-recorders normally allow perfect copies of the original signals to be made (it will be understood that so-called D.~T-recorde:rs are tape/cassette recorders which digitally register audio signals on tape).
According to a first aspect of t:he present invention, a low-frequency disturbance signal (also called first disturbance signal) is added to an (original) audio signal.
Such a first disturbance signal i:a shown in Fig. 2. The signal has a frequency of approximately 2 Hz, although a range of frequencies from about 1 Hz to about 10 Hz may be used. It is only desired that the first disturbance s~~gnal is normally not audible. The amplitude of the second dist:urbance signal is of the same order of magnitude as the amplitude of the audio signal it is to be added to, and is preferably two or three times as large as the latter. The first disturbance signal is, like the second disturbance signal to be d~.scussed later, preferably digitally generated.
The result of the adding of the signals is shown in Fig.
3. The mixed or combined signal of Fig. 3 is obtained by producing the sum of an original audio signal and the first

6 disturbance signal of Fig. 2. When this signal is reproduced from e.g. a CD (Compact Disc), the low-frequency modulation is not audible. However, when the corrtbined signal of Fig. 3 is copied onto tape (regular tape or cassette), thc~ slow "shift" results in the recorded signal (shown in F:ig. 4) being regularly interrupted. This is caused by the change in signal amplitude between the peaks and troughs of the first disturbance signal, which induces an erasing effect in the recorder heads . It will be clear that the resulting signal of Fig. 4 has a very poor sound quality.. The loss-less copying of the signal of Fig. 3 onto tape is thus~effectively prevented by inducing a serious signal distortion.
According to a second aspect of the present invention, a second, high-frequency disturbance signal is applied to the audio signal. Preferably, as shown in Figs. 5-9,, this modulation is applied to the original audio signal through the first disturbance signal, but the second disturbance signal may also be applied independently from, o:r even without, the first disturbance signal..
Fig. 5 again shows the first disi~urbance signal of Fig.
2. This low-frequency signal is mixed with a high-frequency second disturbance: signal which preferably is a sine wave.
This second disturbance signal preferably has an frequency of approximately 20 kHz, but may also have a varying frequency, as will be explained later. The frequE~ncy is chosen such that the second disturbance signal, like the first disturbance signal, is normally non-audible.
The signal of Fig. 6 is the resulting signal after multiplying the first disturbance signal of Fig. 5 with the second disturbance signal (product of the signal amplitudes).
The signal of Fig. 7 is the resulting signal after adding the first disturbance signal of Fig. 5 to the second disturbance signal (sum of the signal amplitudes).
The signal of Fig. 8 consists of a low-frequency first disturbance signa:L combined with bursts of a high-frequency second disturbance signal (the bursts are schematically represented in Figs. 8 and 9 by shaded areas). These. bursts preferably have a duration of about 50 to 200 ms and do preferably occur about the point where the first disturbance WO 99157723 PCT/EP!~9/03231

7 signal is "zero" (average signal value). By u:~ing a discontinuous second disturbance signal, instead of the continuous second disturbance signal of Figs. 6 and 7, a first disturbance signal having a much smaller amplitude can be used to obtain i dentical results . Thus, using a bur;~t-lika second disturbance signal makes it possible to reduce the energy of the first disturbance signal.
The high-frequency bursts are even more effective when they occur in pair's, as schematically represented in :fig. 9.
In the exemplary signal of Fig. 9, two high-frequency burst s (schematically represented as shaded areas) are located just before and just after each "zero" crossing of tr.e low-frequency disturbance signal. It has proven less effective to locate the bursts around the peaks and troughs of the low-frequency signal. The paired bursts of Fig. 9 also allow a reduced low-frequency signal amplitude.
Instead of using bursts, the disturbance sigmal or signals may be applied intermittently. That is, thc~ first and/or second disturbance signals may be mixed w~~th the original audio signal only in certain time periods of e.g.
0.1 s or 1.0 s. These time periods ars spaced apart at a distance of e.g. also 0.~. s or 1.0 s. The resulting audio signal comprises alternating processed and unprocessed periods. As a reault, the eventual disturbance wizen the signal is copied is much more pronounced.
The heads of a tape (or cassette) recorder normally do not reproduce signals having a frequency of about 20 kHz.
Feeding the signal of any of the Figs . 6 - 9 to the rE:cording head of a tape recorder will result i.n a partial demodulation of the high-frequency disturbance signal. This partial demodulation will lead to a distortion of the signal, thus effectively protecting the signal against copying.
It will be understood that the copy protection of the present invention does not affect the bias frequency of tape or cassette recorders. Instead, the audio signal. to be recorded is affected in such a way, that due to the properties of the recorder the: previously u:naudible disturbances become audible.

8 Advantageously, the high-frequency second disturbance signal is frequency modulated by at. least one modulating signal. Alternatively, or additionally, the second disturbance signal is amplitude modulated by the at least one modulating signal. These modulations will result in a further distortion of the (partially) demodulated signal.
The modulating signal may contain spoken mEa sages indicative of illegal copying, such as "this is an illegal copy" or "this recording is copy proi~ected". The modulating signal may also be derived from the original audio signal and may comprise the original signal itself or its inverse (inverted original signal) . Preferably, the modulating signal contains both spoken messages, the original signal and the inverted original signal, each during a certain time period of for example 30 seconds.
In order to produce a copy-protected recording, the modulating signals may be additionally registered on the information carrier (such as a CD) or may be tran;amitted separately via a satellite network, :internet, or the Like.
The disturbance signals described above are in particular suited for the prevention of copying without quality loss to analog recorders (group 1 in F:ig. 1). The (second) disturbance signal. of Fig. 10 is in particular suited for the prevention of copying without qu<~lity loss to digital recorders, such as so-called CD burners.
The disturbance signal of Fig. 10 has a frequency which increases stepwise from 20 kHz to 30 kHz. As the sampling frequency used for CD recordings is about 44 kHz, the disturbance signal of Fig. 10 incre=_ases in frequency from just below half the sampling frequency (the Shannon frequency) to about three-quarters of: the sampling frequency.
As the frequency increases, the amplitude of the signal decreases. This type of signal proves to be highly effective in the prevention of loss-less copying. When mixed with an audio signal (by multiplying or add_ing), the signal of Fig.
11 results. As shown in Fig. 11, the resulting signal has a decreasing amplitude (schematically represented by tJze drawn lines A), effected by the modulating signal. In the signal of

9 Fig. 11, the amplitude variations caused by the frE~quency sweep are clearly audible.
In addition to the frequency sweep, the same modulating signals may be used to modulate the second disturbance signals of Fig. 10 as used for the second disturbance signal of Figs. 6-9. Thus spoken message:, the original signal and/or the inverted original signal may be used to further modulate the signal of Fig. 1Ø
The audio signal fragment shown in Fig. 12 ha:~ three peaks (maxima) and three troughs (mi.nima). The peaks M1 and M3 have relative (positive) signal amplitudes of 30o and 250 respectively. The troughs M2 and M4 have relative (negative) signal amplitudes of 50% and 35o respectively.
In accordance with the present invention, the signal of Fig. 12 is combined (multiplied) with a high-frequency disturbance signal. The resulting combined signal is :shown in Fig. 1.3. Due to the disturbance signal, the audio signal is "chopped". As a result, the energy content of the combined signal is only ha:Lf of that of the original signal. When such a signal is reproduced, the volume (sound level) is a:Lso only half of that of the original signal. In order to compensate for this, the amplitude of the signal may be dour>led, as shown in Fig. 14. This doubled signal has the same: energy content as the original signal and will therefore, when reproduced, generate the same sound volume.
Ta achieve an even better level of copy protection, the maximum amplitude of the "chopped" signal sections ma.y not be just doubled, but may be raised to approximately the highest possible level. Thus, all signal samples in a certain signal section (e.g. .sec:tion S1) are multi~alied by a factor k which is chosen such that the maximum amplitude (Ml) equals 90% or 950 of the highest possible signal level (1000 is also possible but creates the risk of irreversible: signal distortion). It will be understood that far eac:~h signal section a different factor k applies . Preferably, k is greater than or equal to one.
The signal distortion introduced by multiplying the signal samples by a factor k is compensated for by introducing samples having a signal. amplitude with <~ reverse WO 99/57723 PCT/Ef99l03231 polarity. Thus, where the multiplication by k results in signal amplitudes k.v, v being the original amplitude, an additional sample having an amplitude v'= -(k-1).v will result in an unaltered signal level when played back.
5 However, due to the changed signal structure, copying of the thus processed signal will result in distortions.
The system 100 shown by way of example in Fig. 16 is designed for copy protecting audio signals in accordance with the present invention. The system comprises signal sources

10 101-105, an analog to digital convertor ZO&, a signal processing unit (computer) 107 and signal destinations 108-112. The sources :101-104 of analog signals are connected to the signal processing unit 107 via the A/D converter 106, while the digital source 105 (e. g. a. CD player) is directly connected with the signal processing unit 107.
The first source 101 is a programmable wave-form generator for producing the first arEd/or secand disturbance signal. The second and third sources 102 and 1.03 are programmable FM and AM modulators respectively. The fourth source 104 is an analog audio source, e.g. for producing spoken messages. The sources 101-104 may be constituted by software, advantageously stand-alone software. Thus all signal manipulations take place in the digital domain.
By means of the system 100, thE~ method of the present invention may be readily applied.
An audio signal which has been combined w~~th the disturbance signa:Ls according to the present invention may be stored on a commercially available information carrier, such as a floppy disc :having a magnetic recording medium, or a CD
having an optical recording medium.
In accordance with another <~spect of the present invention an additional or alternative form of copy protection is achieved by modifying the registration of digital information on a digital information carrier. In the case of a CD, the digital information is recorded on the CD's surface by making a track of small holes in the surface using a laser beam (the so-called "burning" of the CD). It has been found that briefly interrupting the laser beam, resulting in a number of hole: not being made, provides a very effective

11 copy protection. That is, the CD thus protected may be played normally, resulting in no signal distortion at all, provided the interruptions in the track are short enaugh. He>wever, when the contents (the signal) of the CD are copied t.a e.g.
a hard disc, the resulting digital information is distorted to the point of being useless.
The interruption of the recording or "burning" process of audio CDs can be effected in several ways:
- short interruptions during pau:aes, at the beginning and/or the end of the CD and/or between music pieces;
and/or - short interruptions at the zero crossings of the: audio signals; and/or - very short interruptions, e.g. of about l~,s or evE~n 1 ns (nano-second), having a repetition frequency lying above the audible range, or even above the sampling rate used.
In the case of video recordings, shoat interruptions may be effected at the edges of the video image, that is, outside the visible image. Short interruptic>ns may also be used to copy protect software programs recorded an e.g. CD-R~JMs. In the latter case, pauses (interruptions) can be created.in the software program in order to create "blank spots". This causes uncontrollable digital information inserted in the original software information after <:oping to e.g. hard disc and/or CD ("on the fly"). In thi:~ particular care, the frequency of the interruptions may lie within the audible range.
The arrangement of Fig. 17 comprises a laser section 201, a power supply 202 and data circuits 203 of a CD recorder.
The data circuits 203 provide a digital data stream representing data to be recorded. Accordingly, the laser of laser section 201 is modulated by the data stream when burning (making small holes in) CD 210. During the burning process, the CD is rotated by rotation means (not shown).
Power supply 202 continuously provides power to th.e laser section 201.
According to the present inventian an additional circuit 204 is provided which briefly interrupts the laser, e.g. by briefly interrupting the power supplied to it. To this end, WO 99/57723 PCT/EF'99/03231

12 the interruption circuit: 204 may b~° programmed to cause interruptions on a regular basis or a1. predetermined moments in time. Alternatively, or additionally, the data stream originating from circuits 203 may be :Eed to the interruption circuit 204 to effect data-dependent interruption~~. The latter option enables the interruptions to occur e.g. during pauses or at zero crossings of the .audio or video signal.
Thus a very effective supplementary copy protection may be attached, which may also be used without combining the original signal with non-audible disturbance signals.
Instead of using an additional interruption circuit 204, the hardware and/or software of data circuits 203 may be modified so as to cause interruptions. in the laser beam.
Although the invention has been described primarily with reference to audio signals, it will be understood that its principles can be applied to other signals as well, e.g.
video signals, by mixing normally non-perceivable disturbance signals with the original signal.
It will thus be understood by those skilled in ~.he art that the present invention is not limited to the embodiments discussed above and that many additions and modifications are possible without departing from the scope of the present invention.

Claims (15)

1. Method of producing a copy-protected digital information carrier (210), comprising the step of recording digital information on the carrier (220), characterized by interrupting the record:i.ng during short intervals corresponding to playing times between :L00 microseconds and 1 nanosecond.

2. Method according to claim 1, wherein the interruptions have a repetition frequency lying above the audible range.

3. Method according to claim 1, wherein the interruptions have a repetition frequency lying above the sampling rate used.

4. Method according to any of claims 1-3, wherein the digital information is audio information, and the interruptions are made during pauses.

5. Method according to any of claims 1-4, wherein the digital information is audio information, and the interruptions are made at the beginning and/or the end of the carrier.

6. Method according to any of claims 1-5, wherein the digital information is audio information, and the interruptions are made between music pieces.

7. Method according to any of claims 1-6, wherein the digital information is audio information, and the interruptions are made at the zero crossings of the audio signals.

8. Method according to claim 1, wherein the digital information is video information, and the interruptions are effected at the edges of the video image, outside the visible range.

9. Device for producing a copy-protected digital information carrier (210), comprising:
data circuits (203) providing a digital data stream representing digital data to be recorded;
a laser section (201) being modulated by the data stream when recording the digital data on the carrier, characterized in that the data circuits (203) cause interruptions in the recording by the laser section (201).

10. Device for producing a copy-protected digital information carrier (210), comprising:
data circuits (203) providing a digital. data stream representing digital data to be recorded;
a laser section (201) being modulated by the data stream when recording the digital data on the carrier, characterized by an interruption circuit (204) for briefly interrupting the recording by the laser section (201).

11. Device according to claim 10, wherein a power supply (202) provides power to the laser section (201), and the interruption circuit (204) is adapted to interrupt the power supplied to the laser section (201).

12. Device according to claim 10 or 11, wherein the interruption circuit (204) is programmed to cause interruptions on a regular basis.

13. Device according to claim 10 or 11, wherein the interruption circuit (204) is programmed to cause interruptions at predetermined moments in time.

14. Device according to any of claims 10-13, wherein the data stream originating from the data circuits (203) is fed to the interruption circuit (204) to effects data-dependent interruptions.

15. Information carrier (210), provided with a track of marks representing digital information, characterized in that the tracks are interrupted over short distances to provide copy protection, the short distances corresponding to playing times between 100 microseconds and 1 nanosecond.