Abstract:
A method for recording audio data onto a compact disc so as to discourage unauthorized copying thereof, the method including sampling a source audio data stream to determine at least one source audio data portion thereof the a derivative function of a sequence of samples of the source audio data portion is substantially constant, modifying the source audio data stream by substituting the source audio data portion with a substitute audio data portion, recording the modified audio data stream into at least one frame of the compact disc the frame has a P-channel, and setting the value of the P-channel that corresponds to the substitute audio data portion to 1.

Description:
FIELD OF THE INVENTION 
     The present invention relates to compact disc technology in general, and in particular to methods and apparatus for recording audio data onto a compact disc (CD) so as to discourage unauthorized duplication thereof. 
     BACKGROUND OF THE INVENTION 
     Measures currently known or proposed in the art for preventing the unauthorized duplication of compact discs are generally limited to marking compact discs in order to identify a counterfeit copy and trace it back to its point of origin. The primary disadvantage of such measures is that they provide no technical deterrent to duplicating compact discs. 
     The disclosures of all publications mentioned in this specification and of the publications cited therein are hereby incorporated by reference. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide novel methods and apparatus for recording audio data onto a compact disc so as to discourage unauthorized duplication thereof which overcomes the known disadvantage of the prior art as discussed above. 
     There is thus provided in accordance with a preferred embodiment of the present invention a method for recording audio data onto a compact disc so as to discourage unauthorized copying thereof, the method including sampling a source audio data stream to determine at least one source audio data portion thereof the a derivative function of a sequence of samples of the source audio data portion is substantially constant, modifying the source audio data stream by substituting the source audio data portion with a substitute audio data portion, recording the modified audio data stream into at least one frame of the compact disc the frame has a P-channel, and setting the value of the P-channel that corresponds to the substitute audio data portion to 1. 
     Further in accordance with a preferred embodiment of the present invention the modifying step includes creating the substitute audio data portion by altering the original audio data portion. 
     Still further in accordance with a preferred embodiment of the present invention the modifying step includes altering the original audio data portion by adding a fullscale complement to the original audio data portion. 
     There is also provided in accordance with a preferred embodiment of the present invention a compact disc including at least one frame including modified audio data derived from a source audio data stream having at least one source audio data portion thereof the a derivative function of a sequence of samples of the source audio data portion is substantially constant and the source audio data stream has been modified by substituting the source audio data portion with a substitute audio data portion, and a P-channel that corresponds to the substitute audio data portion the P-channel has a value of 1. 
     There is additionally provided in accordance with a preferred embodiment of the present invention apparatus for preparing audio data prior to being recording onto a compact disc so as to discourage unauthorized copying thereof including intercept apparatus operative to sample a source audio data stream to determine at least one source audio data portion thereof the a derivative function of a sequence of samples of the source audio data portion is substantially constant, modify the source audio data stream by substituting the source audio data portion with a substitute audio data portion, and set a P-channel of a frame of the compact disc into which the substitute audio data portion is to be recorded to 1. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated from the following detailed description, taken in conjunction with the drawings in which: 
     FIG. 1 is a simplified conceptual illustration of a prior art compact disc constructed and operative in accordance with International Electrotechnical Commission Standard No. 908 governing compact disc digital audio systems; 
     FIG. 2 is a simplified block diagram of a P-channel/audio data manipulation system constructed and operative in accordance with a preferred embodiment of the present invention; 
     FIG. 3 is a simplified flowchart illustration of a method of operation of modification apparatus  30  of FIG. 2 operative in accordance with a preferred embodiment of the present invention; and 
     FIGS. 4A,  4 B, and  4 C are conceptual illustrations of an audio data portion of a CD track for a master CD ( 4 A), a CD produced using the method of FIG. 3 ( 4 B), and an unauthorized duplicate of a CD produced using the method of FIG. 3 ( 4 C). 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1 which is a simplified conceptual illustration of a prior art compact disc constructed and operative in accordance with International Electrotechnical Commission Standard No. 908 governing compact disc digital audio systems, herein referred to as “standard 908,” the disclosure of which is hereby incorporated by reference. FIG. 1 shows a compact disc  10  having a spiral recording track  12  onto which audio and control data are recorded. A portion  14  of track  12  is shown enlarged at  16  where track  12  is shown as comprising data sections  18 , where music or other audio data are recorded, as well as non-data sections  20  where control information is stored. In accordance with standard 908 a section of the track known as the P-channel is generally recorded with a value of 0 for data sections and 1 for non-data sections, as shown respectively by reference numerals  22  for data sections  18  and reference numeral  24  for non-data section  20 . Regarding information recorded in a data section, standard 908 dictates that where the P-channel=0 the digital audio information may be passed to a digital-to-analog converter for output via a speaker, and where the P-channel=1 it may not. 
     Reference is now made to FIG. 2 which is a simplified block diagram of a P-channel/audio data manipulation system constructed and operative in accordance with a preferred embodiment of the present invention. As in prior art recording systems, an audio source  26  provides an audio signal input to a data processor  28  which typically includes CD encoding circuitry for preparing the audio signal to be recorded onto the surface of a compact disc in accordance with standard 908. In prior art systems the output from data processor  28  is typically input into a laser beam recorder  30  which in turn controls a stamper  32  for recording the prepared audio signal onto the CD. In accordance with the present invention intercept apparatus generally designated  34  intercepts the prepared audio output from data processor  28 . Intercept apparatus  34  preferably converts the incoming eight-to-fourteen modulated (EFM) signal to a binary signal using a converter  36 , and, at modification apparatus  38 , modifies the audio data and corresponding P-channel using the method described in greater detail hereinbelow with reference to FIG.  3 . The modified signal is then reconverted back to EFM using a converter  40  and input to recorder  30 . 
     A buffer  42  may be used to buffer incoming audio data record blocks during processing. Preferably, a non-data block need not be buffered and may be immediately written out to recorder  30 . Data block processing typically begins with buffering three data blocks. As a fourth data block is received and buffered, modification apparatus  38  retrieves the first data block from buffer  42  for processing. As the fifth data block is received and buffered, the now-processed first data block is sent to recorder  30  and modification apparatus  38  retrieves the second data block from buffer  42  for processing. This process continues until the next non-data block is received. 
     Typical operation of modification apparatus  30  of FIG. 2 is now described with additional reference to FIG.  3 . The method of FIG. 3 as described in detail hereinbelow is preferably implemented through the use of specialized software and/or hardware designed for that purpose using well-known techniques. In the method of FIG. 3 a data block representing a portion of a source audio data stream is received and N samples othe source audio data signal are taken, preferably 7 per block. For each sample n in N a derivative A is calculated using samples n−1 and n+1. An average B is then calculated from the derivatives for samples n−1 and n+1. The derivative A is then compared to the average B. Should A=B the P-channel corresponding to the sample is set to 1 and the source audio data is substituted with false data, preferably the fullscale complement to the sampled signal. After each data block is processed it is input to recorder  30  for stamping. Subsequent data blocks are similarly processed until a non-data block is encountered. 
     Reference is now made to FIGS. 4A,  4 B, and  4 C which are conceptual illustrations of an audio data portion of a CD track for a master CD ( 4 A), a CD produced using the method of FIG. 3 ( 4 B), and an unauthorized duplicate of a CD produced using the method of FIG. 3 ( 4 C). In FIG. 4A a track portion  44  of a master CD is shown having an audio signal  46 . A P-channel  48  is set to 0 as is required and expected of a standard 908 compliant CD. A portion  50  of signal  46 , bounded by dashed lines, is shown as having a constant derivative. FIG. 4B shows a track portion  52  that corresponds to track portion  44  of a CD produced using the method of FIG. 3 above. An audio signal  54  is shown that generally corresponds to signal  46  with the notable exception that portion  50 , represented by a dashed line  56 , has been substituted with a substitute audio data portion  58 , and the portion  60  of the P-channel that corresponds to portion  58  has been set to 1. 
     As was mentioned above, it is a particular feature of standard 908 that where the P-channel=1 in a data section, the corresponding digital audio information may not be passed to a digital-to-analog converter for output via a speaker. Rather, under such circumstances standard 908 dictates that an interpolation be made between the last valid datum where the P-channel=0 and the next valid datum. The interpolated information is then passed to the digital-to-analog converter and output via the speaker. Thus, the substitute audio data portion  58  of FIG. 4B is ignored, and instead an interpolation, substantially equivalent to the original portion  50  of FIG. 4A, is output, thus resulting in little or no net difference in audio quality between the corresponding track port  44  and  52  of FIGS. 4A and 4B. 
     FIG. 4C shows a track portion  62  of an unauthorized duplicate of a CD produced using the method of FIG.  3 . Track portion  62  corresponds to track portion  52  of FIG.  4 B. While an audio signal  64  is identical to audio signal  54  of FIG. 4B, the portion  66  of the P-channel that corresponds to portion  58  has been set to 0. This is because standard 908-compliant CD encoding circuitry does not directly provide for the copying of the P-channel from a source CD that is being duplicated. Rather, the CD encoding circuitry itself decided when and how to set the P-channel. During duplication the CD encoding circuitry merely sets the P-channel=0 while recording to the data are, and therefore the P-channel setting of portion  60  is ignored. Thus, during playback, the substituted audio data portion  58  is provided to the digital-to-analog converter as normal data, resulting in audio distortion and potentially damaging the output circuitry. 
     It is appreciated that the steps of the methods described hereinabove need not necessarily be performed in the order shown, and that in fact different implementations of the steps may be employed to yield similar overall results. 
     It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of the features described hereinabove as well as modifications and variations thereof which would occur to a person of skill in the art upon reading the foregoing description and which are not in the prior art.