| 摘要 |
A digital audio compact disc is copy protected by introducing altered values into the encoded digital audio data. In addition, all of the codewords containing the introduced altered values are changed so that the altered codewords are identified as uncorrectable. This identification of codewords as uncorrectable is used to force the altered data values to be subject to interpolation or other concealment means during playback of the audio data. As a data reader does not utilise error concealment means when reading data it may subject the altered codewords to additional attempts at correction or may pass the digital audio data, incorporating the altered values, unchanged. This means that if a data reader is used as the input to a copier, the copy produced will be degraded. In this respect, the altered values correspond to impulses (82, Fig 8a) superimposed onto the analog audio data (80) and are audible as clicks. Preferably the altered values are positioned so as to correspond to the particular features in the analogue audio waveform. Such features may be peaks, to maximise the loudness of the clicks produced by the introduced impulse. Alternatively the altered values can be placed to correspond to other areas in which the rate of change (gradient) of the waveform is low; or, simply at specific places chosen by reference to waveform shape. Other preferable features include the XOR-ing of at least four bytes of codewords with a value of at least four bytes and changing only parity bytes of a codeword.
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