A copy-protected digital audio compact disc and method for producing same,
such that neither the proper playing of the original nor the enforcement
of the copy-protection depends on the use of any special equipment. A
pattern of latent noise is incorporated into a digital audio CD by
overwriting some of the original audio signal data symbols with
grossly-erroneous values, and then overwriting the corresponding
error-correction parity symbols in such a way as to create an
uncorrectable error in the codewords containing the erroneous values. An
ordinary CD player of such a disc will therefore detect each occurrence of
a latent noise value as an uncorrectable error and will apply
interpolative error-concealment to prevent the output of the error. By
appropriately choosing the locations for the overwriting of the erroneous
values in such a way that the interpolated value will be substantially
identical to the original value, the resulting sound output from the CD
player will match that of the original audio signal, so that an ordinary
CD audio player will properly reproduce the original audio signal from the
protected disc without any superimposed noise. Most commercially-available
CD-ROM drives for computers, however, do not employ error-concealment when
reading a compact disc as a data source. Depending on the copying software
employed with such CD-ROM drives, when an uncorrectable error is detected,
either no data is read from the disc, or only the raw uncorrected data is
read from the disc. Thus, either substantial portions of the original disc
will be uncopyable because of the uncorrectable errors, or the uncorrected
raw data of the original will be copied, which will place audible noise on
the unauthorized copy. To inhibit unauthorized copying via CD-ROM drives
that employ error-concealment, alterations are made to the channel Q mode
and/or CRC to disable the ability of the CD-ROM drive to seek to the
desired data position.
