The ability of a data reader, such as a CD-ROM drive, to access, extract,
or otherwise read the data on a digital audio compact disc provides a
problem for the music industry. A user can use his CD-ROM drive to read
the data from an audio disc into a computer file, and then that data can
be copied. To provide copy protection, errors are deliberately introduced
into the data on a CD, but these errors are of a type which are generally
transparent to an audio player but which will interfere with the reading
of the audio data by a data reader. According to the standards, the data
on a CD is encoded into frames by EFM (eight to fourteen modulation). Each
frame has sync data, sub-code bits providing control and display symbols,
data bits and parity bits, and includes 24 bytes of data, which is audio
data for a CD-DA. The standard requires that 98 such frames are grouped
into a sector. To provide copy protection, each is provided with a
non-standard number of frames, for example, has 99 rather than 98 frames.
Then the S0 and S1 sub-code synchronisation patterns are placed one frame
later than they otherwise would be, but the data within each frame remains
the same. An audio player would divide the 24 bytes of data from each
frame of the sector into 4 byte samples and continue playing the disc,
albeit with an inaccurate time display. However, a data reader used to
read audio data from the CD-DA to enable a copy to be made, would produce
a copy having a degraded quality of sound.
