| 摘要 |
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 f or 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 provid e copy protection, errors are deliberately introduced into the data on a CD, b ut these errors are of a type which are generally transparent to an audio playe r 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 includ es 24 bytes of data, which is audio data for a CD-DA. The standard requires tha t 98 such frames are grouped into a sector. To provide copy protection, each sector 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 da ta from each frame of the sector into 4 byte samples and continue playing the disc, albeit with an inaccurate time display. Howewer, 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.
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