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Compact Disc and Drive Formats Chapter <strong>13</strong><br />
709<br />
Table <strong>13</strong>.12 Green Book/CD-ROM XA (Yellow Book Extensions) Mode 2, Form 1<br />
Sector Format<br />
Sync Header Subheader User Data EDC ECC<br />
12 4 8 2,048 bytes 4 276<br />
Table <strong>13</strong>.<strong>13</strong> Green Book Mode 2 Sector Format Breakdown<br />
Green Book/CD-ROM XA Sectors (Mode 2, Form 2):<br />
--------------------------------------------------------------------<br />
Q+P parity bytes 784<br />
Subcode bytes 98<br />
--------------------------------------------------------------------<br />
Sync bytes 12<br />
Header bytes 4<br />
Subheader bytes 8<br />
Data bytes 2,324<br />
EDC bytes 4<br />
--------------------------------------------------------------------<br />
Bytes/sector RAW (unencoded) 3,234<br />
Table <strong>13</strong>.14 Green Book/CD-ROM XA (Yellow Book Extensions) Mode 2, Form 2<br />
Sector Format<br />
Sync Header Subheader User Data EDC<br />
12 4 8 2,324 bytes 4<br />
Both Mode 2 sector formats add a subheader field that identifies the type of information (such as<br />
audio or video) carried in the user data field. The Form 2 sector lacks the ECC of the Form 1 sector<br />
and increases the size of the user data field instead. This type of sector is for storing audio or video<br />
data that can tolerate errors.<br />
Because they don’t use any third-level error correction, CD-ROMs that use the Mode 2, Form 2 sector<br />
format (such as MPEG video CDs) can hold more user information than other CD-ROM types in the<br />
same number of sectors, and as a result also have a higher data transfer rate of 174.3KB/sec instead of<br />
the standard 153.6KB/sec. Note that Form 2 sectors are never used to store data or program files<br />
because errors can’t be tolerated in that type of information. In that case, the Mode 2, Form 1 sector<br />
format would be used.<br />
For a drive to be truly XA compatible, the audio data written in Form 2 sectors on the disc as audio<br />
must be ADPCM audio—specially compressed and encoded audio. This requires that the drive or the<br />
SCSI controller have a signal processor chip that can decompress the audio during the synchronization<br />
process.<br />
Some earlier drives were called XA-ready, which meant they were capable of Mode 2, Form 1 and<br />
Form 2 reading but did not incorporate the ADPCM chip. This is not a signficant shortcoming, however,<br />
because only certain multimedia titles use the ADPM encoding (with interleaved audio and<br />
video). The main benefit XA brought to the table was the additional sector modes and forms taken<br />
from the Green Book.
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