Talk:Compact Disc subcode

I am a bit confused as to whether the subcode is located as part of the sector of the CD, or if it is located elsewhere. The CD-ROM article under the format section states each sector is 2325 bytes long, but this article makes it sound as if the subcode is in a different place altogether other than those 2325 byte sectors. Totally confused. 71.248.139.39 (talk) 23:51, 2 January 2010 (UTC)[reply]

I hope it is clearer now with the changes I have made, though this particular question should be addressed more fully in the CD-ROM article. But the short answer is that a sector is 2,352 bytes long in terms of its useful data, not on the amount of space it actually takes on a CD. The value 2,352 comes from 98 frames in a sector with 24 bytes of useful data inside each frame. But on a CD, each frame has actually 9 more bytes (33 total), 8 for error correction and 1 with subcode information. So a sector actually has 98*24=2,352 bytes of useful data, 98*8=784 bytes for error correction, and 98 bytes of subcode data, for a grand total of 3,234 bytes per sector. This value, 3,234, is not commonly used since typically the CD-ROM drivers handle and hide the error correction and subcode bytes, so the only thing that is obtainable by the user or a program from the driver is the 2,352 bytes of useful data (there are special ways to get the subcode data, though, if required).

If you want to be even more exact, each sector is not stored in 3,234 bytes on a CD, since (as the CD-ROM article explains), each 33 bytes long frame is actually encoded with eight-to-fourteen modulation, which also adds merge bits, as a second layer of error detection. This effectively adds 9 bits to each byte in a frame, making it 33*(8+9)=561 bits long. Since a 27-bit long sync word is further added at the beggining of a frame, 588 bits (73.5 bytes) are actually used to store a 33 byte frame with 24 bytes of user data on a CD. So in reality, a sector uses up 73.5*98=7,203 bytes of actual space on a CD, though again, there are only 2,352 bytes (which have to be decoded) that actually store user data. Or only 2,048 (2 KB) in the case of CD-ROMs in mode 1 or XA mode 2 form 1, where part of the 2,352 bytes are user for a third layer of error correction and detection. Sega381 (talk) 14:06, 21 May 2013 (UTC)[reply]

Pregaps

How are the strats of pregaps and other INDEXes rather than 01 encoded by those subcodes and how does it comrrespond to the TOC that only marks the locations of INDEX 01? Any info on that?

83.13.239.255 (talk) 07:51, 29 April 2016 (UTC)[reply]

Fairly sure this is wrong at least code CD-ROM etc

I'm not sure about CDDA, but fairly sure our article is wrong for CD-ROMs (and AFAIK this also means for most other stuff except CDDA). Subchannels or control codes are not at the sector level. They are at the section level. Although both are 98 frames, they are very different things. A sector is made up of 98 F1 frames (although the first and last F1 frame can crossover sectors). But a single F1 frame is then spread over 106 F2 frames. And you then just take each 98 F2 frames add the control byte to get a section. So the subchannel is is actually fairly disconnected from a sector and indeed how it's spread will depend on how your F1 frame is located in the sector. Nil Einne (talk) 21:52, 23 November 2020 (UTC)[reply]