What was the purpose of exotic modes and sectors for CD-ROM?

Question

(I ask the question again, one suggested SuperUser, but I was right, it has been closed instantly.)

There are well-known sectors and modes for CD-ROM medias:

• CD-DA:
  • for storing music in a digital form, with 2 channels at 44.1 Khz
• Mode 1 Form 1, Mode 2 Form 1:
  • for storing data, content that requires error correction
• Mode 2 Form 2
  • for storing content were errors are permissible, e.g. video

But there are also less-known sectors and modes for CD-ROM medias:

• Mode 0:
  • they are filled with zeroes, AFAICT, never seen in a burning software
• Mode 1 Form 2:
  • indeed they are valid by the standard but why such specialization ?
• Mode 2 Formless:
  • another special type for which I miss purpose of
• Mixed Mode (for Mode 2, applies to tracks):
  • the ability to mix Mode 2 sectors in a track, for special content apparently

Of all the retro hardware that used CD-ROM, I remember AKAI CD-ROMs for SCSI audio samplers being one of these, these weren't directly readable in a PC, only using special software.

Question:

Do you have any other example of hardware that used non-standard CD-ROM modes and sectors ?

(I hope the question is correct now, let me know how I could improve it otherwise)

Answer 1

Try to approach the classification differently:

1) The CD-DA standard defined an audio sector of 2352 bytes.

2) The CD-ROM standard took 16 bytes from this sector for a 4 byte header and 12 sync bytes. The header allowed random access to this sector (which didn't work precisely enough for audio sectors).

1 byte of the header was for the mode. Mode 0 identified a sector with no data (for a printed medium), Mode 1 2048 bytes of data with error correction (additional to the audio sector error correction), and Mode 2 2336 bytes of data without additional error correction.

So Mode 1 could be used for harddisk-like blocks with a size of a power of 2 and solid error correction (for audio, occasional errors were ok, for data, they were not). Mode 2 was a "do whatever you like" approach, but had the header for precise addressing.

3) The CD-ROM/XA standard refined the CD-ROM standard mode 2 by adding a subheader of 8 bytes (defining the form and type of data in this block). Form 1 has 2048 bytes of pure data just like mode 1 above, but now with 8 bytes less for the error correction (and thus a different error correction). So it's similar to Mode 1. Form 2 has no additional error correction, so it's similar to Mode 2.

The advantage of this format was that different types of data could be interleaved on the same track.

So to answer your questions:

Mode 0: they are filled with zeroes, AFAICT, never seen in a burning software

Doesn't make sense when you burn CDs, only used for printed (mass-manufactured) CDs. Remember, being able to burn a CD was only possible a lot after the CD-ROM standard was published.

Mode 1 Form 2: indeed they are valid by the standard

No, it's not valid by the standard: Mode 1 needs to have a specific type of error correction with a specific layout, and the subheader doesn't fit in.

Mode 2 Formless:

It just means "the 2336 bytes of mode 2 are not subdivided according to CD-ROM/XA". So you have 2336 you can use for whatever you want. Certainly makes sense, though I don't know if there was ever any widespread product that used them for something else, e.g. video.

Mixed Mode (for Mode 2, applies to tracks):

Mixed mode is not restricted to Mode 2. It just means you can have tracks of different type even without CD-ROM/XA. But having e.g. video and audio on different tracks means you can't stream them at the same time, which is what CD-ROM/XA was good for.