Welcome to cdfreaks chemabus.
Ops, not sure if anyone can understand that, but perhaps I can clarify some things…
The quote taken is from the Yellow Book spec equivalent, and basically means is from CD-ROM spec equivalent.
For anyone who doesn’t know yet, a small frame is the physical sector of CDs, i.e. we are looking at a lower level than the logical sector that is normally seen on the PC (2448 bytes per sector).
A small frame layout is like this:
1 synchronisation pattern +
1 byte of sub-channel +
12 bytes of main-channel +
4 bytes of C2 parity +
12 bytes of main-channel +
4 bytes of C1 parity
As you can see a small frame contains a total of 12+12 = 24 bytes of main-channel data. Main-channel is the 2352 bytes per sector part that we see on the PC.
Offset - you’re referring to the read or write offset that we get when reading or writing a CD. A popular subject upon EAC and dBpoweramp users :).
Each Scrambled Sector shall be mapped onto a series of consecutive frames. Each frame consists of 24 8-bit bytes, numbered from 0 to 23. Byte 0 of the Sector shall be placed in byte 4n of a frame, where n is 0, 1, 2, 3, 4 or 5. Consecutive bytes of the Sector are placed in consecutive bytes of the frames. Byte 2 351 of a Sector is immediately followed by byte 0 of the next Sector.
What I understand from this is that this allows the writing of a CD-ROM disc track to start at any of the 6 position choices.
By step you mean unit?
The allowable start positions are in units of 4n (as stated), which is same as CD audio sample units (1 stereo sample = 4 bytes). Since n are only of 6 numbers (as stated) the possible start byte positions are:
40, 41, 42, 43, 44, 45 or
0, 4, 8, 12, 16, 20
@chemabus, you wanted to give example of the first 2 start positions: 0 & 4. Yes, padding by 4 bytes of the 1st example would yield the same as the 2nd.
From my understanding of above quote, only the first byte of the track is affected and rest of bytes of the track are all consecutive. So your idea of padding 20 bytes to start position 4 (2nd example) wouldn’t yield the same as position 0 (1st example). It would instead be position 0 of the next frame - which is a total offset of 24 and not 0.
It is also important to mention that the above only applies to CD-ROM tracks or sectors.
I should also mention that a drive reading CD-ROM sectors will search for and align the data found in the sync and address (sync header) in the 2352 bytes per sector part (main-channel), so the offset is always 0 when looking at the logical level.
Contrast this to CD audio tracks - they don’t have such sync header, and so when a drive read these tracks they search and align to the address found in the sub-channel, but sadly they also introduce an arbitrary but constant offset between main-channel and sub-channel (the so called drive offset).