• CD-DA sector addressing, was Re: playing CDROM music questions

    From Thomas Schmitt@21:1/5 to Nicolas George on Mon Jan 8 20:10:01 2024
    Hi,

    i cannot contribute much to the practical issues with playing music.
    But i'd like to clarify technical properties of CD-DA media:

    Nicolas George wrote:
    compared to data CDs, audio CDs lack one layer of error-correcting code

    True.

    Another drawback is that CD-DA sectors cannot be read by the usual SCSI commands for hard disk reading, but only by the special command READ CD.

    and [lack] the synchronization necessary to tell the
    position of a particular sector.

    Not true. It is possible to read CD-DA with sector granularity.

    Random addressing may be somewhat cumbersome for the drive, because the
    exact sector address is part of the low level data of the sector.
    See MMC-5 4.2.3.5.2 "Mode-1 Q" for how it is implemented on medium and
    6.20 "READ CD Command" for how it is to be operated by software of the
    hosting computer.

    This is why tools like cdparanoia exist: between one read and the next
    they seek back a few sectors and check that the overlap matches.

    If ever it is the drive's firmware which has to guess the position and
    then to pick the sectors with the desired address in Mode-1 Q (or in a
    neighbor if one of the 10% sectors without Mode-1 Q is desired).

    The main reason for specialized CD readers like cdparanoia is rather in
    the fact that data readers like mount(8) want to use the general SCSI disk
    read commands. There are more reasons, as the man page of cdparanoia
    indicates, including workarounds for bugs of antique drives.


    Have a nice day :)

    Thomas

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