Question about the potential for using LibreDrive for CD preservation
Posted: Sat Feb 10, 2024 11:47 am
I've been following (and occasionally contributing to) the Redump project for a while, which catalogues hashes of commercially-released optical discs so that people ripping their own can know that their copies are properly dumped.
One of the issues, though, is that CDs are pretty tricky to guarantee are properly read, for a few reasons. The CD-ROM format is, in some ways, more complicated than modern formats. You can have multiple tracks (including data and audio tracks on the same CD), there were copy protection methods that deliberately created invalid data on discs to check against (so, for example, sometimes the subchannel data mattered), it wasn't unheard of to have data in the leadin or leadout, and so on.
CD preservation is really kind of wild with all the edge cases. Unfortunately, this means that properly reading these discs is pretty challenging as well, and the best drives to do it with (certain Plextor models) haven't been manufactured in nearly 20 years. In particular, this is because the most reliable technique known (aside from being able to read back into the leadin and out into the leadout) is to read the entire disc as audio using the legacy SCSI D8 op code (reading the sector data, C2 error correction data, and subchannel data) and then to convert it to data. That's one of the major things these old Plextor drives support that others don't. https://github.com/superg/redumper is a newer tool that does this using the audio-read technique, for example.
With LibreDrive supporting more direct drive access, is it theoretically possible that it could be extended with CD reads on the same LibreDrives we already have to allow them to do something similar to the old Plextor drives? If so, it could be a huge boon to the preservation community, as right now, a lot of people have to spend a lot of time and money chasing an ever-dwindling supply of old, expensive drives (specific models of them, too) in order to be able to contribute.
One of the issues, though, is that CDs are pretty tricky to guarantee are properly read, for a few reasons. The CD-ROM format is, in some ways, more complicated than modern formats. You can have multiple tracks (including data and audio tracks on the same CD), there were copy protection methods that deliberately created invalid data on discs to check against (so, for example, sometimes the subchannel data mattered), it wasn't unheard of to have data in the leadin or leadout, and so on.
CD preservation is really kind of wild with all the edge cases. Unfortunately, this means that properly reading these discs is pretty challenging as well, and the best drives to do it with (certain Plextor models) haven't been manufactured in nearly 20 years. In particular, this is because the most reliable technique known (aside from being able to read back into the leadin and out into the leadout) is to read the entire disc as audio using the legacy SCSI D8 op code (reading the sector data, C2 error correction data, and subchannel data) and then to convert it to data. That's one of the major things these old Plextor drives support that others don't. https://github.com/superg/redumper is a newer tool that does this using the audio-read technique, for example.
With LibreDrive supporting more direct drive access, is it theoretically possible that it could be extended with CD reads on the same LibreDrives we already have to allow them to do something similar to the old Plextor drives? If so, it could be a huge boon to the preservation community, as right now, a lot of people have to spend a lot of time and money chasing an ever-dwindling supply of old, expensive drives (specific models of them, too) in order to be able to contribute.