I've been playing a bit with reading the Q subchannel in the program area in order to accurately detect pregaps and track indexes. I'm working on OS X.

I seem to have things working, but I am seeing some unexpected results that I can't quite figure out and I wondered if someone with more extensive CD experience would have an idea.

For testing purposes I am using the disc The Very Best of Sheryl Crow, track two. Track two extends from LBA 20,550 to 42,494, corresponding to M:S.F of 04:36.00 to 09:28.44.

I am reading the Q data into a struct that mirrors the 16 bytes returned by CD READ. When I scan the Q subchannel for track two I'm seeing some strange values. Here is my debugging output for Mode-1 Q, filtered to show only ADR=0001b. MSF values in parentheses are relative and absolute, respectively.



So it seems that the pregap is being detected correctly, from block 42,440 to 42,494 (55 frames). EAC reports the Track3 pregap as 0:00:00.55, which matches.

What is not obvious to me is what an index of 5 and an index of 41 are doing in there, and why the 8 bits of zero are not showing up as 0 for two of the sectors. I was under the impression that index points had to be numerically sequential within a track.

Is the disc copy protected?

I suppose it's possible- I'm not sure how to conclusively determine if the disc is copy protected or not. Honestly the thought hadn't crossed my mind, but erroneous subchannel data would seem to be a good indicator that the disc is copy protected.

I've no idea how prevalent that sort of thing is; I tested maybe 6 discs and 5 of them exhibited this behavior with 1 of them appearing "normal".