Al:
"My understanding is that if a CD is in good physical condition, the main reason why it might sound inferior to playback of a bit-perfect computer file is NOT errors that the player can't correct on the fly in a bit-perfect manner, which would therefore require inexact interpolation. My understanding, based on numerous references I have seen in the past, is that for a disk that is in good condition that will happen rarely during the playing of a disk, and not at all in many or most cases."
Well, if that were completely true, tweaks like Lightstop (green dye around outer edge) would have no impact on sound quality. But many have observed they do impact sound (including myself).
I did an extensive in house study myself a number of months back looking at BLER (block error rates) of my vintage 80's CDs vs new SHM Japanese imports. The SHM disks had C1 error counts into the single digits, whereas many of my 80's disks had C1 error rates into the hundreds. Very minor surface scratches can cause a large number of C1 level errors. Sonicly, I could not definitively correlate the number of C1 errors with sound quality, as most of the SHM disks were remasters as well (which introduced other variables). But generally, the SHM disks did sound considerably better than the same material in an early 80's pressing.
With respect to scratches, they all cause some diffraction and light scattering of the laser. I think that is why some of those surface treatments work, as they fill in very small scratches so they do not diffract. A scratch on the laser read side is very unlikely to penetrate the entire thickness of the polycarbonate subtrate, due to its thickness. But a scratch on the label side can penetrate the thin polymer coating that protects the metallization layer.
When I used to order DVDs from Netflix, many where in horrendus shape with tons of surface scratches. I would test them for BLER and find thousands of C1 errors and numerous C2 errors. After applying rubbing compound and wax, I could often get C1 BLERs under 100.
One issue not mentioned wrt CD transports mechanisms is that they are being discontinued by manufacturers. Take the Bryston BCD-1 for example. Phillips and Sony drives for high end audio applications are getting harder to source, and since they do wear out, you are stuck with trying to find replacements after some period of time. Ripped files are generated by computer DVD/CDROM drives which are cheap and readily available. So if there is close sonic parity between CD transports and digital ripped file sources, I think the digital sources will win out in the long run.
