Before I get into the messy bits (ha - bits!), though, a mission statement of sorts - it isn't my goal (at least at this point) to claim that differences can't/don't exist. I've never heard differences between identical sources, but at least one friend of mine who isn't a quack has (or at least thinks he has!). Perhaps such a study can one day be properly conducted, but honestly at this time I don't care to debate the issue.
What I do care about, though, is the notion that differences *have* to exist, and if you don't hear them you're either deaf or have "inferior" equipment. My feeling is, and has always been, that if differences can exist, it's entirely based on your playback system. That is to say, two discs might sound the same on one system, different on another. If you hear differences, great. Just don't look down on others who don't hear differences because you have "golden ears".
Let's get this one out of the way quickly. One theory a lot of people like to trot out is that differences in discs can be due to reflectivity, how well the discs are burned, pinholes, etc. If not burned well, the error correction has to "work harder" and thus the disc won't sound as good. Gold discs are better, because they have more even surfaces and no pinholes.
This is nothing but nonsense.
CIRC (Cross Interleaved Read-Solomon Code) encoding utilized on CDs can correct gaps up to 2.4mm (3,500 bits) in length. Mind you, that doesn't mean "just make it so there isn't a pop", it means "even with a 2.4mm hole the data is still 100% accurate and intact". In the world of pinholes and scratches, 2.4mm is pretty huge. So unless you have a CD in *really* bad shape, chances are you're going to be getting 100% of the data that's on the CD. Quote:
Another suggestion is that sonic degradation is the result of
uncorrectable data errors on replay. This seems initially unlikely, since uncorrectable errors are very infrequent (at least as evidenced by the VALID flag at the player's output), and the sonic degradation described by listeners is continuous in character.
It's in the mastering
If by mastering you mean "the process of playing back an analog tape, converting to digital, and using EQ/compression/etc. to create a master tape", umm, no. Digitized music is a series of numbers. Total silence could be considered 0. A peak at 0dB could be considered 2^16 (or 2^24 if using 24 bits, for example). On a stereo CD, there are 88,200 16 bit numbers per second. Change the mastering and you change those numbers. If the numbers are the same, so is the mastering. There isn't some hidden 5th dimension where the voodoo is stored, at least not in terms of the data itself.
Very unlikely. Jitter is a time based error - instead of sampling at precise intervals, some samples might come slightly before or after they are supposed to. Mind you, this *can* be a problem with DACs. But they key here is CD players don't get timing data off of the CDs themselves. Quote:
A common theory is that physical imperfections in the 'pits' of the CD cause timing variations in the recovery of the EFM data, which are carried through to the player's digital-to-analogue converter (DAC). This 'sampling jitter' could cause significant audible degradation, as has been documented in , .
However, this theory misunderstands the basic architecture of the player: the timing clock of the DAC does not commonly depend on the recovered EFM data timing, but is derived from a free-running and stable crystal oscillator. The data is buffered in memory between the disc-reading electronics and the DAC. This buffer is emptied at the crystal oscillator rate, and kept half-full by varying the rotation speed of the disc; therefore
jitter in the recovered EFM data should not affect the clocking of the DAC.
Hell, even someone who hears differences with green pens couldn't find any changes in jitter:
The jitter measurements that I carried out raised more questions than they answered. As with error rates, none of the devices or treatments reduced jitter in the HF signal. During the testing, I had no idea how much jitter reduction to anticipate, and consequently set the measurement parameters so that even the smallest change would be detected. When I measured no change in jitter, I thought that perhaps the jitter analyzer, or my interpretation of the results, lacked sufficient resolution. However, after completing the jitter measurements, I received the Esoteric PD2, a $4000 CD transport. It had enormously lower jitter than the Magnavox player that I had used in conjunction with the error analyzer. Any concerns about the jitter analyzer's resolution were thus allayed. The PD2's jitter reduction was an order of magnitude greater than what I had been looking for during the tweak-effect measurements.
Side note - this answers a mystery I've always wondered about:
"This could be attributed to the fact that PDO uses a slightly different process in creating the center hole. Instead of molding the disc with the hole, it is punched after the disc has been made."
The leading theory (*only* theory?) seems to be this:
Measurements have confirmed that amplitude modulation of the analogue outputs of many one-box CD players by motor and servo-related interference occur. Furthermore, the resulting spuriae are of a character and at a level which would be consistent with noticeable sound degradation for a critical listener. The disc-dependent modulations are low-frequency, and so produce distortion sidebands close to the stimulus frequency. Masking theory suggests that these would be inaudible. The track-position-dependent modulations are generally higher in frequency and amplitude and, as such, may be noticeable to a critical listener. These effects have not been identified in two-box players, which is not surprising since they appear to be caused by modulation of the reference voltage of the internal DAC by the servo and motor electronics.
If I'm reading all of that correctly, it seems to suggest that physical differences between discs can cause the motor and servo to behave slightly differently, which cause voltage differences in the DAC which then result in audible differences. Now, this doesn't explain everything (notably that the people in the blind tests conducted, albeit a *very* small sample, couldn't accurately differentiate between discs), but it goes farther than just about anything else.
Of course, this all points to two things I've been saying for years:
It's in the player
The study I've referenced firmly suggests that different players may handle discs differently. Some may have problems with "bad" discs, while others (notably those with external DACs) most certainly would not. If you can't hear a difference on your player, it might just mean there *isn't* a difference on your player.
Burning/copying will eliminate physical differences
Let's assume that two discs are digitally identical, but yet have different physical properties. Let's assume they sound different on a certain player. Ok. Well, like it or not, if you copy both of those to your PC, those physical differences VANISH. You're left with nothing but the raw digital data. Play them both back on your computer and you won't hear a difference. Burn them both to CD-R, and (assuming you don't change anything in the burning process) they will both sound the same. Now, will they sound better or worse than the original discs? Who knows?! If the original discs had "bad" physical characteristics, it's quite possible that a CD-R could sound better. Or if the originals were "good" and your CD burner isn't so hot, the CD-R could sound worse. Whatever the case, the data hasn't changed. The physical differences only exist at the last step in the chain, and are eliminated each time a copy is made. Let's say you burn a CD with "bad" characteristics and send it to your friend. He burns another "bad" CD and so on. 100 generations of "bad" copies later, somebody gets it and burns it on a "good" CD burner. Assuming there were no digital errors along the way (unlikely), that CD will sound identical to a first generation "good" burn. The process isn't additive.
I think that's about all for now. I'm sure I'll think of various points later, but in the mean time, discuss and share with all of your friends.