So basically we all agree that jitter may exist internally, but even a hobbyist can build a transport from surplus computer parts that elimiates it to an ARBITRARILY high degree (by repeatedly ripping the track until it gets it right, for some statistical definition of right; bit for bit equivalent n times in a row?).
That's usually *not* what is being talked about when people refer to digital audio jitter. I would call the missing data problem, well, the missing data problem (and I would expect it to produce horrifically bad sound, although maybe it can be interpolated well enough to not suck totally).
What is usually worried about is timing jitter on the digital values. The CD reader produces a continuous stream of numbers. Half the numbers are for the left speaker, half are for the right. For simplicity, I'll just consider one of the speakers.
Each one of those numbers represents a voltage that is to be driven across the speaker wires for a short period of time. A particular number is turned into a voltage by an analog-to-digital converter, that voltage goes to an amplifier that makes it much bigger, and the big voltage goes to the speaker. The speaker position depends on the voltage, and so is determined by the number. Successive numbers are applied to the system one after the other, causing the speaker to move and causing sound to be generated.
Ideally, the numbers will come out of the CD reader at a constant rate (if I remember right, CDs are 44 kHz, which is a number every 22.73 microseconds). Unfortunately, the real world is not ideal. CD readers are mechanical devices and are subject to the usual vagaries of gadgets with moving parts. The disk speed fluctuates, the circuitry has to do varying amounts of error correction, the crystal timebase varies, etc.
The net effect is that the numbers coming out of the CD reader come out at varying rates. Sometimes they come a little faster, sometimes a little slower. Now, this variation isn't enough to screw up the digital interpretation. If it's supposed to be the sequence 4, 5, 6, the analog-to-digital converter will still see 4, 5, 6. It will just see them at slightly wrong times. The effect of this jitter is to cause frequency modulation of the audio signal, which is undesirable. (I don't know how audible it is in practice, but nevermind that.)
The naive (and cheap) way to build the analog-to-digital converter is to feed the numbers directly from the CD reader to the analog conversion circuitry. If you do that, whatever timing jitter is present in the mechanical parts will carry directly into the sound. You can improve the jitter with flywheels, fancier motors, and shock absorbers, but it will still exist. Unfortunately, mechanical solutions are not totally effective and are rather expensive.
The correct solution is to put a FIFO (first-in first-out) buffer in front of the analog-to-digital converter. The input port of the FIFO accepts data from the CD reader as it arrives, and stores it. The output port of the FIFO sends data to the analog-to-digital converter at a constant rate.
Which leads to the next question: how do you clock the FIFO output? If you clock it in sync with the CD reader, you won't have done anything for the jitter. If you clock it with an independent quartz crystal, the crystal will gradually fall behind or work faster than the CD reader; the FIFO will run out of data or overflow. Either way, distortion will have been introduced into the sound.
The clocking solution, as fluffy grue mentioned, is a PLL (phase-locked loop). A PLL is a type of oscillator that produces a clock with the same average frequency as another clock, but with the short-term variations filtered out. With a FIFO + PLL, the analog-to-digital converter gets samples at a constant rate (in the short-term), and the timing jitter is eliminated.
I honestly don't understand why the listening-only audiophiles are so freaked out by jitter. It takes a trivial amount of circuitry to eliminate. Every portable CD player that has 'skip protection' has very good jitter elimination built in. Building a receiver/amplifier with digital inputs that has jitter elimination is dead simple. I mean, if they can put it in a $50 portable player that runs 6 hours on batteries, an $600 receiver ought to be trivial, right? I guess I can understand why artists who are merging different audio streams that are at slightly different clock rates might be nervous, but even that is a fairly simple problem.
I guess I can understand, though. If I had a choice between selling $0.75 worth of PLL/FIFO, or a $6000 rig that floats on air, I'd probably pick the latter. Profit margin, profit margin, profit margin.
I don't want the world, I just want your half.
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