Below is an interesting article on digital jitter which I have pasted below - very relevant to the topic at hand.

Extracted from The Absolute Sound Magazine, March 2008, reproduced at http://www.avcables.co.za/Downloads/...19Apr2008).pdf

A Brief History

The advent of digital audio was heralded by proclamations that the sound-quality variability inherent in analogue systems was a thing of the past. Once an audio signal had been digitized, the conventional wisdom held—it was immune to degradation. If the bits were the same, the sound was the same— digital audio either worked perfectly, or didn’t work at all. This was, at first glance, a startling achievement over analogue systems which introduced slight (or not so slight) cumulative distortions at every turn. But beginning in the mid-80’s critical listeners reported hearing differences in digital audio where none should have existed. Using observational listening techniques, audiophiles noticed musically significant variations between coaxial and Toslink connections, different brands of digital cables; and even in the directionality of digital cables themselves! If the bit streams were identical, then how on earth could the sound change? However, if the sound is different then the signals must also be different, but in what way were the signals different? What was this “X”-factor that caused identical digital bit streams to exhibit an analogue-like variability? The idea that bit streams with the same “ones and zeros” could sound different when converted to analogue by the same DAC was viewed as the epitome of audiophile lunacy.

It was not until the late 80’s that engineers at JVC’s laboratories in Japan proved that timing inaccuracies (called “jitter”) in the DAC process introduced an analogue-like variability in digital playback. The “paradigm shift” that transformed jitter from audiophile lunacy to textbook orthodoxy had begun! By the mid-90’s, jitter, and its audible effects, became established fact. Today it is as though the aging debates of the late 80’s and early 90’s never happened—jitter is now accepted as a source of degradation in digital-audio recording and reproduction.

What exactly is Jitter?

This portion is rather technical, but bear with me and I’ll try my best to make clear what jitter is and how it degrades fidelity. We first need some background. Most of you know that in PCM-encoded audio, an analogue waveform is sampled at some regular interval (44Khz in the case of CD) and each sample generates a binary number that represents the signal’s amplitude at the instant the sample is taken. Sampling is like taking a snapshot of the analogue waveform at precise intervals, and then later reconstructing the original waveform from the snapshots. Each snapshot is a 16-bit binary “word” that represents the analogue signal’s amplitude. For example, a low-level signal might be represented by the binary word 000 000 000 000 0010, and a high-level signal by the binary word 110 110 010 110 101. This series of binary words (44,100 of them every second in the case of CD) is converted back into analogue with a Digital-to-Analogue Converter (DAC) chip. The DAC takes in a word and outputs an electrical current level that is commensurate with the word’s value. That is, if the digital word consists of all -zeros, there is no output current from the DAC. If the digital word is all ones, the DAC outputs maximum current. In a 16-bit system, there are 65,536 possible output currents, corresponding to the 65,536 discrete steps in a 16-bit word. This output current is converted into a voltage that is, after low-pas filtering, a nearly exact replica of the original analogue waveform.

But, what tells the DAC when to convert each sample to an output current? This is where the rubber meets the road. A signal called the “word clock” is fed to the DAC. The word clock is simply a square wave with a frequency of 44.1kHz (in this example). On the square wave’s leading edge, a word is loaded into the DAC; and on the square wave’s trailing edge, that word is converted into an output current as shown on Figure 1. This process is repeated 44,100 times per second (in a non-oversampling system). This is where jitter matters!

If the clock controlling when the samples are converted to analogue isn’t a perfectly precise and stable frequency, the spacing of the “snapshots” of the original analogue signal is wrong, some samples will be too close together, others too far apart, as shown in Figure 2. The result of reassembling the samples with precise timing is a misshapen waveform. Specifically, timing error in the clock translates directly to an amplitude error in the reconstructed signal.

There’s more going on than simple amplitude errors. Jitter also introduces in the reconstructed analogue signal spurious sideband frequencies that are not part of the original signal, and that are not related harmonically to the signal being constructed. Moreover, it turns out that the human ear/brain is astonishingly sensitive to these timing errors in the reconstruction of musical waveforms.

The classic sonic signature of jitter is now well known and documented: Loss of space and depth; softening of the bass; hardening of timbre; a glassy sound on initial transients (most noticeable on the leading edge of upper-register piano attacks); a metallic sheen overlaying the treble; and an overall flattening of the soundstage and homogenization of instrumental images within the stage.

Jitter’s deleterious effects aren’t confined to D/A conversion; jitter in the A/D clock is just as sonically harmful. Unlike D/A jitter, however, A/D jitter is permanently encoded in the digital bit stream and no playback clock, no matter how precise, can undo the damage. That’s one reason why CDs remastered from analogue tapes using modern A/D converters sound better.

It’s also worth noting that jitter matters only when converting digital audio to analogue. You can copy a digital bit stream to a recording device using the inferior Toslink connection with no degradation. But if you use Toslink between the digital source and the D/A converter, you’ll hear jitter’s effect because the bits are now being converted to analogue and the waveforms are being analysed by your brain.