Jitter is quite easy to identify - it comes in many forms: a sudden change in speed of the song, a sudden gruffness in voice or instruments, a sudden skipping of a few notes (like a TT jumping a grove). These are effects you can hear.
Some of that is not, and could not be "jitter".
Jitter also works at a subtle level to change the soundstage, depth, and tonality of the instruments and/or voice.
But that could be
Apparently it can cause actual wrong notes to appear in the music. I take a look at the
Stereophile article, linked to above, from time to time, but have not yet managed to understand it. However, some of the diagrams reveal quite a lot, even to the innumerate such as myself. It seems that jitter can sometimes be heard as a hiss, or can even inject "wrong notes" into the music. The diagrams in the Stereophile article, as well as others, explain this a million times better than I ever could. Possibly this discordant-note injection might explain what you have perceived as a faulty pitch or incorrect tempo? Just making suggestions there.
It seems that the effects of jitter can be subtle, or unsubtle, but they do not extend to dropouts or skipping. I might be better able to suggest what
does when I have managed to cure all the problems with my own system. I am fairly sure that it belongs in the areas of interrupt handling and buffering.
This stuff is hellishlishly difficult to trace, because, unlike in analogue, there is no way that we can listen to what is happening along the path to the DAC, and events that may make a very audible difference to our final sound may be happening in fractions of time that we can not see or measure, or detect with normal system monitoring software.
Jitter is not a PC-only problem. It potentially applies to any DA or AD conversion, and one of the things we will all be reading in the articles we find is that the data on our CDs may very well have been subject to jitter. Somehow, though, we never thought about it much before we started using the PC as source.
Another one for the "Link Library":
Digital Problems, Practical Solutions, Sound On Sound Magazine. It's certainly off-topic to USB cables, but it reading it, along with listening to the provided sound samples, covers a lot of digital audio basics. I had no real idea, before this, for instance, what
dithering is about. Because it illustrates it with sound, rather than maths (the graphs are there too, though) it makes it very clear.
Here's a thing: he regards jitter as a problem pretty much solved
A lot of fuss is still made about jitter, but while it is potentially a serious issue it's rarely a practical problem these days simply because equipment designers and chip manufacturers have found very effective ways of both preventing it and dealing with it.
...
If the clock jitter is entirely random, the resulting distortion will also be random, and a random signal is noise. Since a high-frequency signal changes faster than a low-frequency one, small timing errors will produce larger amplitude errors in a high-frequency signal. So random jitter tends to produce a predominately high frequency hiss. I've yet to hear that on any current digital system, though clocking circuits these days are just too good for this to be a practical problem.
On the other hand, if the jitter variations are cyclical or related to the audio, the distortions will be tonal (similar to aliasing) or harmonic, and they'd tend to be far more obvious and audible. But I've not heard that on any current digital audio system either: other than in very low cost equipment with extremely inferior clocking structures, A-D and D-A jitter just isn't a practical problem anymore.
OK, though, he goes on to say...
Another source of jitter (the strongest source these days) is cable-induced. If you pass digital signals down a long cable (or fibre), the nice square-wave signals that enter degrade into something that looks more like shark fins at the other end, with slowed rise and fall times. This is caused by the cable's capacitance (or the fibre's internal light dispersion), so the longer the cable, the worse the degradation becomes. That's why digital cables need to be wide-bandwidth, low-capacitance types.
This matters because most digital signals incorporate embedded clocks along with the audio data, and that clocking information is determined from the rise and fall between the data pulses. If the clocking edges are vertical, the clocking moments are obvious. However, if the clocking edges slope, the timing point becomes ambiguous and we now have embedded jittery clocks!
When passing digital audio between one system and the next, the precise clock timing actually doesn't matter that much, as long as the average sample rate is the same for both. All that's needed is to be able to determine at each clock moment what the binary value of each bit is in the binary word.
However, when sampling or reconstructing an analogue signal, the clocking moments are critically important, as explained. So if a D-A relies on using the jittery embedded clocking information from its input signal to reconstruct the analogue output, there could be a problem with jitter noise or distortions. Fortunately, most modern D-As incorporate sophisticated jitter-removal systems to provide isolation between the inherently jittery incoming clocks embedded in the digital signal, and the converter stage's own reconstruction clock.
But please note the last line! Modern? He was writing in
2008.
Frankly, though, I do wonder if PC architecture development isn't actually moving away from being suitable for audio. I have experienced many more problems with the
PC equipment I've owned over the past five years than I did with what I had in, say, 2003
--- and that is including using the same sound card in those different computers.
Venkat, yes, digital
is digital, but that absolutely does
not mean that digital audio is without problems!
.
