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Topic: timebase jitter a real problem? (Read 2540 times) previous topic - next topic
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timebase jitter a real problem?

I read this thread http://www.avsforum.com/avs-vb/showthread....threadid=115135 at avsforum. The discussed if a pc could be as good as a standalone-cd at feeding a dac with data.
I assumed it should, but then some sad you could have a problem with jitter. I thought this wouldn't be a problem, but after a while I realized it was not the same jitter that I thought it was i.e. dae jitter.
So I found a page with info on time-base error jitter at http://www.digido.com/jitteressay.html.
As far as I can understand jitter could be a problem,  but  if you use the s/dif output on a soundcard and a ordinary signalcable it should not be a problem (but maybe if the sondcard resample, like the SB?) for a decent dac to put the sample right in time if they have, as I assume, a own clock.

timebase jitter a real problem?

Reply #1
People who are seriously into audio suggest that jitter is the main reason why a standard PC is NOT suitable for digital audio playback.

If you take a (very good) soundcard with a digital output, and connect it to an external DAC, then there will be jitter in the timing of the signal. It'll be worse than a typical CD player, due to the harsh RF environment and (relatively poor) PSU within a PC.

Even if the DAC re-clocks the data, it still has to "keep in time" with the master clock, which sits inside the PC - so there's always the chance that jitter may "get through" to the final audio signal. It can be reduced by re-clocking the data in the DAC, but never eliminated. This is important: I used to believe that if you had enough buffering, you could solve the problem; unfortunately,  however much buffering you have, the DAC still has to slave to the PC. The buffering filters a lot of the jitter out, but there will still be some present.

In practice, some people can hear the effects of jitter, some can't. Re-clocking (or buffered) DACs certainly reduce it.


The ultimate solution is to have the clock in the external DAC - the DAC feeds the clock back to the PC sound card, and the PC sends data out according to this external clock - the DAC has a small buffer to allow for small variations, but it knows that the PC will always keep up, so just concentrates on its own (internal) clock.

There are sound cards which do this, but I don't know how it's done, I don't know which DACs allow this, and I can't point you in the right direction to find anymore answers. Sorry! But I hope someone is going to follow this post with a full answer to those questions!

Cheers,
David.

P.S. whilst I haven't demonstrated to myself that I can hear the effects of jitter, I do have a very "jitter prone" system here at work: The output of the sound card is fed (via a very long wire) to a standard DAC with no buffering. Some days (and specifically times-of-day) it sounds much better than others - this could be purely psychological, but I suspect it has something to do with the amonut of RF noise in the mains effecting the jitter level (and the analgue electronics of course).

 

timebase jitter a real problem?

Reply #2
Just like additive noise, audio distortion due to jitter is a matter of degree. In other words, everyone has an audible jitter threshold level---a level under which jitter cannot be heard/detected.

And also like noise, no digital audio system can completely prevent or eliminate jitter. Every DAC will introduce some into the audio signal. But it certainly can be controlled to remain within inaudible levels.

But, unlike noise---which cannot be truly be eliminated or reduced through post-processing---jitter can be reduced through appropriate buffering and more stable reclocking (before the D/A conversion itself, of course).

So while I agree that PC's are more prone to jitter than dedicatead audio equipment, I am more optimistic than 2Bdecided on the ability of a good outboard DAC to effectively (in the audible sense) eliminate jitter from a PC's signal. I don't agree that it is absolutely necessary to provide an external clock source to the sound card.

Of course, while a good outboard DAC most certainly can do this, whether it does is a different matter.
Michael

 
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