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In Reply to: RE: Jitter is often expressed as a percentage of clock period ... posted by slider on July 30, 2012 at 11:27:25
This has little to do with audio quality IME. Its how much the edges move relative to the ideal, regardless of the sample-rate.
If differences in SQ between 44.1 and 88.2 are small (they are if your DAC does decent digital filtering), then I think the edges jittering are a bigger factor than the improvement due to SR.
Steve N.
Follow Ups:
RE: Jitter is often expressed as a percentage of clock period ..."
Steve, I believe we are in agreement. Let me put it a different way as it may make it clearer to some others.
Unless there's noise shaping, jitter depends on the high frequency content of the music, not the sampling rate. So long as the sampling rate is high enough to capture essentially all the audio content, going to a higher sampling rate isn't going to affect jitter. This is most obvious when looking at a DC (constant) signal, where there will be no effect of jitter whatsoever in the absence of noise shaping. If there is noise shaping, then jitter will appear as a form of noise modulation. Expressing jitter as a percentage of clock rate may be of interest to a communications engineer as it relates to the eye pattern and may affect bit error rate. However, for music reproduction this measure is irrelevant. What matters is the effect of jitter on the analog output of the DAC, which appears as phase modulation of the audio signal and is largely independent of the sampling rate.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Tony, Steve;
Remember audio related jitter error happens at really low frequecies like below 10Hz or 1Hz. While yes this error will effect higher frequency content because they are made up of less samples. The over all jitter will tend to go up as the sample rate goes up.
As slider said this is the way most phase noise measurements are made.
It's actually pretty easy to see by looking at the phase noise of the bit clock coming out of the divider.
Now the big questions is does any of this matter? Really if you look at current DAC chips internally none of this matters. As they simply have what is called a Serial Input Module which takes the data input and outputs parallel samples and also give some indication of the current sample rate then the rest of the dac works off of parallel samples. Really with parts like this the Master Clock becomes the key to really true low jitter output.
Thanks
Gordon
J. Gordon Rankin
As far as I know, no DAC manufacturer has yet to deliver a DAC that makes it unnecessary to tweak the PC or USB cable. So presumably, something really matters. Whether it is jitter or something else, who can say?
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
The clock jitter we most need to reduce is the data-correlated type which is so prevalent in many of the transmission interfaces, and so it doesn't really matter how you measure it in relation to the clock period since it isn't related to clock timing. That said, clock averaging can be a very effective method of eliminating that incoming jitter, and it is based on dividing the high rate clocks down to very low frequencies, and then using the diff signal to regenerate "clean" sample clocks from a low phase noise VCXO.
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