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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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In Reply to: Those points seem arguable to me posted by Wavy Davy on August 28, 2003 at 19:57:53:
> you're saying that because the clock rate is lower that the effect will be less and I'm not sure I buy that since either clock would have the same jitter spectrum from the transport.Correct, the jitter spectrum given the same transport will be identical. However, if you for example took a jittery signal and fed it into an oversampler, the oversampled reconstruction would be distorted in a very different way from that of the non-oversampled one. With the latter, the error is distributed over a larger period, and it does not manifest itself as a distorted waveform, just an early or slightly delayed bit change (the waveform is distorted to hell already because of the 1x OS, but systematically so). My claim is that this is less problematic than what would go on inside an interpolation filter, where the errors is distributed into each interpolated bit level between samples.
And remember, the jitter is described in a statistical spectrum - if the jitter amplitude is correlated to, say, the effect of an external time-varying AC field (a non-guassian probability function), you have problems during reconstruction that you wouldn't be able to predict just by looking at the jitter spectrum. The waveform distortion due to jitter in this case is coupled to the AC field, a rather complex interaction.
Of course there are arguments saying that since non-OS image components are inaudible, the waveform distortion should be inaudible too. I am not saying I understand how jitter rejection behavior correlates to how a non-OS sounds so much more natural; jitter is not well-understood to begin with (The better sound is certainly a combination of effects from rather many factors, including the low bit rate conversion, ZOH reconstruction filter, implementation, post-filtering, etc... How they interact to yield that magic I do not know.). But in practice, the dAck! has definitely proven to be less responsive to those things that seem to make oversampling converters perform better. These include those jitterbug things, bybee filters, etc... Similarly, using a crappy transport with the dAck! v. a pretty good transport does not yield huge improvements like it would a more conventional converter.
Regarding the 240ps figure, it is related to the probability density function of the jitter. Max error is not as straighforward as calculating the error required for some fractional bit error for a single randomly-chosen sample - the RMS value can be the same for different distributions. I calculated the ~240ps value long ago from a formula in my transmission line textbook, I'm sorry but I honestly can't remember the form for max jitter I used off the top of my head. I'll see if I can find the reference for you.In any event, it's not straightforward to determine what this value is because it depends on the PDF, which changes depending on external conditions. RMS value doesn't tell you the whole story.
Here are some figures I turned up in a quick internet search showing a jitter probability function and how it gives rise to bit error. The figure isn't quite right for the non-OS case, since there the ideal sampling instant is left-edge-justified.
-chris
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Follow Ups
- Re: Those points seem arguable to me - csown 23:41:20 08/28/03 (0)
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