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S/PDIF and PLL
S/PDIF and PLL
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Old 10th January 2017, 12:16 AM   #71
abraxalito is online now abraxalito  United Kingdom
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Quote:
Originally Posted by peufeu View Post
This neatly explains why no-one uses 1-bit DACs anymore* : a little bit of wideband jitter (like 100ps) utterly destroys an easily measurable characteristic (ie, SNR).
Hi peufeu - don't overlook that some manufacturers (AKM and Wolfson just to mention two) minimize the issue through using switched-capacitor DACs and output filters. Its the ones that allow off-chip opamps (TI and ADI I'm looking at you) that are going to be most susceptible to jitter. Not that on-chip op-amps don't introduce their own weaknesses....

In the days of 'Bitstream' there was the TDA1547 which had both a S-C 1bit DAC and on-chip opamps (allegedly something similar to 5532).

Thanks a lot for sharing those very illuminating sim results
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Last edited by abraxalito; 10th January 2017 at 12:19 AM.
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Old 10th January 2017, 10:22 AM   #72
TNT is offline TNT  Sweden
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So that is the noise aspect of jitter in different DAC topologies - interesting! But SNR, is it really critical in the listening situation? How about distorsion?

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Old 10th January 2017, 11:06 AM   #73
abraxalito is online now abraxalito  United Kingdom
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SNR if its at a static level doesn't matter too much, what's important for SQ is that the SNR shouldn't change dynamically in a manner correlated with the signal.
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Old 10th January 2017, 02:49 PM   #74
TNT is offline TNT  Sweden
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Quote:
Originally Posted by abraxalito View Post
SNR if its at a static level doesn't matter too much, what's important for SQ is that the SNR shouldn't change dynamically in a manner correlated with the signal.
Good.

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Old 10th January 2017, 08:16 PM   #75
peufeu is offline peufeu  France
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Quote:
Originally Posted by abraxalito View Post
Hi peufeu - don't overlook that some manufacturers (AKM and Wolfson just to mention two) minimize the issue through using switched-capacitor DACs
Yeah, charge transfer DACs are supposed to be a lot less jitter-sensitive... I don't know if WM8840 uses this style or the more classic current DACs, couldn't find any info in the datasheet.

Quote:
Originally Posted by TNT View Post
So that is the noise aspect of jitter in different DAC topologies - interesting! But SNR, is it really critical in the listening situation? How about distorsion?
//
Well, when you FFT a signal, you usually get peaks (harmonic distortion, intermodulation, jitter peaks, etc) and then a noise floor.

The latter is misleading, though. It looks flat and harmless enough, maybe even white noise! Here are several signals whose FFT spectra look rather identical, as you can see the signals are very different, one is harmless white noise, but the others are not, and the last one is even a log frequency sweep !

So, don't trust your noise floor. I's on my todo list to make some measurements on that... someday...
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File Type: png white noise spectra.png (188.3 KB, 99 views)
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Old 11th January 2017, 07:07 AM   #76
TNT is offline TNT  Sweden
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I suppose it has to do with the FFT length? A lot will also look like noise if it is averaged over a long enough period.

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Old 11th January 2017, 07:21 AM   #77
abraxalito is online now abraxalito  United Kingdom
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As jcx often points out, FFTs don't intrinsically average in that the transform as a whole (real and imaginary) is reversible. However when you look at a pure magnitude plot, half the data's been already neglected (phase). FFTs are a rather poor tool for examining noise.
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Old 11th January 2017, 08:52 AM   #78
cdsgames is offline cdsgames  Canada
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One question on the spidif output...I often see the isolation transformer (coaxial) and understand what its used for. However how will the transformer impact the jitter/quality of the spidif signal...anyone knows ?
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Old 11th January 2017, 09:16 AM   #79
peufeu is offline peufeu  France
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Quote:
Originally Posted by TNT View Post
I suppose it has to do with the FFT length? A lot will also look like noise if it is averaged over a long enough period.
For noise, FFT shows power (or amplitude/phase) in each frequency bin, averaged over FFT length. This is why FFT is amazing, since it allows to examine periodic, repeating signals with great precision while rejecting noise from the display.

FFT only reports one value of (phase,amplitude) for each frequency over its length. So, if the signal's phase and amplitude vary, either it does so in a way that maps well to a meaningful visual representation... or it does not.

Anyway. Other measurements (like noise power vs DC offset to check for noise floor modulation, or simply looking at it on a scope) are more useful for noise.

Quote:
One question on the spidif output...I often see the isolation transformer (coaxial) and understand what its used for. However how will the transformer impact the jitter/quality of the spidif signal...anyone knows ?
Theory is simple : a transformer is a bandpass system, so it introduces phase shift and amplitude attenuation (see measurement of 3 transformers, curve 5). Therefore it distorts the waveform. LF highpass effects add LF jitter ; HF attenuation slows down edges and also adds jitter.

Using a simple matching circuit (first attached image) some of this can be corrected. However, theory breaks down when you realize every SPDIF source has a different output circuit. In practice, it has to be fine-tuned to the source while looking at the scope (see scope shots) as the value of CT should be identical to the series capacitor in the source...

Optical is much simpler ! (and much better isolated)
Attached Images
File Type: jpg schema.jpg (24.0 KB, 80 views)
File Type: png RT=75 CT=1n C1=10n TP1.png (24.6 KB, 77 views)
File Type: png RT=75 CT=10n C1=100n C2=1n TP3.png (23.8 KB, 75 views)
File Type: png RT=75 CT=short C1=100n C2=1n TP3.png (28.5 KB, 76 views)
File Type: png transfos dm.png (47.6 KB, 33 views)
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Old 11th January 2017, 09:25 AM   #80
cdsgames is offline cdsgames  Canada
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ok got it. We are designing a spidif board and thinking that instead of isolating at output...we will galvanically isolate the input.
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