Can jitter be measured in S/PDIF cable and correlated to SQ?

Empirical Audio

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Great! Look forward to seeing them

How to correlate the plot shape to auditory perception is difficult but my 2 cents
Working from the ideal single very narrow spike, which signifies that all timing is accurate from sample to sample - which means that all samples are processed at the correct time.
What happens if a sample is mistimed - it's the same as if it was a wrong sample value arriving at the right time.
In the frequency domain, it means that this sample, which should represent the amplitude of a point on a waveform, is slightly wrong in amplitude.

So back to the plots - I reckon that a single but widened spike could translate into constant fluctuations in the frequency waveform - in other words, if timing errors are significant enough (broader spike) then if a pure tone was being replayed it would be smeared away from purity. The wider the spike , the more smeared the tone.
What would be the result of two narrow spikes on the plot on replying this pure tone? I reckon it would produce two clear tones slightly off in frequency from the target tone's frequency

So how would all this be perceived playing complex signals like music? This is where the translation becomes somewhat speculative
I reckon the single broad spike will result in frequencies in the replayed music fluctuating around their recorded values - diffuse & less defined note purity - may well be more noticeable at lower frequencies as, from my reading, we seem to be more sensitive to fluctuations at lower frequencies. At a higher perceptual level, I believe this translates into soundstage definition - less precise

Two narrow spikes would be perceived in a different manner because it's creating two distinctly defined, slightly offset frequencies instead of one specific frequency for each note in the music. This may well be perceived as pleasing, giving a sense of more richness or depth (don't know just guessing on this one) to the sound?

The only way to tease all this out would be a careful & forensic approach trying to isolate just a specific change in jitter to its auditory perception possibly with pure tones, then with complex tones & then with music.

This sort of correlation between measurements & auditory perception is what is badly needed but also very difficult & complex

So far, the two narrow spikes, spaced at 10 psec or so does not seem to be audible when compared to a single narrow spike. It's worse to have a single broader spike for sure.

I have noticed a slight "hardness" to the sound when there are multiple thin peaks spaced out more.

Steve N.
Empirical Audio
 
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Empirical Audio

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Posted the TIE data in the same thread above, but I could not get the spectrum plot to work right. Maybe because it only works after the plots are finished and not on the fly....

Steve N.
 

jkeny

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So far, the two narrow spikes, spaced at 10 psec or so does not seem to be audible when compared to a single narrow spike. It's worse to have a single broader spike for sure.

Steve N.
Empirical Audio

Right but we have to be careful of what the TIE or TDC shows - it's a statistical plot of the mistiming hits - so it tells us how many mistimings occurred 10 psec either side of the target timing. We can't just assume this is happening randomly, there may be mistimings grouping together - i.e as an exaggerated example, all the 10 psec hits that are too late (on the right) may have all happened together in sequential samples - we don't know. If all early hits also happened in sequential samples then the result would be a fluctuating frequency which I believe is more audible than maybe a small frequency smearing.

Obviously, with a cable there isn't any mechanism for such grouping but at sender electronics side there could well be. It would be great to see the plot for no cable i.e directly from the output of your SPDIF transmitter to see what the cable overlays on this

So one narrow spike Vs two narrow spikes Vs one broader spike needs to be considered more & maybe further analysis.
Is there any way that the sample timing data can be analysed to look for pattern in the sequential errors or even exported for further analysis outside the scope?
 

jkeny

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Posted the TIE data in the same thread above, but I could not get the spectrum plot to work right. Maybe because it only works after the plots are finished and not on the fly....

Steve N.

Yea, AFAIK, it only works when capture/plotting task is finished.
 

Empirical Audio

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Right but we have to be careful of what the TIE or TDC shows - it's a statistical plot of the mistiming hits - so it tells us how many mistimings occurred 10 psec either side of the target timing. We can't just assume this is happening randomly, there may be mistimings grouping together - i.e as an exaggerated example, all the 10 psec hits that are too late (on the right) may have all happened together in sequential samples - we don't know. If all early hits also happened in sequential samples then the result would be a fluctuating frequency which I believe is more audible than maybe a small frequency smearing.

Obviously, with a cable there isn't any mechanism for such grouping but at sender electronics side there could well be. It would be great to see the plot for no cable i.e directly from the output of your SPDIF transmitter to see what the cable overlays on this

Great idea. I'll give it a try, but I think I only have a 50 ohm dual male connector. Should be close enough to prevent transmission-line effects.

It may be that the two peaks are just a product of a really small impedance mismatch. I plan to do some fine-tuning and use a long cable to determine how good the match is. I previously used the 1694A to determine the good match, but it may be that the two BNC cables are more accurately 75 ohms. I don't have a TDR, but I have other ways to determine how good the match is.

So one narrow spike Vs two narrow spikes Vs one broader spike needs to be considered more & maybe further analysis.
Is there any way that the sample timing data can be analysed to look for pattern in the sequential errors or even exported for further analysis outside the scope?

I'll look into this.

Steve N.
Empirical Audio
 

jkeny

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Great idea. I'll give it a try, but I think I only have a 50 ohm dual male connector. Should be close enough to prevent transmission-line effects.
Yea, 50 ohm might be OK but then again so close to the SPDIF transmitter it will result in a reflection returning to transmitter end earlier than a long cable & so we might see these reflections in the TIE plot

It may be that the two peaks are just a product of a really small impedance mismatch. I plan to do some fine-tuning and use a long cable to determine how good the match is. I previously used the 1694A to determine the good match, but it may be that the two BNC cables are more accurately 75 ohms. I don't have a TDR, but I have other ways to determine how good the match is.

I'll look into this.

Steve N.
Empirical Audio
Yea, a TDR (Time Domain reflectometer) would show you what signal reflections you have on the cable - lengthening or shortening the cable would probably show reflections arriving back at different times & a difference in the two peaks plot. Just a question - do you have a feel for how these reflections cause timing errors - how does the reflection arriving back at the SPDIF transmitter at the time of transmission of the signal slow it down?
 

jkeny

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BTW, one of the earlier links I gave to those TIE plots was from a DIYAudio thread about minicircuit RF attenuators & their use on SPDIF lines - have you ever tried them? They seem to benefit SQ even for SPDIF transmitters which are within spec (I used them on Hiface USB/SPDIF which was double the SPDIF output spec). Not sure exactly of their action but it may be to do with attenuating reflections, it may be better impedance matching or it may be that reducing SPDIF signal is beneficial in avoiding overshoot & ringing in the SPDIF receiver?
 

Empirical Audio

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Yea, 50 ohm might be OK but then again so close to the SPDIF transmitter it will result in a reflection returning to transmitter end earlier than a long cable & so we might see these reflections in the TIE plot


Yea, a TDR (Time Domain reflectometer) would show you what signal reflections you have on the cable - lengthening or shortening the cable would probably show reflections arriving back at different times & a difference in the two peaks plot. Just a question - do you have a feel for how these reflections cause timing errors - how does the reflection arriving back at the SPDIF transmitter at the time of transmission of the signal slow it down?

Yes, I do have a theory based on analysis that was verified in ABX tests performed by a Canadian audio magazine. See this white paper I wrote:

https://positive-feedback.com/Issue14/spdif.htm

Steve N.
Empirical Audio
 

Empirical Audio

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www.empiricalaudio.com
BTW, one of the earlier links I gave to those TIE plots was from a DIYAudio thread about minicircuit RF attenuators & their use on SPDIF lines - have you ever tried them? They seem to benefit SQ even for SPDIF transmitters which are within spec (I used them on Hiface USB/SPDIF which was double the SPDIF output spec). Not sure exactly of their action but it may be to do with attenuating reflections, it may be better impedance matching or it may be that reducing SPDIF signal is beneficial in avoiding overshoot & ringing in the SPDIF receiver?

Interesting. Do you have a link I can study?

Thanks,
Steve N.
Empirical Audio
 

jkeny

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Yes, I do have a theory based on analysis that was verified in ABX tests performed by a Canadian audio magazine. See this white paper I wrote:

https://positive-feedback.com/Issue14/spdif.htm

Steve N.
Empirical Audio

Thanks, yes, I just had to remind myself of this excerpt "So, how does this affect the jitter? When the first reflection comes back to the DAC, if the transition already in process at the receiver has not completed, the reflection voltage will superimpose itself on the transition voltage, causing the transition to shift in time. The DAC will sample the transition in this time-shifted state and there you have jitter"
 

jkeny

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Empirical Audio

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They are nothing special, Steve, just Pi-Pads which attenuate at different fixed dB & have 75 ohm characteristic impedance
The ones from minicircuits are bnc female to bnc male https://www.minicircuits.com/WebStore/dashboard.html?model=HAT-10-75+
Cost about $20 when I used them but mostly reduced harshness if it was present

Okay, I understand.

BTW, I did manage to get some frequency spectrum plots today and posted them on my forum. The Synchro-Mesh is basically flat from 0-300kHz, no data there.

Here is the jitter histogram:

Synchro-Mesh_Jitter_Sonos.JPG

Here is the spectrum plot:

Synchro-Mesh_spectrum_Sonos.JPG

As you can see, between 0 and 300kHz, it is flat, no samples there.

Steve N.
Empirical Audio
 
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Empirical Audio

Industry Expert
Oct 12, 2017
1,169
207
150
Great Pacific Northwest
www.empiricalaudio.com
They are nothing special, Steve, just Pi-Pads which attenuate at different fixed dB & have 75 ohm characteristic impedance
The ones from minicircuits are bnc female to bnc male https://www.minicircuits.com/WebStore/dashboard.html?model=HAT-10-75+
Cost about $20 when I used them but mostly reduced harshness if it was present

I finally got the spectrum working right, and I had a request to publish the jitter of the Synchro-Mesh reclocker directly into the scope, so here are the plots:

Synchro-Mesh spectrum of Period jitter with no cable:

Synchro-Mesh_spectrum_no_cable.JPG

Synchro-Mesh spectrum of jitter through Empirical Audio 1.25m Standard BNC-BNC cable:

Standard_BNC-BNC_SM_spectrum.JPG

As you can see, it does not change much when the cable is added, except for two small spikes, which are smaller. This might be resulting from the minimum cable length rule that I created. 1.25m-1.5m is optimum in most cases.

I also plotted my new reference silver cable:

Reference_BNC_cable_spectrum.JPG

This plot has more spikes and noise floor lifts, but they are still really low-level. The vertical scale is 2.8psec. This cable has a sweeter more musical sound. It is probably something going on at the low frequencies that is not visible in this plot that accounts for this.

Steve N.
Empirical Audio
 

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