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Does Quality of Coax Input Matter for DACs?

amirm

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The title is much more lofty than this little test but I thought I share the results of testing a DAC with super high quality Coax S/PDIF vs a lower quality one. Question came up in the review of FX Audio DAC-SQ3: "how good is its coax output?" I have not done many measurements of these "digital to digital bridges" so I thought I expand it into an article by itself.

As I showed in the review of DAC-SQ3, it is one of those odd birds with USB input and Coax/Toslink output:

index.php


So if you happen to have a DAC or AVR that you like otherwise but lacks USB input, you can use this box for that (and get a DAC for free). It was not long ago that companies were selling and promoting such bridges. Getting one question out of way, no, the volume control on DAC-SQ3 does NOT change the output of coax output. It is strictly a bit-exact conversion from asynchronous USB to synchronous S/PDIF.

Coax Output Measurements
My Audio Precision analyzer has a clever mode where on digital input it can extract clock jitter and then analyze it using the rest of its tools. So let's use that to measure its own S/PDIF output to input, compared to output from DAC-SQ3:

FX Audio DAC-SQ3 Coax output Jitter Spectrum Measurements.png


This is a wideband measurement going to 110 kHz or so. The signal being played is the j-test which has 12 kHz square wave in it. That shows up as odd multiples of 12 kHz in both measurements. It also has a small square wave running at 250 Hz. This too is showing up in both.

The vertical scale is unusual: it is in seconds. The value though is quite small with the top of the display being around 25 nanoseconds (billtionth of a second). As a way of reference, if you have as sinusoidal jitter that has 250 picoseconds of jitter (trillionth of a second), it can damage the low order bit of 16 bit PCM data! So even small numbers matter.

Given the above, you may be alarmed then that the level of jitter out of DAC-SQ3 would deteriorate even your 16-bit audio. But such is not the case because the clock extraction logic in the DAC receiver acts like a flywheel to filter out high frequency jitter (think of heavy platter of a turntable for you analog heads). That filtering is not perfect though and will depend on quality of the implementation. To test the extent of that, I grabbed the Topping DX3 Pro+ DAC which I just reviewed and fed it both ways: through Audio Precision and then FX-Audio DAC-SQ3. Let's start with our dashboards:

Topping DX3 Pro+ AP Coax Input Measurements.png


This is basically the performance we got with USB input for DX3 Pro+. Let's now feed it data over USB to FX-Audio DAC-SQ3 and the Coax to DX3 Pro:

Topping DX3 Pro+ FX Audio DAC-SQ3 Coax Input Measurements.png


As you see, nothing is out of place despite the DAC-SQ3 having worse quality Coax output. If anything, it eliminated power supply spikes that were visible when AP was driving the DX3 Pro+.

Jitter however is frequency sensitive. The higher it is, the more it is visible. Our dashboard is run at just 1 kHz. So let's up the stakes by running the standard J-test where the primary tone is at 12 kHz (so 12 times magnification of jitter):

Topping DX3 Pro+ FX Audio DAC-SQ3 Coax Input Jitter Measurements.png


We see that the response is identical. There are essentially no jitter components visible down to whopping -150 dB. All that jitter that we saw has been filtered by Topping DX3 Pro+.

Granted, your DAC may not have such good filtering so there is a bit of unknown there. In all the jitter measurements I have made though, hardly any show levels that rise to audibility so generally speaking, you should be OK. You will certainly be fine if you buy highly rated DACs that I measure.

Conclusion
Digital audio is sensitive to clock jitter due to its high bandwidth and bit depth. Thankfully this has been a known problem for decades and DACs routinely have filters that get rid of much jitter. So within reason, I would not worry about the level of jitter we see out of devices like FX Audio DAC-SQ3.

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PeteL

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Those results are expected, but important. Good article. The thing that is left out in your article is that fundamentally, it is not due to "filtering" it is IMO simply the nature of Asynchronous transfer or USB audio class 2. UAC2, up till a point, will not transfer back incoming jitter, it should be immune to it. data is buffered and resynchronized, It's not a filter thing.
Edit: @amirm I am not sur I get it tough, just confirm, when DX3 is Fed trough audio-Precision output, it is fed trough USB in or SPDIF in. if it's from the same input it should be the same result, but if trough USB we should be the same subtle difference that in every jitter test you make is this correct?
 
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amirm

amirm

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The thing that is left out in your article is that fundamentally, it is not due to "filtering" it is IMO simply the nature of Asynchronous transfer or USB audio class 2. UAC2, up till a point, will not transfer back incoming jitter, it should be immune to it. data is buffered and resynchronized, It's not a filter thing.
This has nothing to do with the scenario here and what is tested. Jitter is created by the quality of transmission of S/PDIF over Coax in DAC-SQ3. The filtering is occurring in Topping DX3 Pro+ receiver/ESS DAC it has. It is not fed USB audio to care one way or the other in this testing.
 

PeteL

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This has nothing to do with the scenario here and what is tested. Jitter is created by the quality of transmission of S/PDIF over Coax in DAC-SQ3. The filtering is occurring in Topping DX3 Pro+ receiver/ESS DAC it has. It is not fed USB audio to care one way or the other in this testing.
OK thanks I agree, It's interesting. Can you just confirm the chain for both test to make it clear:
Hypothesis A:
Test 1: Audio Precision USB out-> DAC-SQ3 USB IN -> SPDIF out -> DX3 SPDIF IN -> ANALOG OUT-> AP IN.
Test 2: Audio Precision SPDIF OUT->DX3 SPDIF IN -> ANALOG OUT-> AP IN.

Hypothesis B:
Test 1: Audio Precision USB out-> DAC-SQ3 USB IN -> SPDIF out -> DX3 SPDIF IN -> ANALOG OUT-> AP IN.
Test 2: Audio Precision USB OUT->DX3 USB IN -> ANALOG OUT-> AP IN.

A or B?
Thanks.
 
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amirm

amirm

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AP has no USB output. When measuring USB devices, it uses the USB interface on the computer it is hosted on.

The two scenarios tested are:

AP producing Coax output
FX Audio DAC-SQ3 producing Coax output using its USB input as the PCM data.

In the first test, I analyze the coax output by itself in digital domain. In the second, I use a Topping DX3 Pro to covert it to analog and measure that.
 

PeteL

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AP has no USB output. When measuring USB devices, it uses the USB interface on the computer it is hosted on.

The two scenarios tested are:

AP producing Coax output
FX Audio DAC-SQ3 producing Coax output using its USB input as the PCM data.

In the first test, I analyze the coax output by itself in digital domain. In the second, I use a Topping DX3 Pro to covert it to analog and measure that.
Thanks. There is a bit I am not fully getting but I'm sure Tomorrow I'l get this. I get the same result part, not sure I get the no jitter at all part yet... I'll sleep on it and it will make sense. I still maintain part of what I said about "filtering" I just think it's not the right term even though my comprehension was wrong. In all cases, good stuff.
 
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jannek

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These results are pretty calming at least for jitter reduction in DACs. Are they also relevant for a case where the coax out is used to feed a DSP? Is DSP processing influenced by these jitter values? Or does a DSP do a jitter reduction before processing even if staying in digital domain? Or is a jitter reduction possible at the DAC even after processing?
 
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amirm

amirm

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DSP is orthogonal to this situation. It will get the bits, process them, and put them in a buffer. The output DAC then synchronizes that data with incoming clock. So if jitter filtering is not there, it will induce that in the analog output.
 

jannek

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Would this lead to audible degradations?
 

TabCam

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If I look at the review of the DAC-SQ3, the jitter is nothing to worry about. Are there not far worse devices and more jitter-prone dac's, especially cheap China-tech on Amazon or AliExpress that would make a far more interesting test?
 

jannek

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As far as I found out (at least in my case with a hypex fusion FA123) there seems to be some jitter attenuation in the digital receiver/ sample rate converter used for coax/toslink inputs.
 
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amirm

amirm

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If I look at the review of the DAC-SQ3, the jitter is nothing to worry about. Are there not far worse devices and more jitter-prone dac's, especially cheap China-tech on Amazon or AliExpress that would make a far more interesting test?
Yeh but what would be the reason for any of you to buy such poor devices?
 

Holmz

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Yeh but what would be the reason for any of you to buy such poor devices?

Maybe it is the other way around?

If the device (Hypex) has little buffering and jitter tolerance, then that points to a component that has not been engineered to work with other equipment as well.

So one could feel somewhat confident that that particular piece of gear is somewhat backwards comparable.
 

TabCam

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Yeh but what would be the reason for any of you to buy such poor devices?
Lot's of people buy those for combination of reasons like functionality, availability and price and are far less informed. I think showing them it does matter to buy properly designed devices also sends out a message that it matters in certain circumstances.
 

nyxnyxnyx

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A brief but very helpful test, Amir.
Thank you for revealing more information about this topic and contribute more data for readers in this hobby.

Next, if possible, please do more of "Does X affect/change/impact Y" if you find the idea interesting. Many readers here and outside of ASR surely must have quite a few questions about stuff like this.
 

nyxnyxnyx

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There are other phenomenons and unpopular trends like switching opamps, replacing capacitors with "better" ones, or even chaining the signals of multiple amplifiers together. I don't know exactly why most people do any of those, but it's usually brought up at some point in a typical audio banter.
 
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tonycollinet

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DSP is orthogonal to this situation. It will get the bits, process them, and put them in a buffer. The output DAC then synchronizes that data with incoming clock. So if jitter filtering is not there, it will induce that in the analog output.
By filtering here, are you meaning somthing like a phase locked loop clock generator, with a filter on the phase locking?
 
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