- May 29, 2019
- Bavaria, near lake Ammersee
...the real nice thing about these USB to SPDIF converters is that many feature a built-in signal transformer, which breaks ground loops from digital (Streamer, NAS, PC, etc.) to analogue (DAC, amp) world. Without such (i.e. an USB DAC with identical digital and analog ground) there will always be some hum or noise under certain conditions (grounded amp, grounded devices connected via RCA, phono input/amp...).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:
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:
View attachment 158528
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:
View attachment 158529
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:
View attachment 158530
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):
View attachment 158531
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.
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.
As always, questions, comments, recommendations, etc. are welcome.
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