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Toslink switch and TosLink/Coaxial SPDIF converter - Review and measurements

MRC01

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...
The first toslink/coax adapter that I tried a couple of years ago worked just well enough to realize that it didn't work. It would work fine for a few minutes then start to fail. So don't use this one:

Then I replaced it with this one, which works reliably up to 192-24:

They look the same but in functionality they are not!
...
Well that newer converter just started acting up. Nothing obvious, but the very occasional 'tic' when listening to music, as if it's losing sync. Measuring it with Room EQ Wizard it's obvious. The sweep distortion has these glitches:
1703006748505.png

This converter was clean when I first got it back in March. So, can anyone recommend a converter that is clean and built well enough to stay clean?
 

MRC01

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With more testing, the situation gets interesting.
I am capturing these REW sweeps with a Tascam DA-3000 recorder.
When I play the digital sweep through my DAC/Preamp (Corda Soul) and record its balanced analog output, it is perfect. Same when I record the balanced output of the Topping E70 DAC.
But when I play that digital stream directly to the Tascam's SPDIF input, it must go through a toslink/coax converter. In this case I get the glitches above.
However, the Tascam DA3000 has a built-in sample rate converter. If I enable this, the glitches go away entirely.
This should not make any difference because I am manually setting the DA3000 sample rate to match the source, so there is never any sample rate conversion, even when the converter is enabled.
So here's one theory that fits these observations:
  • The toslink/coax converter box is introducing jitter or sample rate variations.
  • The Tascam DA3000 occasionally loses sync and glitches because the variations are too much for it to adapt to.
  • When you enable the DA3000 sample rate converter, it enables it to adapt to bigger variations in sample rate/jitter.
Put differently, when the DA3000 sample rate converter is disabled, it is very strict about adapting to incoming sample rates. If the clocks are even slightly off, or there is jitter, it loses sync and glitches. When you enable its sample rate converter, it is more forgiving and adapts to bigger variations.

This is similar to the DPLL setting in some DACs that adjusts how much it will adapt to sample rate variations & jitter.
 

MRC01

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This page from the DA3000 manual has a clue:
1703021300688.png

They don't say what the allowable frequency deviation is when SRC (sample rate converter) is ON. But the fact that they say this is the range only when it's OFF, suggests that it is wider when it's ON. Otherwise, why add that note?

So the toslink/coax converter box is adding more than 100ppm of clock drift / jitter. The Corda Soul & E70 can handle this OK (though I did have to increase the E70 DPLL setting), but the Tascam needs to have SRC enabled in order to handle it.
 

MRC01

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I discovered another fix: in the DA-3000 settings, change the Clock Source from "Internal" to "DIN" which means "Digital Input". With this, you don't need to enable SRC (sample rate converter).

I find it interesting that clock sync issues are not a "solved problem" in 2023, but require twiddling with settings. But it does make me glad the settings are there to twiddle so I don't have to put up with the occasional glitch/tic.

Also, the DA-3000 has a handy "Reference Level" setting that changes its analog output voltage. This way you can match it to the voltage sensitivity of your preamp or other downstream device it is driving. The range for digital full scale is +15 to +24 dBu, which in V RMS is 4.36 (consumer) to 12.28 (professional).
 
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