S/PDIF problem. Time synch?

AlanRatcliffe · Jun 30, 2014

S/PDIF is one thing I don't have any experience with and now I'm running into a small but annoying issue and would appreciate any guidance/knowledge.

I'm running the coaxial S/PDIF output from my Vocalist Live Pro into my Focusrite Liquid 56. This is only because I ran out of analog inputs on the Focusrite.

The problem I'm getting is occasional "ticks" in the audio, which I strongly suspect are clock drift between the two units. The noise doesn't appear at the analog inputs.

The Digitech doesn't have any controls or settings for digital synch and also doesn't have Wordclock or any other way of synching with an external source. As I understand it, an S/PDIF signal should include the timing signal, so the two units should synch automatically with no user input.

So, anything I might be doing wrong? Does the cabling make a huge difference? It's pro quality audio plugs and only a short length of good screened cable, but is digital coax cable different? Or is it just the usual case of something labelled "pro" as an indicator that it's anything but?

If all else fails I'll try use the digital output of another, more capable unit like my G-System or Eleven Rack, but would prefer to keep those on the analog inputs which have level meters on the front of the Liquid 56.

AlanRatcliffe · Jun 30, 2014

OK. Seems I was having a senior moment. The source has to be used as the master clock, so I've had to set the synch on the Liquid 56 to "S/PDIF".

This raises more questions though:

Firstly, how advisable is it to use the Digitech clock as master for the Focusrite? I would imagine that jitter will be higher that way round.

Also, what happens if I also use one or two of the ADAT inputs for another digital source? Surely then the Focusrite can only lock to one of the two sources?

Manfred-Klose · Jun 30, 2014

I have used my old line 6 pod with the spdif output for a while with my homemade cable (1 meter of coax cable from yebo and the connectors), and it worked pretty well

I would imagine the clock in the focusrite to be the better, as it being the centerpiece , but then we will be opening up a whole new discussion of clocks.....

AlanRatcliffe · Jul 1, 2014

Thanks for the response Manny

Manfred Klose wrote: I have used my old line 6 pod with the spdif output for a while with my homemade cable (1 meter of coax cable from yebo and the connectors), and it worked pretty well

Yeah - I've been reading up on all things S/PDIF. It seems that audio and S/PDIF cables are not created equal. While you can use audio cable for short lengths, coax cable must be used for longer lengths - it's an impedance thing and longer lengths of cable increase the chances of signal error and clock jitter.

I would imagine the clock in the focusrite to be the better, as it being the centerpiece

Exactly, but unfortunately it has to lock to the source, rather than the other way round. That's the way the S/PDIF interface is designed.

In larger systems I presume you'd use trusty old wordclock to ensure that all digital units are synched together. Unfortunately the Digitech Vocalist in question does not have a Wordclock in though. Oh well, it's basically only for live use anyway and is only intended as a placeholder until I find a TC Helicon Voiceworks Plus or Voicelive Rack at a good price.