Tuesday, June 24, 2014

Jitter in new Toslink/SPDIF link

Now that I've finally implemented a digital link from the kitchen Mac computer to the living room stereo, I'm a bit worried about jitter.  So far I haven't noticed any loss of digital lock, or any bad sound (in fact, it sounds better than anything) but as an audiophile with obsessive tendencies (like most) I nevertheless worry about such things (though not as uselessly as some people, I like to think).

The link is not as simple as I might like.  At minimum, it has to convert the Toslink output of the Mac (conveniently combined with the headphone jack) into coax to run across about 65 feet of coax to the living room (50ft running through ceiling reaching wall jacks, and an additional 15 feet of patch cords…or the ceiling wire might also be 75 ft, I provided both lengths to the electricians and don't remember what they actually used).

Probably most of the degradation would come from conversion into Toslink inside the mac, and then conversion from Toslink to coax, and maybe in the Toslink cables themselves.

But actually, it's more complicated than that, because I need to run the Toslink from the Mac into at least a 3 way splitter.  Currently I use a INDAY 4 way Toslink splitter, which splits off one line for the living room (which gets converted to coax as described above), one line for the kitchen system (a Yamaha AV receiver), and one line for the hard drive recorder (which only accepts analog inputs, so therefore goes through an EMU audio interface repurposed as a DAC).

As I said, everything seems to work perfectly, but the line going to the living room has to pass through these interfaces:

1) The Mac Toslink output
2) 1 foot very high grade plastic Toslink cable
3) INDAY active splitter
4) 12 foot high grade plastic Toslink cable
5) MAudio CO2 converter
6) 2 foot precision coax
8) 50 feet in-wall coax with F terminations
9) F-to-RCA adapter
10) 6 foot RG-6 patch cord with RCA terminations
11) RCA-to-RCA barrel adapter
12) 6 foot Monster Video 3 patch cord with RCA terminations

Now the easiest thing to fix would be 9-12, I can simply get a 10 foot precision video cable terminated with F connector on one end and RCA on the other.  I will be ordering that next month.

But what actually worries me more are the Toslink conversions.  First the digital signal is emitted through a Toslink LED in the Mac.  Count that as 1 hop.  Then in the INDAY splitter, the digital is received and retransmitted.  Count that as 2 hops.  Then, in the CO2 adapter, it goes through one more Toslink receiver.  So count this as 4 optical "hops."  A minimal Toslink connection involves only two hops: sender and receiver.  Each optical hop limits rise time and therefore makes jitter possible.

I could easily change this by running the Toslink from the Mac through a different kind of splitter.  The best kind is the one that I haven't yet found, which would have two optical outputs and one coax output.  Instead, the typical kind of "splitter/converter" has one coax output and one optical output.  That would work just dandy for the living room connection, which would be reduced from 4 hops to 2 hops.  But meanwhile, it would increase the number of hops for the kitchen receiver from 4 to 6, because I'd still have to use the INDAY splitter on the Toslink output of the first converter to get the two Toslink outputs that I need, and likewise the number of hops to the EMU audio interface would increase to 6.  They might not even work with that many hops (though, I expect they would), but at best they would see increased likelihood of jitter, despite the use of very short 1 foot very high grade Toslink cables.

The MAudio CO2 can be used as that new kind of splitter/converter.  By changing some switches, I could change it to take Toslink input and produce both Toslink and Coax outputs.  And there are also other similar splitter/converters available, including one from Calrad.  (MAudio no longer makes the CO2, but there is a Calrad replacement which can also be used as one Toslink to Toslink and Coax.)

But I have seen nothing doing exactly what I want: 1 Toslink to 2 (or 3) Toslink and 1 coax.  There are other variations, however.  Calrad makes an adapter that takes Toslink input and gives as output 1 Toslink, 1 Coax, and one stereo output.  With that unit I would put living room at the minimum of 2 optical hops, and keep the kitchen receiver at 4 optical hops (where it is now).  And I would eliminate the need for the EMU interface.  But that's also a rub, because I doubt the DAC in the Calrad adapter would be as good as the one in the EMU interface (which is professional grade, >110dB S/N, etc) and would also lack the convenient volume control.  Plus, I'd need to use a stereo audio isolation transformer on the analog outputs because otherwise I'd be connecting the grounds of my kitchen hard drive recorder and my living room audio system (very undesirable!).  That isolation transformer would not be as perfect as the optical isolation now used, AND it would further slightly degrade the analog audio sent to the hard drive recorder.

There might be some sort of switch that would do the trick.  This one looks like it might work, except it's unclear whether it actually converts Toslink to coax or merely switches Toslink to Toslink and Coax to Coax.  Plus it's way overcomplicated for what I need, since it also switches and distributes 4 component video inputs, though surprisingly inexpensive also (which raises other doubts).

And I haven't even begun to consider adding yet another digital link to the master bedroom.  I could easily add that on to the system I have now (simply by adding another CO2 converter to the currently unused 4th Toslink output of the INDAY splitter) but it would not work with any of the other adapters or switches I've mentioned so far.

In assessing new digital audio distribution strategies, it might be useful to measure the digital transmission system itself or the jitter it produces.  Here's the best discussion of measuring jitter I've seen.

Here are some very expensive digital analyzers I won't be buying.

Here's a long discussion of transports and jitter--but also looks closely at the SPDIF and Toslink interfaces.

Here's a discussion of Jitter that claims audibility based on AES analytics (not ABX testing) to 120pS for 16 bit and 20pS for 20 bit.  (The ABX testing shows much lower sensitivity up to 10nS, 100 times larger.)  Here's a further discussion of the importance of characteristic impedance by the same author (Steve Nugent of Empirical Audio).

Here is Toslink vs Coax measurement, and the coax is said to be 7 times better.

Here is MSB paper on jitter.

FINALLY, I found what I need, a Toslink splitter with extra coax output, the Inday TLDA-22.

Actually, what would be really perfect would be if it had two galvanically isolated coax outputs, so I could send coax digital to two different rooms.  Given that two galvanically isolated outputs would be pretty hard to do anyway, one coax output is ok.

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