[time-nuts] Sound Cards

Demian Martin demianm at attglobal.net
Tue Jan 13 19:05:45 EST 2009

Magnus Danielson wrote:

> > The digital link in question is S/PDIF; with the current popularity
> > of Home Theater systems cheap cards with digital I/O have become
> > quite prevalent. As an added bonus, S/PDIF can be run over both
> > coaxial and optical media, the latter being attractive in further
> > isolating PC noise from any measurement setup. And of course, a
> > manufacturer's evaluation board is much better documented and more
> > suited to measurement-specific mods than a random sound card.
> The optical link commonly being used for S/P-DIF is TosLink and it seems
> like it can be the cause of many problems. It seems like some care in
> doing the optical link setup is needed. I have never digged into why the
> optical links have that problem. I can only guess, but bad optical
> coupling seems reasonable. The multimode "fiber" seems to be leaving one
> or two things to ask for.
> Cheers,
> Magnus

Without getting too "audiophile" there are several considerations. Toslink
is bandwidth limited- the LED's are not really fast enough for 96K and
really not for 192K sampling. The effect is high jitter on the link or a
lost connection. The high jitter may not matter if the sampling is done with
the external capture system (most of which are much more expensive). SPDIF
or AES/EBU don't have the bandwidth limitation but can have other issues.
The cheap ones don't have transformer isolation, however the transformers
can increase the jitter on the link. You can use external USB capture, I'm
playing with an EMU Tracker pre gadget that seems to do 192K at 24 bits
pretty well and for $125. But I get better results from the ESI Juli@ pci
card for around the same price, very low distortion and noise, and good Alsa
support in Linux. It may be all that's needed.

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