[time-nuts] Sound Cards

[Magnus Danielson]

Demian Martin skrev:
> 
> 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.

I think you meant to say the electrical S/P-DIF and AES/EBU.

> The cheap ones don't have transformer isolation, however the transformers
> can increase the jitter on the link.

Depends on the transformer. The AES-3 electical form of AES/EBU 
(balanced on XLR) has a drawback in that it may be hooked up over lines, 
patchfields and cross-connects not intended for AES/EBU signals. This 
can cause some grief. Installation standards has changed as a result as 
a consequence. For smaller systems may the transformer not be as much of 
an issue as in larger installations. The unbalanced form of AES/EBU, 
AES-3-id, is becomming popular. It should not be confused with S/P-DIF 
since it is distinct in several forms, except for the obvious difference 
in subcode content.

> 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.

It is. Wordclock input?

Cheers,
Magnus