Balanced input/output supported by FreeBSD

Barry Bouwsma freebsd-misuser at
Wed Jan 12 16:35:43 PST 2005

On Wed, 12 Jan 2005 11:38:24 -0800, Dan Albers asked:

> Has anyone out there had any good (preferrably flawless) experiences with 
> soundcards with balanced input/output on FreeBSD? If so, model numbers 

I'm not sure if this would be too far removed from your signal
processing and/or needs, but would an S/PDIF solution combined
with an external box converting balanced/spdif work for you?
This way you would get the immunity to electrical noise and
ground loops offered by optical cables, or the ability to run
long distances, or whatever is most important from balanced
input/output, other than because it's what you have already.

Also, the quality of the soundcard audio should no longer depend
on its electronics, but that of the external converter, and you'd
get better results with, say, a 15-Euro soundcard, as with a much
more expensive pro analog card -- provided you had a decent converter.
This may not be true for all spdif-enabled cards.

As an alternative, would you be able to use a differential/unbalanced
converter (active, I would presume) located at your soundcard, and
a quality soundcard that's known to be supported (in particular, the
noise figures I see from a 1371-based card -- or else 1373, don't
remember) are extremely good, and in the past, I've whipped up a
low-noise op-amp or two to convert to/from balanced when needed)?

If I were to have to set up a sound machine today, I'd use S/PDIF
in order to avoid ground loops, that a year or so ago were making
me think of adding a differential input to handle an unbalanced
audio source with a serious ground loop problem -- probably coax
s/pdif because with my hardware, I was getting a weak optical signal
on input that was causing audible problems, and while I heard the
same on the electrical signal, it was trivial for me to tap into
the signal before it was attenuated to consumer levels and eliminate
that problem, without needing to build an external box.  Said box
would be trivial to build too, and should drive longer lengths than
toslink optical cables, I would think.

This isn't quite a plug-in solution, which may be what you're looking

> please. (We have been using Mia cards on windows machines, but we need to 
> have high-quality balanced I/O for our FreeBSD servers.)

Have you tried these cards with Linux?  There's mention of the codec
AK4528 used in those cards, so if they work, that could be a good
sign, in that there's a datasheet so that it could be possible to
have a FreeBSD driver written.

Also, my question would be, what do your FreeBSD servers do?  If they
are part of the audio chain, you may want to use my earlier suggestions
as last resort, as the bulk of supported cards aren't quite up to
studio quality, though some of them are quite passable, particularly
if the audio gets compressed and limited to hell in the chain.  I'm
not sure what >16bit >48kHz cards have BSD support, either.

While if you're using the FreeBSD boxen to take the final product and
blat it over the internet in some compressed lossy format, then I don't
see any drawbacks to the presently-supported cards, apart from their
unbalanced consumer inputs.

Hopefully someone else will provide a real answer with real products.

Disclaimer:  All my audio tinkering is done as a hobby, where I try
to obtain the best results from cheap^H^H^H^H^Haffordable consumer
products, so I'm not aware of what is out there, nor how the
broadcasting industry has changed in the past 25 years.

barry bouwsma

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