Date: Thu, 13 Jan 2005 01:35:34 +0100 (CET) From: Barry Bouwsma <freebsd-misuser@remove-NOSPAM-to-reply.NOSPAM.dyndns.dk> To: freebsd-multimedia@freebsd.org Cc: Dan Albers <computing@kpfa.org> Subject: Re: Balanced input/output supported by FreeBSD Message-ID: <200501130035.j0D0ZXnY034135@Mail.NOSPAM.DynDNS.dK> References: <20050112120059.64E8D16A4D5@hub.freebsd.org> <200501121138.24718.computing@kpfa.org>
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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 for... > 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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