A buddy and I are both running 64 bit Ubuntu 9.04. (He's trying to set up a Mythbuntu machine.) We're both using Gigabyte GA-MA770 motherboards (his is -DS3P, mine UD3) and the analog outputs are working fine. However, when hooking up the optical digital output (mine to Logitech Z-5500, his to a AV receiver) we get nothing. We're both using GNOME. From what I understand, there's an ALSA manager for configuring the sound driver. I think we have the Realtek Azalia sound chip. That's a common sound device, so Linux support's likely. Where do we go from here?

Does "aplay -D plug:spdif something.wav" work?

Are there any mixer controls that might enable the digital output?
(show the output of "amixer scontents")

The mixer settings were discovered and adjusted and it got stereo working via S/PDIF, but there's an odd result. The AV receiver will do a simulated surround sound from other stereo sources fed in with an optical digital coupling, but a CD played on the Myth box doesn't get the same treatment. For whatever reason the receiver only outputs to the main speakers and the sub. As I understand it, S/PDIF only transmits a stereo signal anyway, but why would the A/V receiver treat the signal from the computer differently than from any other source? Does it have anything to do with ALSA, or how we've set it up, or is this outside anything Mythbuntu/Linux is doing? Are we going to see the same effect when the TV tuner's finally working on ClearQAM? (subject for another thread) Maybe we should abandon the S/PDIF and just hook up via the analog jacks. I'm puzzled because I thought the only reason S/PDIF worked for multi-channel DVDs was by means of Dolby Digital or DTS decoding the multichannel information from the stereo digital signal, and might have expected a CD to play in stereo only. But in this case it's seems the receiver has a default simulated matrix surround, and that's being over-ridden or bypassed somehow in the isolated case of the source being the computer - kicking in when any other source is used.

i have a funny feeling that alsa is outputtng a full surround sound signal in your case but with nothing on the rear channels, so your AV box dosent know to upscale

I see what you mean. In a way, the computer is overriding things. That makes sense. I suppose we could try getting adapters (1/8th in. plug to RCA) and try the analog outputs, but I'd be surprised to see that bypass ALSA and its purist output info. The only thing I'm curious of now is why ALSA would send a full 5.1 (7.1?) signal from a purely 2 channel source like a CD. Too bad no one ever marketed SACD-ROM drives for computers. (;or developed the software for DVD-ROM drives to decode them, if that's possible. I'm a million miles from being capable of hammering out the code to do so, but if that were possible and the skilled developers out there who generously give us Linux distros ever did it, that would be a major incentive for owners of SACDs to abandon Windows for Linux machines. Might breathe new life into a format that needs and deserves it too.)