Routing Stereo to 5.1 With ALSA
ALSA's route_policy duplicate can go beyond simple channel copies, according to a recent engineering note. When used as a slave for a Dmix PCM device, it can mix two-channel CD audio into arbitrary output-channel counts—including 5.1 surround—while still letting other applications access the sound card. This setup reportedly keeps XMMS happy during playback.
The same note flags a quirk with an onboard i810 sound card: it reverses the surround and center channels and applies odd mixing to the center channel for the subwoofer, producing distorted output on rear speakers. The author had not yet worked out a compensation strategy.
A pointer to the ALSA FAQ is available at http://alsa.opensrc.org/faq/.
