'Twas brillig, and cee1 at 20/03/10 14:05 did gyre and gimble:

Hi all, I know alsa support userspace software multi-playback by dmix plugin. How does it realize? I expected a daemon accepting multi-streams which then mixes these streams to a single one -- it seems not the case.

I believe an older version of dmix used to act like a daemon, but the current version no longer does this and instead uses some magic spells that I don't even being to know about.

As far as userspace mixing goes, most linux distros and a lot of embedded systems (e.g. smartphones) now use PulseAudio which does run as a stand alone daemon and opens a single stream to alsa. It does much more than just mixing, but it fits in with your paradigms :)

See http://pulseaudio.org for more info.

Col

cee1 wrote:

I know alsa support userspace software multi-playback by dmix plugin. How does it realize? I expected a daemon accepting multi-streams which then mixes these streams to a single one -- it seems not the case.

The kernel maps the device's DMA buffer into all userspace processes; the processes then cooperate in adding samples to the buffer.

Regards, Clemens