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Hardware mixing, software mixing

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Some soundcards and drivers support hardware mixing of multiple pcm streams which allows more than one app to play sound at the same time. If your soundcard does not support hw mixing (check in the SoundCard Matrix), aoss and DmixPlugin may help you.

There's also dsnoop. And asym to combine the two.

How to setup dmix/dsnoop/asym and aoss

And now follows a little howto on how to setup dmix/dsnoop/asym and aoss:

First, we need to setup a virtual alsa pcm device that knows how to handle sharing playback and capture streams. I assume that your \~/.asoundrc is empty, but if you already have an .asoundrc, I would suggest renaming it to .asoundrc.bak. I also assume that your soundcard is card 0 in your system and that its optimal sampling rate is 48000hz (check this with cat /proc/asound/cards)

Ok, now let's look at an example \~/.asoundrc:

pcm.my_card {
    type hw
    card 0
    # mmap_emulation true

pcm.dmixed {
    type dmix 
    ipc_key 1024 
    #  ipc_key_add_uid false   # let multiple users share
    #  ipc_perm 0666           # IPC permissions for multi user sharing (octal, default 0600)
    slave {
    pcm "my_card" 
    #   rate 48000
    #   period_size 512

pcm.dsnooped {
    type dsnoop 
    ipc_key 2048 
    slave {
    pcm "my_card" 
    #   rate 48000
    #   period_size 128

pcm.asymed {
    type asym 
    playback.pcm "dmixed" 
    capture.pcm "dsnooped"

pcm.pasymed {
    type plug 
    slave.pcm "asymed"

pcm.dsp0 {
    type plug
    slave.pcm "asymed"

pcm.!default {
    type plug
    slave.pcm "asymed"

What we have now is a virtual alsa pcm device called asymed. This device is capable of mixing several playback streams and sharing one capture stream amongst several applications. To get automatic sample-rate conversion, etc, we defined the device pasymed which uses alsa's plug plugin.

We also defined a device called !default. This is equivalent to pasymed. The special name !default makes this device the default device for all well-coded alsa apps (sadly not many are well coded).

And last we defined a device called dsp0. This device is used by the aoss script from the alsa-oss package. We'll say more about this later.

First, we test this basic setup with the standard alsa aplay tool. You will need a .wav file for this test. If you have none, create one from an mp3 with the following command:

mpg123 file.mp3 -w file.wav

With this file.wav we test the pasymed device now:

aplay -D pasymed file.wav

This should playback file.wav. It should work even if you run this command in two different terminals at the same time, because the sound from each one will be mixed by the pasymed device. Because we defined the default alsa device !default to use asymed, you should also be able to run the command without the -D pasymed parameter:

aplay file.wav

Not all apps understand the default device though. mplayer for example is one of them. To test this setup with mplayer use:

mplayer -ao alsa:device=pasymed file.avi

Next you should test whether all of your desired alsa apps work with this setup. Some will need to be told explicitly to use pasymed. Others will happily use the default. If some alsa apps behave badly with pasymed (e.g. crackles, stutter), check the old DmixPlugin page. It has quite a bit of troubleshooting advice. Tips: look at the sample rates of the slave, maybe experiment with the period_size parameter, etc.

I assume that your alsa apps work. Btw: you can also run some soundservers like artsd with alsa-support. artsd can be configured to use pasymed, too. I don't know about the esd soundserver.

You probably also have some OSS apps. The trick to get OSS apps to use our pasymed device is to call them via the aoss script from the alsa-oss package. This tweaks the OSS app to use the dsp0 device defined above in .asoundrc.

Older versions of aoss used to have problems with OSS apps that used fopen() and related functions to access the /dev/dsp device files. This has been fixed. See the aoss page for details.

Some apps use mmap'ed audio data transfer. If your app complains about not being able to use mmap(), then play around with the mmap_emulation setting in the my_card definition.

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Category: Howto