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Playing stereo on surround sound setup (Howto)

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Can I play stereo files over all speakers of my 4.0, 5.1 or 7.1 surround-sound setup?

Yes, you need to edit your .asoundrc file for this to work. I quote Takashi's email from the alsa-user mailing list:

On 28 Jan 2004 Florian Schmidt wrote: Hi, as far as i understand it, the copy plugin does nothing but copy its input to its output. I wonder if it would be easily possible to extend this plugin to copy to multiple slave PCMs. This way it would be (easily?) possible to "distribute" a stereo signal to 4 or even 6 channels. The route and plug plugins already can do that, for example...

pcm.ch40dup {
    type route
    slave.pcm surround40
    slave.channels 4
    ttable.0.0 1
    ttable.1.1 1
    ttable.0.2 1
    ttable.1.3 1

pcm.ch51dup {
    type route
    slave.pcm surround51
    slave.channels 6
    ttable.0.0 1
    ttable.1.1 1
    ttable.0.2 1
    ttable.1.3 1
    ttable.0.4 0.5
    ttable.1.4 0.5

and you can play two channel WAV using...

  aplay -Dch40dup 2ch-stereo.wav
  aplay -Dch51dup 2ch-stereo.wav

for 4.0 and 5.1 surrounds, respectively. Of course, this is just a copy of the left/right samples and no Dolby Prologic effect is included ;)

On 28 Jan 2004 TakashiIwai wrote more about this issue: Route (and plug) plugin already can do that. for example, in your question, multiple slave PCMs are assumed. In such a case, combine the multiple PCMs via the multi plugin at first, then use the route plugin over it.

See .asoundrc and alsa-lib.

To use the ch40dup or ch51dup devices with XMMS go to Preferences (Ctrl+P), choose the ALSA output plugin in the Output Plugin box, choose Configure and fill in the device of your choice at Audio device (it won't be in the list).

Hello. I've been having problems with the line-input capture. I'd like to take this stereo input and send it over my surround setup on my sounblaster live 5.1. MPlayer and XMMS already play in surround without a script like the above one, but the line in does not. Do I need to use a script like the one above in order to sort this out? Cheers

Instead of worrying about the ttable thing, you can also just use thise code in your .asoundrc:

pcm.duplicate {
    type plug
    slave.pcm "surround51"
    slave.channels 6
    route_policy duplicate

You just have to adjust the channels to the number of channels you have. Then just choose the duplicate plug in your audio player, and there you are. You can even set the slave as a dmixer, so you can have software mixing at the same time as you are 'expanding' a stereo file to all your speakers, like e.g. this:

pcm.dmixs51 {
    type dmix
    ipc_key 1024
    slave {
        pcm "hw:0,1"
        rate 48000
        channels 6
        period_time 0
        period_size 1024
        buffer_time 0
        buffer_size 4096
pcm.duplicate {
    type plug
    slave.pcm "dmixs51"
    slave.channels 6
    route_policy duplicate

Note that in this case, I use the pcm "hw:0,1", this may be "hw:0,0" for your soundcard (mine is a CMI8378).

Sebastian Brocks

If you use duplicate instead of the ttable routing then your center (and LFE) channel will be a copy of one of the input channels. Using ttable (as above) you can mix the left and right channels to get a more correct center.

One can use:

mplayer -af pan=2:1:0:0:0 file.ogg # left
mplayer -af pan=2:0:0:0:1 file.ogg # right

to test how the left and right channels are mixed.

You never want to route left or right channels to the LFE output. That one is used for effects. Modern 5.1 systems either use a low-pass filter themselves to drive a subwoofer or play bass using the satellites. If you do route to the LFE, you should get an awful lot of bass and/or distortion from the high frequency sounds.

The ttable example is corrected, and this is another reason why the duplicate method should not be used.

--Linuxfreck 11:01, 5 February 2008 (EST)

Just to add, if you want the duplication across RL and RR done in amarok as described above for xmms, go to amarok->settings->configure amarok->engine. You should have your output plugin set to alsa. In the section called ALSA Device Configuration, add plug:ch51dup to both Mono and Stereo boxes instead of default.

The volume of the speakers can be adjusted with alsamixer (or kmix) using center (for front speaker) front (for FL and FR) and surround (for RL and RR).

I admit setting the .asoundrc to duplicate the sound across all 6 channels is a lot more convenient since it does it for any app that uses alsa and doesn't require application level configuration. Is there any way to get alsa to use ch51dup for all applications (games, movies)?


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