[linux-audio-user] Common linux audio layer

[Dave Phillips]

Greetings:

  I've been following this thread since it began. IIUC the original 
question was something like: "I want to run a number of audio apps 
simultaneously but [Linux, ALSA, SCO] won't allow it on my system".

  Again, if I understand the issue correctly, it seems that we're 
discussing audio stream multiplexing. Now, on cards like the SBLive we 
get what's called hardware mixing, a nicely transparent technology that 
lets me run multiple soundapps without doing anything extra to make it 
happen. You *can* do this in software, and as Lee points out the ALSA 
dmix plugin will do it.

  My laptop includes a NeoMagic 256A/V audio/video chip, i.e., no 
hardware mixing. So under normal circumstances I get to use one soundapp 
at a time. Here's my $HOME/.asoundrc file that gives me software audio 
multiplexing :

#########################################################
#
# asoundrc file to demonstrate use of ALSA dmix plugin
#

pcm.!default {
    type plug
    slave.pcm "dmixer"
}
 
pcm.dmixer  {
    type dmix
    ipc_key 1024
    slave {
        pcm "hw:0,0"
        period_time 0
        period_size 1024
        buffer_size 4096
        rate 44100
    }
    bindings {
        0 0
        1 1
    }
}
 
ctl.dmixer {
    type hw
    card 0
}

##########################################################


An example usage might look like this:

    xine -A alsa foo.mpg
    aplay -f cd -D default foo.wav

Performance is improved on my machine (a PII 366) if I change the sample 
rate to 32000. At 44.1 kHz the little box just barely keeps up whie 
playing a movie at the same time I run a major soundapp. There are 
probably other interesting tweaks, but this example might get someone 
started in the right direction.

I think the Windoze system audio mixer does the same thing as dmix but 
rather transparently. It is a pain to have to write a specific asoundrc 
file, but it's not an awful pain...

(dp thanks Takashi Iwai for explaining dmix to him over and over again...)

Best,

dp



tim hall wrote:

>Last Saturday 08 January 2005 18:53, Lee Revell was like:
>  
>
>>Hasn't anyone been listening?  dmix can do this now.  It's a bug in your
>>distro if this blocking behavior is the default.  There's no excuse for
>>it.
>>    
>>
>
>It might bear a little further explanation. Does anyone have any pointers to 
>useful documentation? If this is the same as the dbus-1 daemon then it's only 
>just made it into DeMuDi and I for one haven't figured its function yet.
>
>Yeah yeah, I'm Googling already ...
>
>cheers
>
>tim hall
>http://glastonburymusic.org.uk
>
>
>  
>