Better Linux Sound Managment With ALSA, Part 2 - page 2

Testing Sound Channels

How do you know what type of output your sound card supports? aplay tells the tale:

$ aplay -L
front:CARD=V8237,DEV=0
    VIA 8237, VIA 8237
    Front speakers
surround40:CARD=V8237,DEV=0
    VIA 8237, VIA 8237
    4.0 Surround output to Front and Rear speakers
surround41:CARD=V8237,DEV=0
    VIA 8237, VIA 8237
    4.1 Surround output to Front,
        Rear and Subwoofer speakers
surround50:CARD=V8237,DEV=0
    VIA 8237, VIA 8237
    5.0 Surround output to Front, 
        Center and Rear speakers
surround51:CARD=V8237,DEV=0
    VIA 8237, VIA 8237
    5.1 Surround output to Front, Center, 
       Rear and Subwoofer speakers
iec958:CARD=V8237,DEV=0
    VIA 8237, VIA 8237
    IEC958 (S/PDIF) Digital Audio Output

If you ever need to specify the PCM device, this is where you find it: front, surround40, surround41, and so forth are PCM devices that should have been created automatically at startup. You could test these with aplay, like this:

$ aplay -D front test.wav
Playing WAVE 'test.wav' : Signed 16 bit Little Endian,
 Rate 44100 Hz, Stereo

You may also also test playback by using the card+device numbers. This is sort of a "raw" test, since it doesn't depend on having a correct PCM devices configuration, but uses the card IDs assigned by the kernel. aplay -l shows the "raw" card+device numbers. 0,0 is the first device, 1,0 is the second device, and so on:

$ aplay -D hw:1,0 test.wav

If you have a second sound device, such as a USB headset, you can configure the PCM device names either globally in /etc/asound.conf or per-user in ~.asoundrc. Something as simple as this works just fine:

pcm.v8237 {type hw card V8237}
ctl.v8237 {type hw card V8237}
pcm.headset {type hw card Headset}
ctl.headset {type hw card Headset}

For pcm.v8237, the pcm part is required. The v8237 can be anything you want, and the same goes for the ctl lines. type hw card is required; V8237 and Headset come from the output of aplay -l. You could also use asoundconf:

$ asoundconf list
Names of available sound cards:
V8237
Headset

Now instead of using aplay -D hw:1,0 test.wav I can use aplay -D headset test.wav. You want to use the names (V8237, Headset) in your configuration file instead of the kernel IDs (0,0- 1,0), because the kernel IDs can change, especially udev devices.