Addressing Individual Audio Channels of a Multichannel ALSA System

Mostafa — Mon, 03 Oct 2005 22:17:00 +0000

At first I thought I could just use one of the audio editors to create a 5.1 channel sound file and blank all the channels that I don’t want. Sweep claims to be a multichannel audio editor but at the moment it’s not possible to edit the individual audio channels. So, I tried out Audacity. Although Audacity allows the editing of the individual channels, it’s not possible to save the edited file in a multichannel format – it’s invariably down-mixed to stereo. So, custom sound files were no longer an option ..

At this point, I started looking at the ALSA PCM plugins. The route plugin seemed like the perfect candidate. In order to use the plugin I had to create the plugin definitions in the file ~/.asoundrc:

[darkknight@darkworld ~]$ cat .asoundrc
pcm_slave.rt {
        pcm surround50
        channels 5
}

pcm.front_l {           #Front Left
        type route
        slave rt
        ttable.0.0 1
}

pcm.front_r {           #Front Right
        type route
        slave rt
        ttable.0.1 1
}

pcm.front_c {           #Front Center
        type route
        slave rt
        ttable.0.4 1
}

pcm.rear_l {            #Rear Left
        type route
        slave rt
        ttable.0.3 1
}

pcm.rear_r {            #Rear Right
        type route
        slave rt
        ttable.0.2 1
}

pcm.front_both {        #Front - Both
        type route
        slave rt
        ttable.0.0 1
        ttable.0.1 1
}

pcm.rear_both {         #Rear - Both
        type route
        slave rt
        ttable.0.2 1
        ttable.0.3 1
}

The plugins can be used with the aplay utility:

[darkknight@darkworld ~]$ aplay -D front_c /usr/share/sounds/alsa/Front_Center.wav
Playing WAVE '/usr/share/sounds/alsa/Front_Center.wav' : Signed 16 bit Little Endian, Rate 48000 Hz, Mono

Of course it makes sense to use only Mono files with the plugins since the mappings are always carried out from channel 1. It’s the ttable entry that’s responsible for the mapping. The first value is the source channel (0 = channel 1, 1 = channel 2, etc.). The second value is the channel to route to. The third value controls the volume; a value of 0 means 0% volume and a value of 1.0 means 100% volume.

With the new plugin definitions, it’s even easier to balance the speaker levels. The following command will alternate between the front speakers and the center speaker until interrupted:

[darkknight@darkworld ~]$ while [ 1 ]; do /usr/bin/aplay -D front_both -q /usr/share/sounds/alsa/Noise.wav;
/usr/bin/aplay -D front_c /usr/share/sounds/alsa/Noise.wav; done

Audigy 2 ZS on FC4

Mostafa — Tue, 23 Aug 2005 12:07:00 +0000

I’ve finally got a setup that sounds just as good as it does on Windows. I’ve tested the surround system with a DVD and so far it sounds pretty good. I think the trouble was that there wasn’t any signal generator to let me balance the speakers properly. But today I discovered that there is an alsa subdirectory under /usr/share/sounds that contains a noise file. Playing that in a loop:

$ while [ 1 ]; do aplay /usr/share/sounds/alsa/Noise.wav; done

I finally managed to get that elusive speaker balance through alsamixer. Now, all that’s missing is CMSS3D

This site might be useful.

Random Wisdom » alsa

Addressing Individual Audio Channels of a Multichannel ALSA System

Audigy 2 ZS on FC4