Ecasound Multichannel Audio with ALSA Multi-Device

This article explains how to output multi-channel surround sound from Ecasound to an ALSA multi-device setup. It covers the compatibility between Ecasound and ALSA's pcm.multi plugin, provides the necessary ALSA configuration syntax, and demonstrates the exact Ecasound command-line parameters required to map and route surround channels across multiple physical or virtual audio interfaces.

Ecasound fully supports outputting multi-channel surround sound to an ALSA multi-device configuration. Because Ecasound interacts with ALSA at the standard PCM layer, any logical device defined in your ALSA configuration (~/.asoundrc or /etc/asound.conf) can serve as an Ecasound output target, including complex multi-channel device bindings.

Configuring ALSA pcm.multi

To output surround sound across multiple stereo cards or distinct hardware ports, you must aggregate them into a single ALSA virtual multi-channel device using the multi plugin.

A sample 4-channel configuration combining two stereo sound cards (hw:0,0 and hw:1,0) looks like this:

pcm.multichannel {
    type multi
    slaves.a.pcm "hw:0,0"
    slaves.a.channels 2
    slaves.b.pcm "hw:1,0"
    slaves.b.channels 2

    bindings.0.slave a
    bindings.0.channel 0
    bindings.1.slave a
    bindings.1.channel 1
    bindings.2.slave b
    bindings.2.channel 0
    bindings.3.slave b
    bindings.3.channel 1
}

pcm.surround_out {
    type route
    slave.pcm "multichannel"
    ttable.0.0 1
    ttable.1.1 1
    ttable.2.2 1
    ttable.3.3 1
}

The route plugin wrapper (pcm.surround_out) ensures proper channel mapping and format negotiation for applications like Ecasound.

Routing Ecasound to the ALSA Device

Once the ALSA multi-channel device is configured, launch Ecasound and point the output to the virtual ALSA PCM name using the -o alsa,devicename syntax.

To route a 4-channel surround file to the surround_out device defined above:

ecasound -i surround_track.wav -f:s16_le,4,48000 -o alsa,surround_out

To route discrete mono or stereo inputs into specific surround channels, configure distinct chains in Ecasound and assign them to the aggregate device:

ecasound \
  -a:front -i front_stereo.wav -f:s16_le,2,48000 \
  -a:rear  -i rear_stereo.wav  -f:s16_le,2,48000 \
  -a:front,rear -f:s16_le,4,48000 -o alsa,surround_out \
  -a:front -chorder:1,2,0,0 \
  -a:rear  -chorder:0,0,1,2

In this command:

• -f:s16_le,4,48000 sets the global stream format to 16-bit, 4 channels, at 48 kHz.
• -a:front,rear groups both processing chains to a single 4-channel ALSA endpoint.
• -chorder sets the channel assignment: the front chain occupies channels 1 and 2, while the rear chain routes to channels 3 and 4.

Important Considerations

When binding separate physical sound cards together using ALSA pcm.multi, both cards should ideally share a common hardware word clock. If separate consumer sound cards run on independent internal clocks, sample drift will occur over time, eventually causing buffer under-runs or synchronization loss. For multi-channel outputs spanning a single multi-channel audio card or synchronized professional interfaces, playback remains stable indefinitely.