Linux, Audio stack overview (WTF !?)

Even if the linux audio stack is not as complicated as the graphics stack, I still had a hard time to know how to tweak my config from just reading the official documentation. I will try to cover different views of the system.

• The stack from application to hardware
• The configuration mechanism
• The logical device hierarchy
• The common intefaces ALSA presents to an application

The stack from application to hardware

Mismatch between configuration and logical pcm views

An example of configuration

  pcm.slave0 {
    type slave
    pcm "hw:2,0"
  }

  pcm_slave.slave1 {
    pcm "hw:2,0"
  }

  pcm.alsa_test {
    type plug
    #slave slave0       # KO pcm.slave0 {type slave}
    #slave slave1       # OK pcm_slave.slave1
    #slave.pcm "hw:2,0"
    #slave.pcm "dmix:2,0"

    #slave.pcm "pcm.default"        # not the same as cards.pcm.default
    #slave.pcm "cards.pcm.default"  # works but has never been defined

    #slave.pcm "cards.USB-Audio.pcm.iec958.0:2,4,130,0,2"
    #slave.pcm "cards.USB-Audio.pcm.front.0:2"
    #slave.pcm "cards.USB-Audio.pcm.default:2"

    #slave.pcm "pcm.iec958:2,0"
    #slave.pcm "cards.pcm.iec958:2,0"
  }

Interfaces

• /proc/asound lists information about the hardware
• /dev/snd the pcm,ctl and other hardware devices
• /var/lib/alsa/asound.state status of hardware controls
• aplay to list alsa devices and play audio to a specific device

Unresolved questions

• The configuration seems to have some undocumented mechanisms
  • How do we know that foo.bar.pcm.thingy is a pcm interface ?
  • Is there something special about this identifier pcm_pluginType
  • The cards node has special behaviour, it somehow points to an existing pcm, and it is used for autoloading via cards.@hooks
• The link between configuration and hardware is not clear
  • Alsa package contains configuration for each card or card family but somehow there is automatic discovery to select the features in the config that are actually supported

Sources

• The different layers of the stack
• A helpful overview
• And of course the always helpful arch wiki