The Advanced Linux Sound Architecture (ALSA) subsystem provides audio and MIDI capabilities to Linux systems, including a user space library to simplify application programming (alsa-lib) and support for the older Open Sound System (OSS) architecture through legacy compatibility modes. Specifically for system-on-chips, the architecture defines an ALSA system-on-chip (ASoC) layer which provides optimized support for embedded devices.

The ASoC layer divides an embedded audio system into:

  • Machine driver, with machine-specific code

  • Platform driver, with platform-specific audio interface support such as I2S, AC97, or PCM

  • Codec driver, with platform-independent code

The ASoC layer is described in detail at the Linux kernel documentation.

Audio interfaces

The ConnectCore 6 SBC supports the following audio interfaces:

  • HDMI audio

  • SGTL5000 audio

Audio usage from user space

Listing the available audio devices

To list the available audio devices on the system:

~# aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: sgtl5000audio [sgtl5000-audio], device 0: HiFi sgtl5000-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: imxhdmisoc [imx-hdmi-soc], device 0: i.MX HDMI Audio Tx hdmi-hifi-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Selecting the default audio device

Framebuffer-based systems

To change the default audio device on the system, modify (or create if it does not exist) the file /etc/asound.conf indicating the sound card’s index or name. For instance, to set HDMI as default audio output:

/etc/asound.conf
1 defaults.pcm.card imxhdmisoc
2   defaults.ctl.card imxhdmisoc

X11-based systems

In this case, pulseaudio manages the audio sinks. You can list the available audio sinks with the following command:

# pactl list short sinks
0    alsa_output.platform-sound-hdmi.19.analog-stereo   module-alsa-card.c    s16le 2ch 44100Hz    SUSPENDED
1    alsa_output.platform-sound.18.analog-stereo        module-alsa-card.c    s16le 2ch 44100Hz    SUSPENDED

To change the default sink, you also use the pactl command using the sink index number or the sink name:

# pactl set-default-sink 1
# pactl set-default-sink alsa_output.platform-sound.18.analog-stereo

To make this configuration permanent across reboots, edit the file /etc/pulse/default.pa and add the following:

/etc/pulse/default.pa
1  set-default-sink alsa_output.platform-sound.18.analog-stereo

Playback and recording

You can access the sound driver using the ALSA API. The ALSA utilities package alsa-utils also offers user space applications:

  • aplay: for playback

  • arecord: for recording

  • alsactl: for configuration

  • amixer: for specific control setup

Several predefined configuration files are stored at /var/lib/alsa/:

  • asound.inline_play.state: for recording from LINE-IN and playback

  • asound.inline.state: for recording from LINE-IN only (no playback)

  • asound.micro_play.state: for recording from MIC and playback

  • asound.micro.state: for recording from MIC only (no playback)

  • asound.play.state: for playback only

To enable a configuration profile, run a command like this:

~# alsactl restore -f /var/lib/alsa/asound.inline.state