1. Article purpose
This article explains how to play audio through the ALSA audio framework in Linux® OS context. The examples below, show how to play audio on the different audio hardware interfaces of the STM32MPU boards.
2. Audio playback overview
The ALSA framework exposes audio devices associated to the board audio hardware interfaces.
The application audio streams are routed by default through the Pipewire[1] sound server. Pipewire exposes audio profiles, which are mapped on the ALSA sound card audio devices. The Pipewire server provides a command line interface to list audio profiles and to select one, in order to record from a specific audio interface.
 Information
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| In this article, the PulseAudio 'pactl' command is used in the examples. This is possible because the PipeWire sound server maintains backward compatibility with the former PulseAudio sound server through the pipewire-pulse package. |
The audio playback examples in following sections are given for ALSA utilities. However, the audio playbacks can be launched by other applications, such as gstreamer multimedia framework.
3. Examples
3.1. Audio playback on headset output
3.1.1. Playback on ALSA device
Run audio playback on 'playback_codec' ALSA device:
| Information
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| 'playback_codec' is an alias defined in /etc/asound.conf, for headset output device. |
aplay -D playback_codec /usr/share/sounds/alsa/Front_Left.wav
3.1.2. Playback via Pipewire
- Configure Pipewire
Change the Pulseaudio active profile of the sound card, to 'analog-stereo' profile:
| Information
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| The Pulseaudio analog-stereo profile is the default profile. So next command is not required after boot. |
| Information
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| Example below is given for 'alsa_card.platform-sound' soundcard. Check available sound cards and associated output profiles with "pactl list cards" command. |
pactl set-card-profile alsa_card.platform-sound output:analog-stereo
- Run audio playback:
| Information
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| The Pipewire device is the default one, so "-D pipewire" option can be omitted in the playback command. |
aplay /usr/share/sounds/alsa/Front_Left.wav
3.2. Audio playback on HDMI output
3.2.1. Playback on ALSA device
Run audio playback on 'playback_hdmi' ALSA device:
| Information
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| 'playback_hdmi' is an alias defined in /etc/asound.conf, for hdmi output device. |
aplay -D playback_hdmi /usr/share/sounds/alsa/Front_Left.wav
3.2.2. Playback via Pipewire
- Configure Pipewire
Change the Pulseaudio active profile of the sound card, to 'hdmi-stereo' profile:
| Information
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| Example below is given for 'alsa_card.platform-sound' soundcard. Check available sound cards and associated output profiles with "pactl list cards" command. |
pactl set-card-profile alsa_card.platform-sound output:hdmi-stereo
- Run audio playback:
aplay /usr/share/sounds/alsa/Front_Left.wav
3.3. Audio playback on S/PDIF output
| Information
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| S/PDIF ALSA device supports only 32-bit audio streams. As 32-bit wave files are not provided in sample sound files, speaker-test is used instead of aplay, for convenience, here. Speaker-test generates a 440Hz sine wave in following examples. |
3.3.1. Set IEC958 status bits
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| Example below is given for sound card index 0. Check sound card index with "aplay -l" command. |
Example: set IEC958 mode bit.
iecset -c 0 -n device=0 pro on
3.3.2. Playback on ALSA device
Run audio playback on 'playback_spdif' ALSA device:
| Information
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| 'playback_spdif' is an alias defined in /etc/asound.conf, for S/PDIF output device. |
speaker-test -D playback_spdif --channels 2 --format S32_LE -f 440 -t sine -l 1
3.3.3. Playback via Pipewire
- Configure Pipewire
Change the sound card active profile, to S/PDIF 'iec958-stereo' profile.
| Information
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| Example below is given for 'alsa_card.platform-sound' soundcard. Check available sound cards and associated output profiles with "pactl list cards" command. |
pactl set-card-profile alsa_card.platform-sound output:iec958-stereo
- Run audio playback:
speaker-test --channels 2 --format S32_LE -f 440 -t sine -l 1