How to record audio

Applicable for STM32MP13x lines, STM32MP15x lines

1 Article purpose[edit]

This article explains how to record audio via the kernel ALSA audio framework in Linux® OS context. The examples below, show how to record audio from the different audio hardware interfaces of the STM32MPU boards.

2 Audio record overview[edit]

The ALSA framework exposes audio devices associated to the board audio hardware interfaces.

The application audio streams are routed by default through the PulseAudio sound server. PulseAudio exposes audio profiles, which are mapped on the ALSA sound card audio devices. The PulseAudio server provides a command line interface to list audio profiles and to select one, in order to record from a specific audio interface.

Warning white.png Warning
The applications interacting with Weston framework have to be launched as "Weston" user instead of "root" user. The Pulseaudio commands can, for instance, be launched as follows:
su -l "weston" -c "pactl info"

The audio record examples in following sections are based on ALSA utilities. Some input paths are configured through ALSA controls. These configurations are detailed in sound card configuration article. If an error is issued when running an example, please refer to Audio troubleshooting grid article for debug.

3 Examples[edit]

3.1 Audio record from headset microphone input[edit]

3.1.1 Record from ALSA device[edit]

Start audio record from 'record_codec' ALSA device:

Info white.png Information
'record_codec' is an alias defined in /etc/asound.conf, for headset microphone input device.
 arecord -D record_codec -f S16_LE -d 10 /tmp/rec.wav

3.1.2 Record via PulseAudio[edit]

  • Configure Pulseaudio :

Change Pulseaudio default input source to 'analog_input':

Info white.png Information
The Pulseaudio analog_input is defined in /etc/pulse/system.pa configuration file.
 pactl set-default-source analog_input
  • Start audio record :
Info white.png Information
The Pulseaudio device is the default one, so "-D pulse" option can be omitted in the record command.
 arecord -d 10 /tmp/rec.wav

3.2 Audio record from digital microphone input[edit]

Warning white.png Warning
The support of digital microphone input is board dependent. Please, check available inputs with "arecord -l" command.

3.2.1 Record from ALSA device[edit]

Start mono audio record from 'record_dfsdm0' ALSA device:

Info white.png Information
'record_dfsdm0' is an alias defined in /etc/asound.conf, for digital microphone U1 input device.
 arecord -D record_dfsdm0 -r 16000 -f S32_LE -c 1 -d 10 /tmp/rec.wav

3.2.2 Record multiple digital microphones from a virtual ALSA device[edit]

Start stereo audio record from 'multi' ALSA device:

Info white.png Information
'multi' device, based on ALSA multi plugin[1], has to be defined in /etc/asound.conf file.
 arecord -D multi -r 16000 -f S32_LE -c 2 -d 10 /tmp/rec.wav

3.2.3 Record via PulseAudio[edit]

  • Configure Pulseaudio

Change Pulseaudio default input source to 'dmic1_input':

Info white.png Information
The Pulseaudio dmic1_input is defined in /etc/pulse/system.pa configuration file.
 pactl set-default-source dmic1_input
  • Start audio record :
Info white.png Information
The Pulseaudio device is the default one, so "-D pulse" option can be omitted in the record command.
 arecord -r 48000 -f S32_LE -c 2 -d 10 /tmp/rec.wav

3.3 Audio record from S/PDIF input[edit]

Warning white.png Warning
The support of S/PDIF input is board dependent. Please, check available inputs with "arecord -l" command.

3.3.1 Get IEC958 status bits[edit]

Some restrictions may apply to IEC958 control, depending on the SPDIFRX device configuration as it is explained in SPDIFRX device tree configuration and SPDIFRX Linux driver articles.

 amixer -c STM32MP1EV cget iface=PCM,name='IEC958 Capture Default'

3.3.2 Record from ALSA device[edit]

Start audio record from 'record_spdif' ALSA device:

Info white.png Information
'record_spdif' is an alias defined in /etc/asound.conf, for S/PDIF input device.
Info white.png Information
A S/PDIF signal must be available on S/PDIF RCA input connector before starting the record command. The record rate must be set according to S/PDIF signal sampling rate.
 arecord -D record_spdif -f S32_LE -c 2 -r 48000 -d 10 /tmp/rec.wav

3.3.3 Record via PulseAudio[edit]

  • Configure Pulseaudio

Change Pulseaudio default input source to 'iec958_input':

Info white.png Information
The Pulseaudio iec958_input is defined in /etc/pulse/system.pa configuration file.
 pactl set-default-source iec958_input
  • Start audio record :
Info white.png Information
The Pulseaudio device is the default one, so "-D pulse" option can be omitted in the record command.
 arecord -r 48000 -f S32_LE -c 2 -d 10 /tmp/rec.wav

4 References[edit]