Audio

This chapter describes the common information and instructions of Audio on IoT Yocto, such as listing the audio hardware, using a USB audio device, and so on. Audio on different platforms may have some platform-specific instructions or test results. For example, you will have different audio interfaces on different platforms. Please refer to the platform-specific section for more details.

ALSA

Audio System Overview

../../../../_images/sw_rity_app-dev_audio_evk_alsa-overview.svg

ALSA framework on Yocto Linux

  • ALSA Application

    ALSA applications are audio software based on the Linux system, utilizing ALSA-provided drivers and libraries for sound recording, playback, and management. For detailed information, please refer to ALSA Applications.

  • ALSA Library

    The ALSA library API is the interface to the ALSA drivers. For detailed information, please refer to ALSA Library API.

  • ALSA Framework Layer

    This layer comprises the PCM interface, MIDI interface, and Control interface. Please refer to the links below for detailed information.

  • ASoC Framework Layer

    ASoC layer enhances audio support for embedded systems by enabling codec independence for driver reuse, simplifying I2S/PCM setup between codecs and SoCs, and providing DAPM for optimal power efficiency. For detailed information, please refer to ASoC Layer.

Hardware Driver

For more ALSA information, please refer to ALSA project and ALSA unofficial wiki.

ALSA Mixer Control

ALSA mixer control is a component of the ALSA sound system in Linux that provides an interface for adjusting sound settings on a software level. It allows users and applications to control the audio hardware’s mixer settings, such as volume levels, mute states, and other audio parameters that the sound card’s physical or software mixer supports.

Below are the tools for interacting with ALSA mixer controls.

  • alsamixer

    A text-based user interface tool that provides a visual representation of mixer controls. Users can navigate through different controls and adjust them using keyboard commands.

  • amixer

    A command-line tool that allows users to modify mixer settings using command-line arguments. It is useful for scripting and automation of audio settings.

  • alsactl

    A system utility for managing sound card settings. It can be used to save and restore mixer settings across system reboots.

ALSA mixer controls are usually linked to specific components of the audio hardware. For instance, a sound card may feature distinct controls for headphone output and speaker output. Each control may contain one or more elements representing adjustable parameters, like volume sliders or mute/unmute switches. Each control element has a unique identifier (ID) that can be specified in the mixer command.

In the ALSA system, the mixer control names typically follow a naming convention that divides a control name into three main components: source, flow, and function. This naming convention assists in clarifying the function and operation of sound card mixer controls, making it easier for users to understand and manipulate them. However, this is not mandatory. The sound card manufacturers and driver developers might have their own styles.

Mixer controls naming rules

Component

Description

Common Example

Source

Describes the origin of the audio signal. It usually identifies where the sound is coming from.

Master: The main volume control.

PCM: Digital audio signal, such as sound transmitted through the audio chip’s PCM interface.

Line: Analog signal source, like a line input.

Flow

Defines the direction of the audio signal flow related to the control.

Playback: Also known as output, it’s the path of audio flowing to speakers or headphones.

Capture: Also known as input, it’s the path of audio coming from microphones or other input sources being recorded.

Bypass: Typically refers to directing the input audio straight to the output without any processing.

Function

Describes the control’s action or purpose.

Switch: A toggle control used to mute or enable the corresponding audio path.

Volume: A volume control that increase or decrease the amplitude of the audio signal.

Mux: Multiplexer, often used to select one active input source among multiple audio sources.

Here are the examples on Genio 700-EVK:

  • Headset Volume, Lineout Volume: control the audio device’s volume

  • HDMI_OUT_MUX, DPTX_OUT_MUX: select the output device

  • O176 I070 Switch, O177 I071 Switch: configure the audio data path

The asound.state file functions as a storage for ALSA’s mixer control settings. It contains the current configurations or default values for all mixer controls on the sound card, enabling the system to restore these settings during boot or when a user wants to reload specific audio configurations.

Typically, the configuration syntax looks like this.

state.[CARD_NAME] {
    control.[CONTROL_NUM] {
        iface [INTERFACE_TYPE]
        name ['CONTROL_NAME']
        value [CONTROL_VALUE]
        comment {
            access ['ACCESS_TYPE']
            type ['VALUE_TYPE']
            count [COUNT]
            ...
        }
    }
    control.[ANOTHER_CONTROL_NUM] {
        ...
    }
    ...
}

The elements here can be interpreted as:

  • CARD_NAME: Name or identifier of the sound card.

  • CONTROL_NUM: A unique numeric ID for the control.

  • INTERFACE_TYPE: Such as MIXER or PCM, indicating the type of interface the control is on.

  • CONTROL_NAME: The name of the control.

  • CONTROL_VALUE: The actual value set for the control, which might be numeric for volume controls or on/off for switches.

  • ACCESS_TYPE: The access permissions of the control, for example, read/write.

  • VALUE_TYPE: The type of value, for example, INTEGER, BOOLEAN, ENUMERATED.

  • COUNT: Indicates how many independent values the control has; for example, a stereo control would have two values, one for left and one for right, thus the count would be 2.

The image below shows the appearance of the ‘Headset Volume’ mixer control on Genio 700-EVK in the asound.state file.

../../../../_images/sw_rity_app-dev_audio_evk_asound-state.svg

Headset Volunme mixer control

Mixer Control Usage Command

  • To view or modify the asound.state:

    cat /var/lib/alsa/asound.state

  • To store the current mixer state in the /var/lib/alsa/asound.state configuration file.

    alsactl store

  • To restore the mixer state from the /var/lib/alsa/asound.state configuration file and to apply it to the system.

    alsactl restore

Audio Interfaces

The audio system provides audio data exchange between the device and external audio components. Audio interfaces play a crucial role in enabling high-quality sound recording and playback. This section aims to offer a thorough guide to the integrated audio interface on the SoC. The dai-link name is the name used by the software driver during integration.

The following table will show the interfaces present on the SoC MT8370/MT8390.

MT8370/MT8390 Audio Interfaces

Audio Interfaces

Mode

Master/Slave

Sample Rate (kHz)

Channel Count

Maximum Bit Resolution

Dai-Link Name

I2SO1

I2S

Master

8 ~ 192

2

32

ETDM1_OUT_BE

I2SO1

TDM

Master

8 ~ 48

16

32

ETDM1_OUT_BE

I2SO2

I2S

Master/Slave

8 ~ 192

8

32

ETDM2_OUT_BE

I2SO2

TDM

Master/Slave

48

16

32

ETDM2_OUT_BE

TDM IN

I2S

Master/Slave

8 ~ 192

2

32

ETDM1_IN_BE

TDM IN

TDM

Master/Slave

48

16

32

ETDM1_IN_BE

I2S IN

I2S

Master/Slave

8 ~ 192

8

32

ETDM2_IN_BE

I2S IN

TDM

Master/Slave

8 ~ 48

16

32

ETDM2_IN_BE

I2S IN

TDM (Direct Path)

Master/Slave

8 ~ 48

16

32

ETDM2_IN_BE

PMIC DL

8 ~ 192

2

24

ADDA_BE

PMIC UL

8 ~ 192

2

24

ADDA_BE

PDM1

Master

8, 16, 32, 48

2

24

DMIC_BE

PDM2

Master

8, 16, 32, 48

2

24

DMIC_BE

PDM3

Master

8, 16, 32, 48

2

24

DMIC_BE

PDM4

Master

8, 16, 32, 48

2

24

DMIC_BE

PCM (with SRC)

Master/Slave

8, 16, 32, 44.1, 48

2

24

PCM1_BE

SPDIF OUT

Master

32, 44.1, 48, 88.2, 96, 192

2

24

SPDIF IN

Slave

32, 44.1, 48, 88.2, 96, 176.4, 192

2

24

AUDIO IN

Slave

8 ~ 192

8

24

HDMITX

Master

8 ~ 192

8

32

ETDM3_OUT_BE

DPTX

Master

8 ~ 192

8

24

DPTX_BE

The following table will show the interfaces present on the SoC MT8395.

MT8395 Audio Interfaces

Audio Interfaces

Mode

Master/Slave

Sample Rate (kHz)

Channel Count

Maximum Bit Resolution

Dai-Link Name

I2SO1

I2S

Master

8 ~ 384

2

32

ETDM1_OUT_BE

I2SO1

TDM

Master

8 ~ 48

16

32

ETDM1_OUT_BE

I2SO2

I2S

Master/Slave

8 ~ 384

8

32

ETDM2_OUT_BE

I2SO2

TDM

Master/Slave

16

16

32

ETDM2_OUT_BE

I2SO2

TDM

Master/Slave

48

16

32

ETDM2_OUT_BE

TDM IN

I2S

Master/Slave

8 ~ 384

2

32

ETDM1_IN_BE

TDM IN

TDM

Master/Slave

16

16

32

ETDM1_IN_BE

TDM IN

TDM

Master/Slave

48

16

32

ETDM1_IN_BE

I2S IN

I2S

Master/Slave

8 ~ 384

8

32

ETDM2_IN_BE

I2S IN

TDM

Master/Slave

8 ~ 48

16

32

ETDM2_IN_BE

I2S IN

TDM (Direct Path)

Master/Slave

8 ~ 48

16

32

ETDM2_IN_BE

PMIC DL

8 ~ 192

2

24

DL_SRC_BE

PMIC UL

8 ~ 192

2

24

UL_SRC1_BE/UL_SRC2_BE

PDM1

Master

8, 16, 32, 48

2

24

DMIC_BE

PDM2

Master

8, 16, 32, 48

2

24

DMIC_BE

PDM3

Master

8, 16, 32, 48

2

24

DMIC_BE

PDM4

Master

8, 16, 32, 48

2

24

DMIC_BE

PCM (with SRC)

Master/Slave

8, 16, 32, 44.1, 48

2

24

PCM1_BE

SPDIF OUT

Master

32, 44.1, 48, 88.2, 96, 192

2

24

SPDIF IN

Slave

32, 44.1, 48, 88.2, 96, 176.4, 192

2

24

AUDIO IN

Slave

8 ~ 192

8

24

HDMIRX

Slave

192

8

24

MULTI_IN1_BE

HDMITX

Master

8 ~ 192

8

32

ETDM3_OUT_BE

DPTX

Master

8 ~ 192

8

24

DPTX_BE

MT6365 is a power management system chip optimized for handsets and smart phones. This codec is used with the MT8370, MT8390, or MT8395 on the MediaTek platform. The following table will show the interfaces present on the PMIC MT6365.

MT6365 Audio Interfaces

Audio Interfaces

Master/Slave

Sample Rate (kHz)

Channel Count

Maximum Bit Resolution

ACCDET

NA

NA

NA

NA

Playback (Earphone)

Master

8, 11.025, 12, 16, 22.05, 24, 32, 44.1, 48, 96, 192

2

24

Playback (Speaker)

Master

8, 11.025, 12, 16, 22.05, 24, 32, 44.1, 48, 96, 192

1

24

Record (Earphone)

Master

8, 16, 32, 48, 96, 192

2

24

Record (AMIC)

Master

8, 16, 32, 48, 96, 192

3

24

List Sound Devices

You can use aplay and arecord to list available sound devices.

Note

The following commands will use Genio 700-EVK as an example.

aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: mt8390evk [mt8390-evk], device 0: DL2 Playback (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 1: DL3 Playback (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 2: DL6 Playback (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 3: DL7 Playback (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 4: DL8 Playback (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 5: DL10 Playback (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 6: DL11 Playback (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0

arecord -l
**** List of CAPTURE Hardware Devices ****
card 0: mt8390evk [mt8390-evk], device 7: UL1 Capture (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 8: UL2 Capture (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 9: UL3 Capture (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 10: UL4 Capture (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 11: UL5 Capture (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 12: UL6 Capture (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 13: UL8 Capture (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 14: UL9 Capture (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: mt8390evk [mt8390-evk], device 15: UL10 Capture (*) []
  Subdevices: 1/1
  Subdevice #0: subdevice #0

You can find the corresponding device nodes under /dev/snd/.

ls -l /dev/snd
total 0
drwxr-xr-x 2 root root       60 Apr 28 18:24 by-path
crw-rw---- 1 root audio 116, 18 Apr 28 18:24 controlC0
crw-rw---- 1 root audio 116,  2 Apr 28 18:24 pcmC0D0p
crw-rw---- 1 root audio 116, 12 Apr 28 18:24 pcmC0D10c
crw-rw---- 1 root audio 116, 13 Apr 28 18:24 pcmC0D11c
crw-rw---- 1 root audio 116, 14 Apr 28 18:24 pcmC0D12c
crw-rw---- 1 root audio 116, 15 Apr 28 18:24 pcmC0D13c
crw-rw---- 1 root audio 116, 16 Apr 28 18:24 pcmC0D14c
crw-rw---- 1 root audio 116, 17 Apr 28 18:24 pcmC0D15c
crw-rw---- 1 root audio 116,  3 Apr 28 18:24 pcmC0D1p
crw-rw---- 1 root audio 116,  4 Apr 28 18:24 pcmC0D2p
crw-rw---- 1 root audio 116,  5 Apr 28 18:24 pcmC0D3p
crw-rw---- 1 root audio 116,  6 Apr 28 18:24 pcmC0D4p
crw-rw---- 1 root audio 116,  7 Apr 28 18:24 pcmC0D5p
crw-rw---- 1 root audio 116,  8 Apr 28 18:24 pcmC0D6p
crw-rw---- 1 root audio 116,  9 Apr 28 18:24 pcmC0D7c
crw-rw---- 1 root audio 116, 10 Apr 28 18:24 pcmC0D8c
crw-rw---- 1 root audio 116, 11 Apr 28 18:24 pcmC0D9c
crw-rw---- 1 root audio 116, 33 Apr 28 18:24 timer

There are some default plugin devices specified in /etc/asound.conf. Please refer to Asoundrc for more details. For example, the following configuration creates an alias name, jack_speaker, which represents /dev/pcmC0D0p. Then, you can play the music with the name jack_speaker by aplay -D jack_speaker.

pcm.jack_speaker {
       type plug
       slave {
               pcm "hw:mt8390evk,0,0"
               channels 2
       }
}

For examples of playback and capture, please refer to each platform section.

USB Audio

On the IoT platform, it supports USB audio. You can simply plug in a USB audio device (e.g. USB headset) and check its ID and name before playing or recording something.

aplay -l
**** List of PLAYBACK Hardware Devices ****
card 1: H540 [Logitech USB Headset H540], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

arecord -l
**** List of CAPTURE Hardware Devices ****
card 1: H540 [Logitech USB Headset H540], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

You can see there is card 1 with the CAPTURE device and the PLAYBACK device.

To play the music:

aplay -D hw:1,0 sample.wav

To record the voice:

arecord -D hw:1,0 -r 48000 -f S16_LE -c 2 sample.wav

Loopback Test

There’s a loopback audio test that directly transfers the input voice to the output audio.

For instance, you can use it to transfer sound from the Earphone-In Jack to the Earphone-Out Jack.

arecord -D jack_mic -r 48000 -f S32_LE | aplay -D jack_speaker -r 48000 -f S32_LE