Audio function

2025-11-26

This document provides a concise guide to operating the Linux audio system. We utilize a dual-architecture audio backend of ALSA and PulseAudio, offering a complete audio solution from the bottom layer to the application layer. On the hardware side, it fully supports external microphone input and external speaker amplification output, and integrates a standard 3.5mm audio interface, enabling full-duplex communication for headphone playback and microphone input. On the software side, the following audio configuration commands are provided to ensure perfect initial audio output. Through this document, you will learn how to use basic command-line tools to quickly complete daily operations such as audio device checks, recording and playback, and volume control, resolving most audio-related problems without delving into complex configurations.

Quick Start

This section will explain how to connect external speakers and headphones, and then check the system configuration using commands to ensure proper collaboration between the hardware and software layers.

Speaker and headphone hardware interfaces

View sound card information

Enter the following command in the terminal to check the sound card's mounting status:

root@qcm6490-idp:/opt# cat /proc/asound/cards
 0 [qcm6490idpsndca]: qcm6490 - qcm6490-idp-snd-card
                      qcm6490-idp-snd-card

Enter the following command in the terminal to view the list of allocated PCM streams:

root@qcm6490-idp:/opt# cat /proc/asound/pcm
00-00: CODEC_DMA-LPAIF_RXTX-RX-0 multicodec-0 :  : playback 1
00-01: CODEC_DMA-LPAIF_RXTX-TX-3 multicodec-1 :  : capture 1
00-02: CODEC_DMA-LPAIF_VA-TX-0 va_macro_tx1-2 :  : capture 1

Without sound card information and PCM stream lists, both ALSA and PulseAudio functions will be unavailable.

Function usage

VLC Applications

Play

Locate the VLC media player application in the application interface and double-click to open it.

Then click the media option, select the Open File option, and double-click the audio file you want to play in the opened folder to complete the audio playback.

Play in command format

root@qcm6490-idp:/opt# cvlc /path/to/your/audio.mp3

Replace /path/to/your/audio.mp3 with the path to your audio file.

ALSA Architecture

ALSAOverview

ALSA is the underlying audio architecture and driver core of the Linux system, directly managing and controlling the computer's audio hardware. On one hand, it provides native drivers for sound cards, enabling direct reading and writing of physical audio channels (such as external microphones, headphones, and speakers); on the other hand, it provides standardized API interfaces for applications to complete the most basic audio capture and playback. ALSA is the cornerstone for building all advanced audio services (such as PulseAudio).

Audio recording

1.External microphone audio recording

root@qcm6490-idp:/opt# systemctl stop pulseaudio
root@qcm6490-idp:/opt# tinymix set "VA DMIC MUX0" "DMIC0"
root@qcm6490-idp:/opt# tinymix set "VA_AIF1_CAP Mixer DEC0" "1"
root@qcm6490-idp:/opt# tinymix set "VA_DEC0 Volume" "100"
root@qcm6490-idp:/opt# agmcap test1.wav -D 100 -d 101 -c 1 -r 48000 -b 16 -i "CODEC_DMA-LPAIF_VA-TX-0"
pcm_plug_open: dlopen successful for libagm_pcm_plugin.so
Capturing sample: 1 ch, 48000 hz, 16 bit
diag: Diag_LSM_Init: invoked for pid: 1599 with init_count: 0
diag:successfully connected to socket 17
diag: Diag_LSM_Init: done for pid: 1599 with init_count: 1
diag: Successfully registered commands with the driver

PulseAudio may occupy the audio device, causing direct hardware access (such as agmcap) to fail. Stopping it ensures exclusive access.
-c 1 Mono recording
-r 48000 Sampling rate 48 kHz
-b 16 Sampling depth 16-bit
-i "CODEC_DMA-LPAIF_VA-TX-0" specifies the input device interface as the transmit (recording) channel.
To stop recording, press Ctrl+C, which will generate an audio file named test1.wav in the current directory.

root@qcm6490-idp:/opt# ls
cni  containerd  test1.wav

2.Headphone recording

To record using a 3.5mm headphone, please enter the following command.

root@qcm6490-idp:/opt# systemctl stop pulseaudio
root@qcm6490-idp:/opt# tinymix set "TX DEC0 MUX" "SWR_MIC" 
root@qcm6490-idp:/opt# tinymix set "TX SMIC MUX0" "ADC1" 
root@qcm6490-idp:/opt# tinymix set "TX_AIF1_CAP Mixer DEC0" "1" 
root@qcm6490-idp:/opt# tinymix set "ADC2 MUX" "INP2" 
root@qcm6490-idp:/opt# tinymix set "ADC2 Switch" "1" 
root@qcm6490-idp:/opt# tinymix set "ADC2_MIXER Switch" "1" 
root@qcm6490-idp:/opt# tinymix set "TX_DEC0 Volume" "80" 
root@qcm6490-idp:/opt# tinymix set "ADC2 Volume" "20" 
root@qcm6490-idp:/opt# agmcap test2.wav -D 100 -d 101 -c 1 -r 48000 -b 16 -i "CODEC_DMA-LPAIF_RXTX-TX-3"
pcm_plug_open: dlopen successful for libagm_pcm_plugin.so
Capturing sample: 1 ch, 48000 hz, 16 bit
diag: Diag_LSM_Init: invoked for pid: 1384 with init_count: 0
diag:successfully connected to socket 17
diag: Diag_LSM_Init: done for pid: 1384 with init_count: 1
diag: Successfully registered commands with the driver

To stop recording, press Ctrl+C, which will generate an audio file named test2.wav in the current directory.

root@qcm6490-idp:/opt# ls
cni  containerd  test2.wav

Audio playback

1.Headphone playback

To play using 3.5mm headphones, please enter the following command.

root@qcm6490-idp:/opt# systemctl stop pulseaudio
root@qcm6490-idp:/opt# tinymix set "RX_MACRO RX0 MUX" "AIF1_PB" 
root@qcm6490-idp:/opt# tinymix set "RX_MACRO RX1 MUX" "AIF1_PB" 
root@qcm6490-idp:/opt# tinymix set "RX INT0_1 MIX1 INP0" "RX0" 
root@qcm6490-idp:/opt# tinymix set "RX INT1_1 MIX1 INP0" "RX1" 
root@qcm6490-idp:/opt# tinymix set "RX INT0 DEM MUX" "CLSH_DSM_OUT" 
root@qcm6490-idp:/opt# tinymix set "RX INT1 DEM MUX" "CLSH_DSM_OUT" 
root@qcm6490-idp:/opt# tinymix set "RX_COMP1 Switch" "1" 
root@qcm6490-idp:/opt# tinymix set "RX_COMP2 Switch" "1"
root@qcm6490-idp:/opt# tinymix set "HPHL Switch" "1" 
root@qcm6490-idp:/opt# tinymix set "HPHR Switch" "1" 
root@qcm6490-idp:/opt# tinymix set "HPHL_RDAC Switch" "1" 
root@qcm6490-idp:/opt# tinymix set "HPHR_RDAC Switch" "1" 
root@qcm6490-idp:/opt# agmplay test.wav -D 100 -d 100 -i "CODEC_DMA-LPAIF_RXTX-RX-0"

The audio will stop automatically after playback ends. To stop it earlier, press Ctrl+C.

2.Speaker playback

Enter the following command to complete audio playback

root@qcm6490-idp:/opt# systemctl stop pulseaudio
root@qcm6490-idp:/opt# tinymix set "AUX_RDAC Switch" "1" 
root@qcm6490-idp:/opt# tinymix set "RX_MACRO RX0 MUX" "AIF1_PB" 
root@qcm6490-idp:/opt# tinymix set "RX INT2_1 MIX1 INP0" "RX0" 
root@qcm6490-idp:/opt# tinymix set "LO Switch" "1" 
root@qcm6490-idp:/opt# gmplay test_loop.wav -D 100 -d 100 -i "CODEC_DMA-LPAIF_RXTX-RX-0"

PulseAudio Architecture

PulseAudio Overview

PulseAudio is a cross-platform, network-enabled audio service that accepts audio input from one or more audio sources (processes or input devices) and then redirects the audio to one or more slots (sound card, remote network PulseAudio service, or other processes). It interacts with the lowest-level ALSA (Advanced Linux Sound Architecture) and provides a unified interface for applications, enabling advanced features such as multi-channel audio mixing and audio forwarding.

Equipment Viewing and Management

1.View available output devices (Sinks)

To view the currently available audio output devices on your system and their details, you can use the following command:
This command lists the index and name of all available sinks (output devices). Example output:

root@qcm6490-idp:/# pactl list short sinks
0       low-latency0    module-pal-card.c       s16le 1ch 16000Hz       SUSPENDED
1       deep-buffer0    module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
2       offload0        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
3       voip-rx0        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED

The asterisk (*) indicates the current default output device.
For more detailed device information, please use:

root@qcm6490-idp:/# pactl list sinks

2.Viewing Input Devices (Sources)
Similarly, you can view audio input devices (such as microphones):

root@qcm6490-idp:/# pactl list short sources
0       low-latency0.monitor    module-pal-card.c       s16le 1ch 16000Hz       SUSPENDED
1       deep-buffer0.monitor    module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
2       offload0.monitor        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
3       voip-rx0.monitor        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
4       regular0        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
5       regular2        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
6       voip-tx0        module-pal-card.c       s16le 1ch 48000Hz       SUSPENDED

For more detailed device information, please use:

root@qcm6490-idp:/# pactl list sources

recording

1.Onboard DIMC recording

root@qcm6490-idp:~# pactl set-source-port 5 speaker-mic
root@qcm6490-idp:~# parec -d 5 --file-format=wav output.wav

2.Headphone recording

root@qcm6490-idp:~# pactl set-source-port 5 headset-mic
root@qcm6490-idp:~# parec -d 5 --file-format=wav output1.wav

Play

1.Speaker playback

root@qcm6490-idp:~# pactl set-sink-port 0 speaker
root@qcm6490-idp:~# paplay output.wav

2.Headphone playback

root@qcm6490-idp:~# pactl set-sink-port 0 headset
root@qcm6490-idp:~# paplay output1.wav

Advanced features

Sink Explanation

root@qcm6490-idp:~#  pactl list sinks short
0       low-latency0    module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
1       deep-buffer0    module-pal-card.c       s16le 2ch 32000Hz       SUSPENDED
2       offload0        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
3       voip-rx0        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
root@qcm6490-idp:~#

1.low-latency0

low-latency0 is used for real-time audio playback, such as music, games, notifications, etc., requiring ultra-low latency.
Features:
Extremely low latency, suitable for applications with high real-time requirements.
Supports PCM format (uncompressed audio)
It supports both speakers and headphones.

2.deep-buffer0

deep-buffer0 is used for audio playback that requires a large buffer, such as streaming media and internet radio, allowing for a larger audio buffer and reducing stuttering.
Features:
Higher latency, but more stable audio, suitable for environments with unstable networks.
It supports both speakers and headphones.

3.offload0

Offload0 is used to offload compressed audio (such as MP3 and AAC) to the hardware, allowing it to be decoded and played directly by the hardware, thus saving CPU resources.
Features:
It only supports compressed formats (such as MPEG and AAC), and does not support PCM.
Suitable for playing compressed audio for extended periods (such as in music players).

4.voip-rx0

The voip-rx0 is specifically designed for audio reception in VoIP calls (such as internet telephony and video conferencing).
Features:
Optimized voice call latency and echo suppression.
It is automatically activated only in VoIP applications (such as Skype and Zoom), and will not be used for regular audio playback.

sourceExplanation

root@qcm6490-idp:~# pactl list sources short
0       low-latency0.monitor    module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
1       deep-buffer0.monitor    module-pal-card.c       s16le 2ch 32000Hz       SUSPENDED
2       offload0.monitor        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
3       voip-rx0.monitor        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
4       regular0        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
5       regular2        module-pal-card.c       s16le 2ch 48000Hz       SUSPENDED
6       voip-tx0        module-pal-card.c       s16le 1ch 48000Hz       SUSPENDED
root@qcm6490-idp:~#

1.low-latency0.monitor

low-latency0.monitor is suitable for real-time recording, such as recording game sound effects, music player output, etc.
Features:
Extremely low latency, suitable for real-time monitoring of audio output.

2.deep-buffer0.monitor

deep-buffer0.monitor is suitable for long-duration recordings, such as streaming media and internet radio.
Features:
It has a relatively high latency, but the audio is stable, making it suitable for environments with unstable networks.

3.offload0.monitor

offload0.monitor is used to record the audio output of offload0 Sink, that is, to record the compressed audio (such as MP3, AAC) of the hardware unloading.
Features:
Activated only when playing compressed audio.
Suitable for recording compressed audio over long periods of time.

4.voip-rx0.monitor

voip-rx0.monitor is used to record the voice of the other party in a VoIP call or video conference.
Features:
Suitable for recording the other party's voice during VoIP calls or video conferences.

5.regular0和regular2

regular0 and regular2 are used for directly recording audio from a microphone or line input.
Features:
A universal audio input source suitable for most recording scenarios.

6.voip-tx0

voip-tx0 is used for audio transmission in VoIP calls, that is, recording your microphone input for VoIP calls.
Features:
The sound quality and latency of voice calls have been optimized.

Troubleshooting

The speaker cannot play properly

1.View the default audio playback sink

root@qcm6490-idp:~# pactl get-default-sink
low-latency0
root@qcm6490-idp:~#

If it is not low-latency0 or deep-buffer0, please follow these steps to set the default sink.

root@qcm6490-idp:~# pactl set-default-sink low-latency0
root@qcm6490-idp:~# pactl get-default-sink
low-latency0
root@qcm6490-idp:~#

2.Check the sink's default output port.

Use pactl list sinks to check if the Active Port value of the default output sink is "speaker". If not, set it to "speaker".

root@qcm6490-idp:~# pactl set-sink-port 0 speaker
root@qcm6490-idp:~#

If the above steps are configured correctly, the problem of the speaker not playing music will be resolved.

Resources and Support

Detailed introduction to PulseAudio
linux-PulseAudio
Pulseaudio explanation

Ethernet

USB OTG

Quectel Pi H1

Audio function

Quick Start

Speaker and headphone hardware interfaces

View sound card information

Function usage

VLC Applications

Play

Play in command format

ALSA Architecture

ALSAOverview

Audio recording

Audio playback

PulseAudio Architecture

PulseAudio Overview

Equipment Viewing and Management

recording

Play

Advanced features

Sink Explanation

sourceExplanation

Troubleshooting

The speaker cannot play properly

Resources and Support