Sitara SDK Linux Audio ALSA User Space

Sitara Linux Software Developer's Guide

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Audio: ALSA User Space

Introduction

ALSA provides a number of useful command line utilities that can be used to gather information about sound cards, playback and record audio, and configure sound cards. It is the foundation upon which audio frameworks, such as GStreamer and PulseAudio, are built. In other words, GStreamer and PulseAudio are actually using the ALSA API to communicate with audio hardware.

Furthermore, ALSA exposes the functionality of the kernel drivers to userspace via an API, which can be utilized to develop custom audio applications.

Asoundrc

Asoundrc (or asound.conf) is a configuration file that is not technically needed for audio to work in linux, but it is a powerful tool to tailor to the audio needs of a specific system. Routing and sample-rate conversion are examples of properties that can be configured through Asoundrc.

For more information, please visit the official page:

• Asoundrc on AlsaProject

Command Line Utilities

This section lists a number of useful command line utilities for ALSA. A further demonstration of their uses can be found on this wiki page.

aplay

aplay can be used to playback audio. Furthermore, it can provide the user with some basic information about playback devices detected by the system.

Please consult the aplay manual to learn more about this tool: aplay man page

Example usage (play back of a wave file):

aplay audio.wav 

arecord

arecord can be used to record audio. Furthermore, it can provide the user with some basic information about recording devices detected by the system.

Please consult the arecord manual to learn more about this tool: arecord man page

Example usage (recording a CD quality wave file for 10 seconds):

arecord -d 10 -f cd -t wav audio.wav 

Device Interface

The operational interface in /dev/ contains three main types of devices: 

• PCM devices for recording or playing digitized sound samples,
• CTL devices that allow manipulating the internal mixer and routing of the card, and,
• MIDI devices to control the MIDI port of the card, if any.

Device Interface

Name	Description
/dev/snd/controlC0	Control devices (i.e. mixer, etc)
/dev/snd/pcmC0D0c	PCM Card 0 Device 0 Capture device
/dev/snd/pcmC0D0p	PCM Card 0 Device 0 Playback device

Proc Interface

The /proc/asound kernel interface is a status and configuration interface. A lot of useful information about the sound system can be found in the /proc/asound subdirectory.

See the table below for different proc entries in /proc/asound:

Proc Interface

Name	Description
cards	List of registered cards
version	Version and date the driver was built on
devices	List of registered ALSA devices
pcm	The list of allocated PCM streams
cardX/ (X = 0-7)	The card specific directory
cardX/pcm0p	The directory of the given PCM playback stream
cardX/pcm0c	The directory of the given PCM capture stream

ALSA API

Any application that requires audio in linux will call the ALSA API in order to interact with the sound drivers. The ALSA API is standardized and can be used in any linux system, including Sitara devices.

For a complete overview of the ALSA API, refer to the official documentation: ALSA C Library Reference

Commonly Used APIs

Some of the commonly used APIs to write an ALSA based application are:

Commonly Used APIs

Name	Description
snd_pcm_open	Opens a PCM stream
snd_pcm_close	Closes a previously opened PCM stream
snd_pcm_hw_params_any	Fill params with a full configuration space for a PCM
snd_pcm_hw_params_test_ <<parameter>>	Test the availability of important parameters like number of channels, sample rate etc. For e.g. snd_pcm_hw_params_test_format, snd_pcm_hw_params_test_rate etc.
snd_pcm_hw_params_set_ <<parameter>>	Set the different configuration parameters. For e.g. snd_pcm_hw_params_set_format, snd_pcm_hw_params_set_rate etc.
snd_pcm_hw_params	Install one PCM hardware configuration chosen from a configuration space
snd_pcm_writei	Write interleaved frames to a PCM
snd_pcm_readi	Read interleaved frames from a PCM
snd_pcm_prepare	Prepare PCM for use
snd_pcm_drop	Stop a PCM dropping pending frames
snd_pcm_drain	Stop a PCM preserving pending frames

User Space Interactions

This section depicts the sequence of operations for a simple playback and capture application.

Playback:

Record:

Archived

Sitara Linux SDK 08.00.00.00