add audio support in docker container for gazebo simulation

[jacknlliu]

audio and contact force are very import signals for physical interaction for human.

Human process the audio signal faster than image.

ALSA lib can't work in container even though PulseAudio(server) has been installed in the host.

AL lib: (WW) alc_initconfig: Failed to initialize backend "pulse"
ALSA lib confmisc.c:768:(parse_card) cannot find card '0'
ALSA lib conf.c:4292:(_snd_config_evaluate) function snd_func_card_driver returned error: No such file or directory
ALSA lib confmisc.c:392:(snd_func_concat) error evaluating strings
ALSA lib conf.c:4292:(_snd_config_evaluate) function snd_func_concat returned error: No such file or directory
ALSA lib confmisc.c:1251:(snd_func_refer) error evaluating name
ALSA lib conf.c:4292:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:4771:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2266:(snd_pcm_open_noupdate) Unknown PCM default
AL lib: (EE) ALCplaybackAlsa_open: Could not open playback device 'default': No such file or directory

[Err] [OpenAL.cc:84] Unable to open audio device[default]
 Audio will be disabled.

Solution:

docker run --device /dev/snd \
	          -e PULSE_SERVER=unix:${XDG_RUNTIME_DIR}/pulse/native \
	          -v ${XDG_RUNTIME_DIR}/pulse/native:${XDG_RUNTIME_DIR}/pulse/native \
	          --group-add $(getent group audio | cut -d: -f3) 

In container, we also need the following command to make sure the access to /dev/snd/*, if your user in container is not created by docker run option --user or USER in Dockerfile

sudo usermod -aG audio <your_user>

# or directly change the permission of /dev/snd/*
# sudo chmod a+rwx /dev/snd/*
# this will take effect directly, without restarting your container.

And then restart your container.

Reference

• No sound when running Chrome in Docker container on Ubuntu 14.04 #85
• audio in docker container
• How to play sound in a docker container
• run apps using audio in a docker container
• No sound on Arch Linux (ALSA)
• Difference between docker run --user and --group-add parameters

[jacknlliu]

Pulseaudio vs. ALSA

Pulseaudio is a sound server which allows multiple sound sources to be fed through to one or more sinks. AFAIK Alsa only works with one source at a time.... Actually I think that is wrong and Alsa does support multiple concurrent sources.

Found this diagram showing what it does:

PulseAudio basically sits atop ALSA, and use it internally. ALSA is unable by itself to be used by multiple applications, so PulseAudio provides this functionality among others.

Reference

• Pulseaudio vs ALSA
• What is the relation between ALSA and PulseAudio sound architecture?