2.6.2. How Does It Work?

Fedora uses the Advanced Linux Sound Architecture system (ALSA), which has replaced the Open Sound System (OSS) used in older Linux kernels. The ALSA interface is generally more advanced than the OSS interface; however, OSS is used on many Unix systems, so ALSA also provides an OSS-compatible sound interface for the convenience of cross-platform software developers.

ALSA uses devices in the /dev/snd directory such as /dev/snd/controlC0 , which is used by the Volume Control and KMix tools to control the first sound card (C0). Devices for OSS compatibility are in the /dev directory and include /dev/dsp , /dev/audio , and /dev/mixer .

Most sound chips have several inputsin some cases, a few dozen inputswhich are routed through various sound paths to arrive at one or more outputs. Most of these inputs have a description assigned by the chip designer, but it's not necessary for the sound card designer to use a particular input for its designated purpose, and it's also not guaranteed that the system builder will connect a given signal source to the appropriate input on the sound card. Since the ALSA drivers generally use the designations provided by the chip documentation, you may find situations where the Video control manages the CD-ROM volume, or the Headset control affects the main speaker output.

It's not uncommon for different sound card models to use the same chipsets, with the support circuitry for some features left off of the budget models. In these cases, ALSA has no idea which features are wired up and which ones have been omitted, which explains why there are so many controls that don't do anything.

The Soundcard Detection tool is a Python script named system-config-soundcard . This script configures the file /etc/asound.conf with the selected default PCM device.

When the system is shut down, the script /etc/rc.d/init.d/halt saves the sound configuration (including mixer settings) to /etc/asound.state . The state is restored by the Udev subsystem using the program /etc/dev.d/sound/alsa.dev when the sound devices are detected during system boot.