Sound Software

Sound on Linux is the absolute biggest bunch of bullshit ever.

Terminology

• HDA: Intel High Definition Audio specification
• ELD: EDID-Like Data. Structured capability data returned by graphics-oriented devices.
  • ELD can also mean Enhanced Low Delay (in for instance AAC-ELD)
• PD/ELDV: Presence Detect/ELD-Valid bits aka "Unsolicited Responses".
  • PD indicates a change in presence or the ELDV bit
  • ELDV indicates new ELD information

Kernel

• Determine: are there still ALSA hardware drivers inferior to their OSS equivalents? Assuming no...
• OSS emulation: kernel, library, or neither?
  • Kernel: snd_pcm_oss, snd_mixer_oss, snd_seq_oss provide /dev/dsp, /dev/seq, /dev/mixer
  • Library: aoss sets LD_PRELOAD to include libaoss.so
  • Neither: force fail on attempts to use OSS
    • Might be better if OSS-only code is old and crappy, as suggested by OSS-only nature, as it'll eliminate such code. Might also be better if OSS compatability introduces any kinds of issues. Might be worse if OSS code is better-written than ALSA code alternatives, and OSS compatability introduces no problems.

/proc/asound

• /proc/asound/cards: Maps physical devices to ALSA card numbers

[eschatomatic](0) $ cat /proc/asound/cards
 0 [Intel          ]: HDA-Intel - HDA Intel
                      HDA Intel at 0xe8260000 irq 43
 1 [NVidia         ]: HDA-Intel - HDA NVidia
                      HDA NVidia at 0xe3080000 irq 17
[eschatomatic](0) $ 

• /proc/asound/cardX: Device details indexed by ALSA card number, including codec and ELD dumps
  • NOTE: X being loaded is typically required for ELD detection!
• /proc/asound/devices: Maps functional devices to ALSA card:dev number pairs and describes function
• /proc/asound/modules: Maps kernel modules to ALSA card numbers
• /proc/asound/pcm: Maps card:dev numbers to allocated PCM streams

ALSA configuration

• What the fuck ought go in /etc/asoundrc? It's a complete enigma.

Sound Servers

• pulseaudio -- crashes mpd every time I try to use it (no longer true as of Fall 2008)
• To drive ALSA through pulseaudio by default:

pcm.!default {
	type pulse
}

ctl.!default {
	type pulse
}

• jack -- low-latency, professional software; it frightens me
• esd -- deprecated, replaced by pulseaudio's esd-compat module

Hardware

• PulseAudio can supposedly use a plugin together with HAL to drive dynamic devices

See Also

• A Guide to Linux Sound APIs by Lennart Poettering of PulseAudio
• Linux Audio: It's a Mess from LWN
• PulseAudio -- The Perfect Setup
• Ubuntu has a HOWTO on ... fixing pulseaudio. Yikes.
• mpd, the music player daemon
• The Intel High Definition Audio specification
• The ALSA Wiki
• The computeraudiophile.com Computer Audiophile FAQs