Sound Software

Sound on Linux is the absolute biggest bunch of bullshit ever, closely trailed by sound on computers and sound itself.

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
• EAX: Environmental audio eXtensions
• S/PDIF: Sony/Phillips Digital Interconnect Format (IEC 60958 type II / IEC 958)
  • 10m max cable distance
  • TOSLINK F05 / EIAJ-Optical and coax RCA connections
  • Carries DTS and Dolby Digital compressed audio for >2 PCM (IEC 61937)
• AES3: commercial-grade
  • 3-pin XLR (balanced) and BNC (unbalanced) connections
• CDDA: CD Digital Audio, 2-channel 44.1k PCM
• DAT: Digital Audio Tape, a 48k/44.1k/32k medium using 4mm magnetic tape

Hardware

I recommend the Xonar Essence ST PCI card, making use of the File:Pcm1792a.pdf DAC, for an in-machine solution.

USB DACs

• Watch for true USB Audio Class 2.0 and solid asynchronous solutions
• "The D-A Diaries" is an excellent article regarding the difficulties of producing accurate sound via USB output.
  • Essentially: USB has a clockless wire, thus there must be clocks on each side, which *will* drift
  • If the USB device doesn't have its own crystal, it might even time based off AC power
• USB Audio Class 1 is limited to the 24bit, 96KHz maximum supported by USB 1.x (12Mbps)

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 ought go in /etc/asoundrc? It's a complete enigma.
  • FIXME

HDMI

• FIXME

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 (somewhat outdated) Linux High-Quality Audio HOWTO
• The computeraudiophile.com Computer Audiophile FAQs
• "Pros and Cons of USB DACs" on head-fi.org
• "Configuring Digital Sound" on the MythTV wiki
• The Well-Tempered Computer and the Digital Audio Blog
• ...and, of course, stereophile.com