NetBSD audio – an application perspective

who am I?

• nia@NetBSD.org
• nia on freenode, etc
• Developer since 2018
• First talk :-)

audio options for NetBSD in pkgsrc

• Use NetBSD native audio (sun audio/audioio.h)
• Or OSS emulation layer
  Basically a wrapper around sun audio in the kernel
  Incomplete and old version, but works for simple stuff

Many many abstraction layers available:

• OpenAL-Soft
• alsa-lib (config file required)
• libao, GStreamer (plugins!)
• PortAudio, SDL
• PulseAudio, JACK
• ... lots more!? some obsolete stuff (esd, nas?)

advantages of using NetBSD audio directly

• Low latency, low CPU usage
  Abstraction layers differ in latency (SDL2 vs ALSA/OpenAL)
• Query device information
  Is /dev/audio1 a USB microphone or another sound card?
• Avoid bugs from excessive layering
• Nice API, well documented
  [nia note: I had no idea how to write audio code. I read a man page and now I do.]
• Your code might work on illumos too

[nia note: SDL2 seems very sensitive to the blk_ms sysctl being high or low, with other implementations there seems to be a less noticable difference. I don't know why.]

first test case: RetroArch

• Frontend for libraries implementing emulators or game logic.
• I learned NetBSD had its own non-OSS audio API.
• I wanted to increase support for it.
  RetroArch was a straightforward target
  They accepted my patch
• My code also works on illumos.

differences from the OSS implementation?

• Kernel calculates block size for the user's global latency
• Simpler and probably results in more desirable outcomes
• Less ioctls
  OSS - pass values using setters, e.g. SNDCTL_DSP_SETFMT
  Sun - pass one struct to the kernel with AUDIO_SETINFO

[nia note: Both Sun and OSS involve opening a device in /dev and performing standard read(2) and write(2) operations on it. The difference is how you tell the kernel to interpret the data.]

second test case: aiomixer

• Decided to use my experience after RetroArch to write a mixer
• State of mixers is lacking
  A lightweight user-friendly alternative to mixerctl was needed
• Do more than change volume - DACs
  Your laptop has a headphone port and speakers
  Should be possible to select between them
  And control them independently
• ... would be nice to have more readable DAC names

pkgsrc-wip - wip/aiomixer

github repository- https://github.com/niacat/aiomixer

third test case: SDL2

Popular in applications, but high latency on many platforms.

SDL already targeted NetBSD directly, but with many problems:

• Tried to use non-blocking I/O
  Workaround for the bad old days where opening /dev/audio
  locked it system-wide.
• Calls to SDL_Delay(1) everywhere
• Bad quality output - jittering, poor performance
• Buggy initialization

etlegacy patched to use OpenAL.

reimplementation

I reimplemented it using current docs and advice on SDL's bugzilla.

• Blocking I/O - all calls to SDL_Delay(1) removed
• Much better performance, jittering gone
• Actually usable now
• Submitted to upstream for inclusion in the next release

fourth test case: cubeb (Firefox/SeaMonkey)

cubeb is a portable audio library written in C and C++

• mainly notable because Mozilla uses it
• mozilla recommends PulseAudio
• ... but upstream accepts patches for new backends, if tests pass

Poor native audio support - only an unofficial OSS backend

• No microphone support
• No device selection
• Unmaintained
• FreeBSD removed it, we found bugs after audio2 merge

writing a cubeb backend for NetBSD audio

• Detect devices (e.g. USB microphones)
• Make them available as outputs or inputs
  Give them nice, user-friendly names
  Uses NetBSD APIs, e.g. AUDIO_GETPROPS, AUDIO_GETFORMAT
  But most of it still works on illumos
• Support more channels
• Support 96000 Hz and other fancy sample rates
• Need to convert floats in cubeb - no support in kernel currently
• cubeb uses threads - very different I/O to other code
• I want the OSS backend gone
• Done! Merged by upstream!

[nia note: It's now one of the best supported APIs you could use. SDL2 is now very good too, but it's not as good for recording and doesn't have multi-device support.]

questions?

I might be able to answer them

thanks for listening

I'm looking for a job btw

some annotations in response to questions and other comments made during the talk, marked "nia note"