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OSS Emulation in Alsa
One of the aims of ALSA is to provide full OSS compatibility for OSS applications. ALSA has an OSS emulation which supports all of the OSS devices listed below.
There are two different OSS emulation methods in ALSA (and several not in ALSA itself; see the "Alternatives" section below):
- Kernel-level OSS emulation via the
aossscript from the
The latter has the advantage of being able to use any device from ALSA's PCM plugin layer during the emulation. This enables software-based mixing of streams for OSS apps. See the DmixPlugin page for more details of how to do software mixing.
Note that neither method works for all OSS apps.
Issues and Limitations
Particular issues include:
- the OSS sequencer
/dev/sequenceronly half works (playback works but recording from MIDI keyboards is broken -- see Alsa-devel 2004/02/02 2002/8/26 and 2003/12/7).
- apps that use libc's FILE* functionality (fopen(), etc..). A fix for this should now be in alsa cvs (as of what version/date?)
- things that use OSS via memory mapping (mmap). See ALSA PCM proc commands for some hints on this.
- The ICE1712 chipset supports only an unconventional format, interleaved 10-channels 24bit (packed in 32bit). Therefore you cannot mmap the buffer in a conventional (mono or 2-channels, 8 or 16bit) format using OSS emulation.
- Some USB devices support only 24bit format packed in 3bytes. This format is not supported by OSS and no conversion is provided by kernel OSS emulation. You can, however, use the user-space OSS emulation via libaoss instead.
OSS Emulation Topics
- List of Supported OSS devices (such as /dev/dsp)
Kernel module device mapping options control which ALSA PCM and MIDI devices OSS devices like /dev/dsp get mapped to.
Some ALSA PCM proc commands can be used to control behaviors important for OSS emulation. Check these out if your problems may be related to:
- apps that uses MMAP
- sample rate conversion
- fragment size
- i/o blocking issues
- apps that can't handle duplex streams properly
Takashi Iwai's documentation on the OSS MIDI emulation may be found at OssSequencerEmulation.
- All about Mapping oss mixer controls to alsa mixer
Much of this page and it's sub-topic pages is derived from http://alsa-project.org/\~iwai/OSS-Emulation.html from the official ALSA docs. It may be more frequently updated or contain more extensive information than this wiki.
DmixPlugin includes some tips on using dmix with OSS.
OSS and dmix by Richard is an a step by step guide on how to use xmms while playing Unreal Tournament. It probably will also help with Enemy Territory, Quake3, and other things that use OSS, fopen(), and/or mmap.
ALSA is not the first sound system (and probably not the last) to have a need to capture and convert output from programs written for OSS. If ALSA's OSS Emulation is not doing it for you, you can look into:
- the artsdsp wrapper for KDE's arts sound server
- the esddsp wrapper for esd (Enlighted Sound Daemon) sound server
- the libjackasyn and bio2jack libraries for use with jackit (Jack Audio Connection Kit) (bio2jack requires recompiling)
Retrieved from "http://alsa.opensrc.org/OSS_emulation"