10 Oct
2003
10 Oct
'03
3:38 p.m.
-------- directBOX Reply ---------------
From: david(a)olofson.net
To : linux-audio-dev(a)music.columbia.edu
Date: 10.10.2003 19:17:01
On Friday 10 October 2003 17.41, hexe_2003(a)directbox.com wrote:
[...]
But well, If I synthize ( ? spelling) two signals and
want to send
them at the same time to the device, my first idea'd be:
1.) Open /dev/dsp, maybe O_NONBLOCK | O_WRONLY
2.) thread
3.) two threads calculate the both signals
4.) sync them
5.) two threads write their signals to /dev/dsp
6.) threads close
certainly some of this. I mean mp3blaster, for example, have
several pids while playing audio streams, and it is basicly
OSS-programming and not alsa.
Am I so wrong ? ;)
Yep! ;-)
Synths, trackers, games and other "polyphonic things" at some point
mix everything into a single stream that is sent to the audio device.
In general, all audio processing is done in a single thread, but in
some cases, it can be pretty hard to get the processing deterministic
enough that it can run in the audio thread without causing drop-outs.
This is especially troublesome if you need low latency, as in games
and other interactive applications.
This is probably why mp3blaster is using multiple threads; one for
each mp3 stream (decoding into raw audio data) and one for mixing and
output. There is most probably substantial buffering between the
decoding threads and the mixer thread, as this would allow solid
playback with less buffering (ie less laggy mixing controls) between
the mixer and the audio device.
//David Olofson - Programmer, Composer, Open Source Advocate
..- Audiality -----------------------------------------------.
| Free/Open Source audio engine for games and multimedia. |
| MIDI, modular synthesis, real time effects, scripting,... |
`-----------------------------------> http://audiality.org -'
--- http://olofson.net --- http://www.reologica.se ---
--------------
quiet interesting ;)
Could you give me a short idea howto mix two signals/stream/oscilators/whatever ??
If it is not pthread, what is it ?
Sascha Retzki
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13 Oct
13 Oct
8:49 a.m.
New subject: [linux-audio-dev] Oss & threading -- is it possible
On Friday 10 October 2003 19.19, hexe_2003(a)directbox.com wrote:
[...]
quiet interesting ;)
Could you give me a short idea howto mix two
signals/stream/oscilators/whatever ??
The actual mixing is just a matter of adding the signals together.
You'll have to make sure the result doesn't overflow (wrap), but
there's no single correct way of doing that. You can play safe and
scale to 1/voices, use a compressor/limiter (dynamics processor) or
just set a sensible level and clip the signal (to avoid wrapping),
just in case.
I guess the most confusing part for some people (depends on your
background) is to get multiple "units" working together, in parallel,
generating/processing N samples at a time. Functions that generate as
many samples as they feel like and then expect to send the output
somewhere through a write() style API won't work (well) in single
threaded engines. You want everything to process/generate N samples
upon request, so you can just run all units in the net once per block
of audio.
If it is not pthread, what is
it ?
Platform and API dependent. Some audio APIs will run your audio engine
as a callback, so you don't have to (and generally shouldn't) set up
any audio threads of your own.
Usually, there will still be a thread (managed by the API or driver
subsystem), but in some cases, the callbacks actually come directly
from the driver and run in interrupt context. This is common on
systems without real multitasking, like Win16 and Mac OS "Classic".
//David Olofson - Programmer, Composer, Open Source Advocate
.- Audiality -----------------------------------------------.
| Free/Open Source audio engine for games and multimedia. |
| MIDI, modular synthesis, real time effects, scripting,... |
`-----------------------------------> http://audiality.org -'
--- http://olofson.net --- http://www.reologica.se ---
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hexe_2003＠directbox.com