[SDL] audio documentation...

[David Olofson]

On Wednesday 21 April 2004 11.56, redman wrote:
> hi ppl.,
>
> i'm new to the list (and sdl) and i've got a question., :)
>
> does anyone have some good docs about the audio part of sdl?
> i'm having some speed issues and i'm not sure what's causing it.
> (working on windows / msdev6 / c)
>
> i do some calcs in the callback function but at a certain point
> (ammount of calcs) the buffer
> starts skipping., but i'm SURE i have loads of cycles left to do it
> ,..,

This is because you're not on a real time operating system... There is 
a hard deadline for delivering the audio buffer (ie returning from 
the callback), and if you miss it, audio skips. Meanwhile, a standard 
OS scheduler and some background system load will cause the audio 
callback to start anywhere from a few µs through tens or 
(occasionally) hundreds of ms late, wasting the cycles you were going 
to use for processing, or even causing audio drop-outs before you get 
a chance to do anything at all. Not much you can do about that, short 
of switching to Linux/lowlatency or BeOS. :-/

What you *can* do is increase the buffering (latency) so that the 
scheduling jitter accounts for a smaller part of the audio buffer 
period. That way, you can use more CPU power for audio with less 
drop-outs, but on an OS like Windows, there is no way ever you're 
going to get anywhere near 100%, and/or totally eliminate the 
drop-outs when doing low latency audio. (I think you can get pretty 
close with Win2k or XP, a decent proffesional sound card and running 
your audio code in kernel space as Kernel Stream filters, but that's 
insane for "consumer" multimedia such as games.)


[...]
> so i have basically two questions
> 1 where should i generate the data that would go into the buffer

In the callback. (Otherwise, you'll need extra buffering and thread 
safe communication between the audio engine and the callback. That 
can make sense in some cases, but *definitely* not when you want 
interactive low latency sound effects.)


> 2 when should i call SDL_LockAudio() / SDL_UnlockAudio()

Preferably never. I would recommend using some sort of lock-free 
communication between the main thread and the audio callback context 
instead, to avoid increasing the callback "scheduling" jitter.


//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 ---