[SDL] accurate audio timing

[David Olofson]

On Friday 04 January 2008, Richard Henwood wrote:
[...]
> In short, if I want to make a reliably timed, cross-platform
> metronome, which route should go; threads, callback, something else?

For the low level side, just use the callback. Throwing extra threads 
in the mix is just asking for trouble, and doesn't really add 
anything in most cases.

Timing can be derived from SDL_GetTicks(). It's "only" millisecond 
accurate, but unless you're running an RTOS or a properly 
configured "multimedia OS" (BeOS, OS X, Linux/lowlatency, preemptive 
Linux 2.6 etc), your input event timing won't be nearly as accurate 
anyway.

Basically, what you do is timestamp events as they arrive, and then 
you calculate start times (audio buffer offsets) based on those 
timestamps and the callback timing.

Theoretically, you can just timestamp the callbacks with 
SDL_GetTicks(), but as there will likely be substantial scheduling 
jitter in most cases, you should probably calculate callback 
timestamps based on the sample rate instead, and just use 
SDL_GetTicks() to nudge the "running audio time" to stay in sync.


The real problem, however, is not input timing or input/audio sync, 
but rather that SDL has no way of reporting the actual 
callback->output latency. The best you can do is a qualified guess 
based on the SDL audio buffer size you get (which may differ from 
what you ask for!), and it might be a good idea to provide some way 
for advanced users to tweak it manually.


All that said, note that you don't really need this for games 
generally; a constant latency below some 50 ms is sufficient, and you 
can't do anything about the latency anyway. (Can't count on anything 
below 100 ms or so working on every system, so you need to make it 
user configurable to go any lower.) The problem with musical 
applications (as you've probably realized), is that in addition to 
constant latency, they need to synchronize and/or correlate input 
events, internal events and video output with audio output.


You might want to look at DT-42, which is a minimalistic tracker style 
drum machine/sequencer that deals with most of these issues - or at 
least, tries to, within the limits of the SDL API:
	http://olofson.net/mixed.html


//David Olofson - Programmer, Composer, Open Source Advocate

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