Re: Sync between MIDI and audio

[Steve deRosier <derosier@pianodisc.com>]

Paul,

Ah, well that explains it.  Thanks.  Can you point me to some info on 
how I would link the two clocks (I guess it would be to make the seq use 
the PCM clock)?

As far as using kernel 2.5/2.6 or the backport of the hi-res timers 
code... unfortunately this isn't an option for me.  I can't change the 
kernel version and even if I could, the rest of the people wouldn't 
allow it at this point.  But, even if I used the hi-res stuff, wouldn't 
I still have the same problem with having 2 clocks and those drifting 
apart over time?

Thanks,
- Steve

Paul Davis wrote:
>>The problem: the MIDI and audio start synced up, but after a while they 
>>start to drift further and further apart.  I've checked my stuff 
>>exaustively to make sure that it's not the fault of my program, so I'm 
>>figuring it is caused by me not setting up something properly in alsa to 
>>make the two streams synced up.
> 
> 
> you either have to force the sequencer queue to use the PCM device as
> a clock, or you have to use the RTC as the clock for both (which will
> cause audio glitches every so often since it drifts w.r.t. the audio
> clock). you can't expect this to "just work" - you've created a system
> with two clocks (the sequencer queue and the PCM device), and such
> systems will never stay in sync.
> 
> this is a very tricky area of linux audio/MIDI programming, and my
> personal advice would be to use kernel 2.5/2.6 or the backport of the
> hi-res timers code. you can then get usec-accurate scheduling of
> events (in SCHED_FIFO tasks, anyway) and forget the ALSA
> queue. hopefully, the ALSA sequencer will at some point take advantage
> of the new timers. even so, its still not a simple thing to do.
> 
> --p
> 



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