Re: alsa timer slippage

[Jan Depner <eviltwin69@cableone.net>]

If I'm not mistaken the timing for your audio is coming from your sound
card not your system clock.  The gettimeofday is from the system clock. 
They probably won't match.  Of course, I could be totally in the dark
;-)

Jan


On Sun, 2003-12-14 at 08:07, Chris Cannam wrote:
> 
> While trying to track down the source of some poor timing in 
> sequencing, I've noticed that my ALSA sequencer queue timer has a 
> tendency to fall suddenly behind.
> 
> I have a little test program (available on request) that just starts a 
> queue and every second or so compares the queue timer against real 
> time as returned by gettimeofday().  It doesn't mind if the two don't 
> quite match, but it does complain if the difference between the two 
> timers changes dramatically between two sample points.  When I run 
> it, it never lasts for more than about a minute before the ALSA queue 
> timer suddenly slips by anything from 10 to 60 milliseconds.
> 
> This is a non-low-latency kernel so I'm not surprised that there may 
> be some occasional timing issues, but 60ms is a lot on an unloaded 
> machine, and I am vaguely surprised that the timer doesn't notice 
> it's fallen behind and recover -- instead all events on the queue 
> continue to be delivered late forever.  This obviously makes for some 
> disconcerting audible effects.
> 
> The system is SuSE 9.0 on a dual 2GHz Athlon using SuSE's stock SMP 
> kernel.  I have tried both ALSA 0.9.6 (from SuSE) and 1.0.0rc2 
> drivers and libraries.  I haven't managed to reproduce it using a 
> PlanetCCRMA SMP kernel on the same machine, nor on my uniprocessor 
> laptop.  I've surveyed a few other people on rosegarden-devel and 
> nobody's corroborated my findings, so I guess it might be related to 
> using a dual-processor machine.
> 
> Any thoughts on this, anyone?  I'm finding it a little depressing that 
> I can't play even a minute of 4/4 beats from an ALSA test program 
> without the timing slipping audibly at least once.  I'm ready to 
> delve cluelessly into the timer code to take a look, but (glancing at 
> alsa-kernel/core/timer.c) I'm not at all sure how far I'd get...
> 
> 
> Chris
> 
> 
> 
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