Re: ALSA sample rate conversion and general performance improvements.

[Christian Henz <chrhenz@gmx.de>]

On Wed, Mar 29, 2006 at 09:52:39AM -0500, Paul Davis wrote:
> On Wed, 2006-03-29 at 11:03 +0100, James Courtier-Dutton wrote:
> > Takashi Iwai wrote:
> > > At Wed, 29 Mar 2006 00:24:20 +0100,
> > > James Courtier-Dutton wrote:
> > >   
> > >> Hi,
> > >>
> > >> I have had some conversations with various people regarding some general
> > >> problems some audio developers have with the current ALSA sound model.
> 
> Hi James. Not me by any chance? :)
> 
> > >> What about doing the following?:
> > >> We implement a general timer in the alsa core based on something like
> > >> gettimeofday(). I.e A high resolution timer.
> > >> Each time a period interrupt happens, the current hw pointer is read,
> > >> and recorded along with the current value of the timer.
> > >> This could be used a bit like NTP to correct the generally timer rate.
> > >> This gives us a general high resolution timer corrected to mirror the
> > >> PCM clock on the sound card.
> 
> to correct something said later, this should be done using a DLL, not a
> PLL. subtle but significant difference.
> 
> > >> Then, from this high resolution timer, we could derive any rate
> > >> interrupt we need for really excellent sample rate conversion, and sub
> > >> sample accurate DAC/ADC positioning.
> > >> One could then read the DAC/ADC positioning really quickly by just
> > >> reading the high resolution timer, instead of accessing IO Ports on the
> > >> sound card hardware.
> > >> This would also be a fairly minor change, as none of the current kernel
> > >> sound card hw drivers would need changing. We would just have to
> > >> slightly modify the period_elapsed callback to do a gettimeofday() call
> > >> as well as it's current hw_pointer read.
> 
> i think its more than a minor change. the real benefit of this model
> (which in my eyes correctly emulates the CoreAudio HAL) is that *if* you
> have a schedulable high res timer (which linux, appallingly, still does
> not have; we'll accept HZ=1000 for now, i guess), you don't need the
>

Hmm, I though that we DO have schedulable high res timers
now with the recent hrtimers merge? At least thats what I 
got from watching the video of the talk on RT patches at 
FOSDEM (http://free-electrons.com/community/videos/conferences).
I might have to watch it again to see if that only applied to
the -rt patch...


cheers,
Christian


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