[LAD] [64studio-devel] [64studio-users] Correlation of alsa -p value and hw MIDI jitter

[Ralf Mardorf]

On Wed, 2010-07-14 at 14:12 -0700, Devin Anderson wrote:
> On Wed, Jul 14, 2010 at 12:43 PM, Ralf Mardorf
> <ralf.mardorf at alice-dsl.net> wrote:
> > On Wed, 2010-07-14 at 12:30 -0700, Devin Anderson wrote:
> >> On Wed, Jul 14, 2010 at 10:29 AM, Ralf Mardorf
> >> <ralf.mardorf at alice-dsl.net> wrote:
> >>
> >> > Hi :)
> >> >
> >> > delayed by a thunder-storm I could do another test.
> >> > --snip--
> >>
> >> So, what you're saying is that your MIDI device and software synth
> >> sync up less and less as you raise the period size.
> >
> > Yes :).
> >
> >> I had presupposed
> >> before that your MIDI device was triggering *after* your software
> >> synth, but it occurs to me that it might be the other way around.  Do
> >> you hear the audio from your software synth first, or from your MIDI
> >> device?
> >
> > I can't say it today, now I do some office work. I had the impression
> > that it might vary. Sometimes the virtual drum sampler and sometimes the
> > standalone drum sampler was played earlier, I need to check this ASAP.
> > For older tests with my USB MIDI device it was exactly that way, that
> > jitter had positive and negative delay. At least the recorded waveforms
> > of external MIDI equipment (when I used USB MIDI, now I'm using PCI
> > MIDI), were recorded by Qtractor, before theoretically the MIDI event
> > was send ;). Note! Qtractor had no latency compensation, all recorded
> > audio of external MIDI instruments should have (positive) delay, but
> > negative delay.
> 
> If it ends up being the case that your MIDI device is being triggered
> before your software synth, then I'm guessing that the issue here is
> not MIDI jitter.  I'm guessing the issue is that the latency that's
> imposed by JACK on incoming and outgoing audio is not imposed on
> incoming and outgoing ALSA MIDI.  So, while the audio coming out of
> the software synth is delayed by a certain amount of frames imposed by
> JACK, the audio coming out of your MIDI device is only delayed by the
> latency of the ALSA drivers, the latency of the MIDI ports, the
> latency of your MIDI device.
> 
> This would certainly explain why the problem gets worse as you raise
> the period size, and could explain why you had positive and negative
> delay in your older USB MIDI tests, as the reported MIDI jitter in
> your tests was *far* worse in your older tests than it is now.
> 
> At the moment, I happen to be doing some work in JACK 2 that could
> potentially solve this issue by enabling MIDI to sync more closely
> with audio, so I'm very curious to know if my suspicions are correct.
> Please keep me updated. :)

Should I build JACK dummy packages for 64 Studio and daily get JACK2
from svn co http://subversion.jackaudio.org/jack/jack2/trunk/jackmp ?

I wonder if this should be cross-posted to LAD?

On LAD and the 64 Studio list are people with much knowledge and your
reply might hit the nail on the head.

- Ralf