Re: [Jack-Devel] extra latency compensation was Re: is this latency coming from ALSA, jack, or ardour?

On Sun, 2011-04-17 at 22:33 -0500, Chris Caudle wrote:
> OK, understanding extra latency compensation better now, but still a bit
> confused by the results.  I can't seem to reconcile the results from
> jack_iodelay with the track to track delays I see in Ardour 3.
> 
> On Fri, April 15, 2011 2:04 pm, Chris Caudle wrote:
> > OK, so now I connected jack_delay out to the card physical output port,
> then connected the card physical input port to jack_delay in, and got
> this:
> >
> > 4155.593 frames     94.231 ms
> 
> I haven't really optimized this setup yet, so I'm running 1024
> frames/period, 3 periods/buffer.
> 
> So according to jack_iodelay, the additional latency not due to the period
> buffer is 3131 frames (4155 - 1024 frames per period).
> 
> So 3131 frames is about 71 ms at 44100 frames/second.  I don't see
> anywhere near 71ms offset when I record one track out of Ardour, and
> physically loop back to a second track in Ardour.  Does that make any
> sense? Shouldn't that extra latency show up in a time offset between
> tracks?
> I tried using that 3131 value, and used 1566 for the values passed to the
> -I and -O arguments, but it made alignment between tracks noticeably
> worse.  And the delay became negative, the audio on the second track was
> 70 ms ahead of the first track.  That matches pretty closely the 3131
> frames number, but the direction implies that the correction wasn't
> needed.  How could that be?
> 
> I changed the jackd settings to use 512 frames per period, 2 periods per
> buffer, and jack_iodelay reports 1595 frames latency. 1595-512 = 1083
> frames "extra" latency.
> 
> I expected the "extra" latency to be due to the latency in the AD
> conversion and DA conversion, and to be constant for a given sample rate. 
> Why such a large difference in extra latency when the period size is
> changed?
> 
> These results are confounding my expectations, and I have to think that
> either there is something fundamental I missed in the setup, or the test
> methodology has a big hole I can't see.  Anyone else see what is going on
> that I'm missing?

You're missing the periods/buffer value, and the additional period of
latency you get from JACK2's asynchronous mode.

4155 - (1024 * (3 + 1)) = 59 frames
1595 - (512 * (2 + 1)) = 59 frames

Peter.

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