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

On Sun, Apr 17, 2011 at 10:33:58PM -0500, Chris Caudle wrote:
 
> 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).

No, it is 

4155 - 3 * 1024 = 1083    for jack1, or
4155 - 4 * 1024 = 59      if you use jack2 in its default mode.

 
> 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.
 
No, 

1595 - 2 * 512 = 571      for jack1, or
1595 - 3 * 512 = 59       if you use jack2 in its default mode.


These results make me think that you are indeed using jack2 in its
default mode wich adds 1 extra period of 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 are missing the correct way to calculate the extra latency.

   measured_latency - n * p           for jack1 or
   measured_latency - (n + 1) * p     for jack2 in its default mode. 

where n and p are the values given to the -n and -p options. 

Ciao,

-- 
FA

1303119381.32397_0.ltw:2,a <20110418093557.GA4954 at linuxaudio dot org>