[pulseaudio-discuss] Latency problem with long latencies

[Georg Chini]

Hello,

I am still working on module-loopback and hit a problem that I cannot 
explain.
When running a HDA card with long latency (333ms) I see that the resulting
latency is not stable but varies around 300 usec. What is worse, the changes
in latency are not even reported, module-loopback sees a completely stable
value ( +/-10 usec).
My setup: I have a bitscope dual channel USB oscilloscope attached to 
another
machine. This device also has a function generator that I set to 1kHz 
rectangle.
The signal is input to the HDA card and fed to the output of the same 
card via
module-loopback. Input and output signal are measured using the 
oscilloscope.
The movement of the edge of the output signal now shows the "latency jitter"
which can be made visible by using the overlay mode of the oscilloscope.

The result for HDA (module-loopback running with 500ms adjust time and 1s
latency) is shown at http://georg.chini.tk/hda_333ms_3.png. Some of the 
original
input signal is somehow fed into the output signal, don't know where 
this happens,
so please ignore it as it has no impact on the result.

At the bottom you see the input signal with a completely stable edge.
On top there is the output signal. You can see that the latency of the 
output signal
is spread over 300 usec with a spacing of a quarter of the sample time.
Module-loopback is doing no regulation at all because - as said above - 
it does
not see those latency changes.

For small latencies (20ms and smaller) I still see a double line. For 
USB devices
the effect is much worse, but difficult to demonstrate because USB 
devices are
less stable anyway and module-loopback's regulation spoils the effect.

Does anyone have an idea why this happens and if it is a problem of 
pulseaudio
or of the alsa driver? I think it is pulseaudio, because with 
zitaj-bridge and Jack
I only see a single line.

Regards
              Georg