[PD] drop out problem

Roman Haefeli reduzierer at yahoo.de
Sat May 17 20:08:26 CEST 2008

hi all

i am setting up a box for an installation. it runs two instances of pd:

pd -rt -jack (with only very few externals loaded)

pd -nrt -noaudio (with quite a lot of externals loaded, such as Gem,
comport, wiimote and others).

both instances talk with each other over two [netsend]/[netreceive]
connections (one for each way). the main part of the tcp connection
bandwith is tracking data sent from the tracking patch to the audio
patch at fixed rate, where each message is 12 floats long. 

all that runs fine and smooth on my own mobile box:
pentium M 1.7GHz
ubuntu dapper
pd 0.40.3 over jack (48KHz, 1024 frames/period, 2 periods/buffer )
cpu usage is around 80%

the same two patches on the installation box cause lots of drop-outs:
AMD Athlon XP 2.8GHz
ubuntu gutsy with rt-kernel and everything setup for low-latency
pd 0.40.3 (with same jackd settings)
cpu usage around 60%

funny enough, that the much faster machine with the more appropriate
setup (rt-kernel) is having the drop-outs.
usually i would suspect [netsend] or [netreceive] to be the cause of the
problem. but in this case the data rate sent over the tcp socket is
constant, whereas the dropouts only happen, if the tracking data changes
values, but not, if the stream from the tracking patch keeps sending the
same values. the major part of the audio patch is just some synth stuff,
that is constantly running, so there aren't big jumps in cpu usage
expected. from what i can tell, the only thing, that happens in the
audio patch, when the received stream changes its values, is, that
around 10 [partconv~]  objects switch tables from a preloaded set of
around 1400 tables, where each table is 512 samples long. i don't have
an explanation, why this could affect audio (and why it doesn't on my
slower mobile box), but this is what i observed. i am happy with any
possible explanation of that behaviour and even more with an idea how to
solve it. i really would like to avoid not to have to use my personal
laptop for the installation.



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