[pulseaudio-discuss] Glitches on null sink?

marcin at saepia.net marcin at saepia.net
Fri May 20 12:28:52 PDT 2011


I continue to develop my radio audio routing software so I continue to
encounter strange bugs.

General idea that lies behind my project is to split some processing
parts into separate processes. There are many reasons why this
architecture has to be like this, it something fundamental and won't

For processing individual elements I use GStreamer (no external
bindings, my code is in C).

There're many advanced dedicated input sources, such as
RTP/VoIP/shoutcast. All of them load their own null sink module, and
perform output on one. Other modules that broadcast the signal to the
FM/net just leech from their ".monitor" PA sources. To this moment
everything works flawlessly, even if I load huge amount of sources.

The problem is when I try to perform some audio processing or/and
mixing. Currently I do that in the following way:

1. Load another null sink module for mix output.
2. Run GStreamer's pipelines (each is also a separate process), e.g.:
 a) pulsesrc device=source1.monitor ! processor1 ! processor2 !
pulsesink device=mix
 b) pulsesrc device=source2.monitor ! processor3 ! processor4 !
pulsesink device=mix

It generally works ok, but from time to time, output (captured from
mix.monitor) becomes full of glitches. Sounds like something lost
synchronisation. And it's really hard to reproduce. Sometimes I run
that kind of pipeline and it works for a long time and then it starts
doing weird things, sometimes it starts it from the beginning.

When it starts producing glitches, pulsesrc element in the GStreamer
pipeline throws an error that it cannot read samples fast enough. What
is interesting, when it starts doing that, any other audio that I pass
to this "mix" sink also sounds bad from the begin. Only reloading
module helps.

My theory is that I have to use module-combine to perform such mixing.
Am I correct?

I wanted to avoid that as I am unable to dynamically add its slaves,
but if it's the only way to perform such mix, I will do that, and
reload it when my dedicated sources' list changes... It's not too
pretty as it would produce short drops, but it's still acceptable.

I just want you to ask if I am seeking for the problem in the correct
place, just before I spend a week on refactoring my code. Maybe
pipeline like shown above (pulseaudio -> GStreamer -> pulseaudio) also
is a wrong approach?

Thank you,


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