plug:iec958 problems

[Martin Soto <soto@informatik.uni-kl.de>]

Hi everyone!

I'm writing a DVD player application based on the GStreamer media
framework (www.gstreamer.net), and, of course, ALSA sound output should
be supported. In particular I'm trying to use the iec958 plugin to
output AC3 sound directly to an external receiver.  In principle, this
works ok, except for a little but annoying detail: when the machine is
under even moderate load, I start hearing short interruptions in the
sound.

Some investigation  of the problem makes me thing it doesn't lie in my
code, because I can trigger it as well with pure vanilla ac3dec -C. All
I need is a constant moderate load, like that generated by the DVD
reading process (in my pretty old AMD Athlon 700, this causes a load of
about 25%) and then suddenly creating a load peak, which I achieve by
quickly moving a window on my desktop. Sound output interrupts suddenly,
some times even for about half a second.

Of course, the first thing I thought was that it had something to do
with data starvation. This doesn't seem to be the case either.
Specially, it called my attention that the API isn't reporting any
buffer underruns. ac3dec doesn't either show any messages when the
problem shows up. Additionally, putting the corresponding thread or
process in SCHED_FIFO mode doesn't either help. I could, for example,
reproduce the problem while running ac3dec in SCHED_FIFO policy,
priority 99

As an additional note, this only seems to happen when using plug:iec958.
Software decoding the AC3 sound and plying through ALSA, works
flawlessly, despite of the fact that the machine load is higher.

I'm using kernel 2.6.5 with the included version of ALSA. My alsa
library is 1.0.3b, from the Debian package, and I'm testing with a
SBLive! card attached to the receiver with an optical link through a
simple third party SBLive! add-on. I have seen the same problem with 2.4
kernels, together with self-compiled recent (post 1.0) ALSA versions.

Any clue?

M. S.
-- 
Martin Soto <soto@informatik.uni-kl.de>



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