[mythtv-users] WriteAudio: buffer underruns without prebuffering pauses (SOLVED - almost)

Tom Greer trgreer at gmail.com
Thu Feb 22 13:59:59 UTC 2007


Last night I had an opportunity to experiment. Here are my results.

My baseline setup was:
Extra Audio Buffering: On
Aggressive Sound Card Buffering: Off
XVMC: Off

With this setup, on my test recording (The Closer episode from TNT-HD
@ 1080i) I incurred a "WriteAudio: buffer underrun" at a rate of just
under 1 per secord.  The CPU ran at about 93% busy.

Turning on Extra Audio Buffering resulted in prebuffering pauses at a
rate of about 2 per second - and the audio buffer underruns
disappeared.  CPU: 93%

Turning on the Aggressive Buffering made no difference with or without
Extra Audio Buffering and with or without XVMC.

Turning on XVMC dropped the CPU load to about 30%; but the number of
audio buffer underruns (with extra buffering off) or the number of
prebuffering pauses (with extra buffering on) was unchanged.

Next I changed the "NVAGP" option in my xorg.conf file from 2 to 3 to
force NvAGP to load in place of AGPGART.  Result: no change.

Next up: I modified the "Enable OpenGL vertical sync for timing"
option from on to off.  Result: no audio buffer underruns and no
prebuffer pauses.  CPU load increased to 95+%.

YES!!!

One final test.  The Fifth Element recorded @ 1080i always seems to
work the system harder than any other recording.  Audio stuttering
returns.

AAARGH!!!

At least this setup let's us watch most HD recordings.  I'm sure it
will work well on chick flicks - and only fail on action adventure
movies...

I'm mystified as to why XVMC was successful in dropping the CPU load
but did not make the prebuffering pauses/audio buffer underruns go
away.  But I am not a fan of XVMC (choppy fast forward/rewind, video
glitches, other anomolies) anyway, and I am not inclined to spend a
lot of time trying to make it work.  Of course, if someone has a
simple fix, I'd love to hear it.

My next step: Convince wifey to let me move this frontend box to that
old projection TV in the living room and build a new box with more CPU
horsepower for the plasma screen.


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