[mythtv-users] Sound problem with AC3 passthrough

[Rob Baumstark]

Over the past week or so I've been teaching myself linux/MythTV, and
I've been making slow but steady progess, usually solving a problem a
day.  I've gotten to the point now where virtually everything is
working they way I want (or I haven't worked on it yet and don't
expect it to work), except for sound.  I'm running a single
frontend/backend box with a firewire connection to a DCT-6200 as my
only tuner.  I've even managed to get the firewire connection stable,
though its changed nodes on me a couple times now.

Now so that the problem makes sense, I'm gonna start with a couple
definitions.  Good and Bad channels.  Good channels have AC3 audio
streams @ 48khz, and record/play perfectly - some examples are
standard-def movie central, and all my high-def channels (which
include 5.1 and 2.0 AC3 streams).  Bad channels are the basic cable
channels - these come over firewire with a 2.0 channel 32Khz AC3
stream.

My problem seems to be related to the 32Khz sample-rate of the AC3 in
the bad channels - my HTPC wants to resample it up to 48Khz, but at
the same time it wants to leave it as AC3 and pass it through.  This
results in the audio playing back at super-high pitch (chipmunk mode,
whatever you wanna call it), though still at the right speed (it stays
in sync with the video) and it tends to make the video rather choppy.
If I disable AC3 passthrough then bad channels playback nicely, but
good channels all get downmixed to stereo.

I can also replicate the problem in mplayer with a small mpeg file
from test-mpeg2 from a bad channel.  "mplayer -vo xv test.mpg" plays
back perfectly, and my receiver indicates its getting 48Khz PCM.
"mplayer -vo xv -ac hwac3 test.mpg" plays back choppy with super-high
pitch audio, and my receiver indicates its receiving a dolby-digital
2.0 stream.  I'm afraid the receiver only displays the sample-rate on
PCM streams.

The only solution I can think of would involve modifying MythTV to
detect the sample-rate of AC3 audio and automatically toggle
passthrough for me.  Or add an option to every channel to enable to
disable AC3 passthrough.  Neither are options I really like - though I
realize there are limitations in alsa with digital sample rates, I
don't see why an existing AC3 stream can't just be passed through
unmodified.