[mythtv-users] Sound problem with AC3 passthrough

[Kevin Kuphal]

Rob Baumstark wrote:
> 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.
>   
Have you looked at the Digitial Sound HOWTO on the Wiki?

Kevin