[mythtv-users] 32khz AC3 Streams was: Sound problem with AC3 passthrough

[Rob Baumstark]

It's been most of a week with no new replies around here.  I've spent
the time googling around and asking everyone I can find about my
problem, and I still haven't come up with a solution.  I've pretty
much narrowed it down to the fact that ALSA and/or the nForce2
driver/hardware can't do what I want it to do - pass a 32khz AC3
stream to my receiver.  PCM audio (from mp3's, etc) is correctly
resampled to 48khz and passed out as PCM over spdif.  48khz AC3
streams are correctly passed through over spdif as well.  32khz AC3
streams attempt to pass-through because they're AC3, but get screwed
up in the attempt to resample to 48khz.

I'm pretty sure at this point that what I need is MythTV to check the
sample-rate of AC3 streams before it attempts to pass them through,
and automatically disable AC3 passthrough for streams that are not
48khz.  I have yet to see an AC3 stream thats not at 48khz and that
also contains more than 2 channels, so there would be no loss of
surround-sound with such an option.

I realize it's a fair bit to ask, as it seems to be a rare problem.
But I just spent an hour looking at the audiooutput code, and with
more than 10 years since I've coded in C/C++ I'm just getting more
confused.  And I think that as firewire recording becomes more stable
(DVB may also be a source of 32khz AC3, I don't know) more and more
people are going to start encountering this issue.

On 5/31/06, Rob Baumstark <rbaumstark at gmail.com> wrote:
> > Have you looked at the Digitial Sound HOWTO on the Wiki?
> >
> > Kevin
>
> Yes - it was what got my digital sound working in the first place.  As
> I had mentioned, sound works, digital sound works, PCM over digital
> works (but limited to stereo), AC3 passthrough works.  But AC3 encoded
> at 32khz causes a lot of wierd issues.  I've spent at least 5 hours
> today on the wiki, the mailing-list archives, and google, and the
> closest thing I found to my problem was a person at the AVS forums
> complaining about 32Khz AC3 causing him problems too, but he didn't
> have a solution either.
>
> Also - I forgot it in my original post, but it might be useful to
> someone...  This box is a Athlon 2500+ on a Abit-NF7 (nForce2 /
> soundstorm chipset).  I don't see how the rest of the hardware in
> there could be related to this problem, but if anyone wants full
> details, or output captures of mythfrontend or mplayer playing
> captures from good or bad channels, etc. just ask me and I'm happy to
> provide.
>