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

Rob Baumstark rbaumstark at gmail.com
Thu Jun 15 00:26:23 UTC 2006


Actually I've come across a temporary workaround.

If I just turn on time-stretching to .9, my "chipmunk" channels play
normally.  This means I often have to adjust time-stretch between
channels as I surf around - but thats much better than exiting live TV
to enable/disable passthrough as I surf around.  And like I said at
the top - its a workaround, not a fix.

I'm convinced that this is either an issue with MythTV's AC3 handling
(and there's discussion over in the -dev list about passthrough too,
though it doesn't touch this particular issue with it), and/or an
issue with my sound hardware - other cards may be able to pass the
32khz AC3 properly (I base this on the ALSA sound cards page I
mentioned earlier in this discussion - I have no proof of any card
actually doing this properly though).

On 6/14/06, N Dugas <normdugas at yahoo.ca> wrote:
> Rob Baumstark wrote:
> > 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.
> >>
> >
> > _______________________________________________
> Just catching up on my emails.  I had the same problem as you did.  I
> have an nforce 2 mobo and digital coax worked fine but when I enabled
> AC3 to SPDIF, I'd get chipmunks.  I then bought a Turtle Beach Riviera
> and had the same results with that.  I was recording analog channels via
> firewire from my DCT-6412 but I've since re-installed my Hauppauge
> PVR-350 and record analog channels from that.  No more chipmunks on new
> analog recordings.  I wish I could offer something that works properly
> rather than working around the issue.
>
> Norm
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