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

[N Dugas]

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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