[mythtv-users] Force AC3 for HD-PVR?

Phil Linttell phil.linttell at rogers.com
Tue Jun 23 11:50:02 UTC 2009

I have an SA4250HD, and I don't see any way to force the box to output 
AC3 instead of AAC for stereo.  However, you can set the HD-PVR to 
encode AC3 (even for stereo channels) as follows:

v4l2-ctl --device=/dev/video0 --set-ctrl=audio_encoding=4

It appears to get reset on system boot (or back-end restart?), so you 
may want to add it to rc.local.  Experiment first, however.  It would be 
nice if we could set this in the MythTV recording profile for the HD-PVR.

I was trying to force the audio to AC3, as I noticed that the PS3 (DLNA 
front-end) doesn't seem to like stereo AAC stereo streamed from MythTV - 
I get no sound at all.   However, after setting the audio encoding as 
above, I recorded three programs in a row that would normally have been 
in stereo AAC, and the PS3  identified the audio as AC3 and played it 
fine - making it a pretty nice front-end for HD-PVR recordings.

A caution though.... on my system (trunk 20737) the AC3 audio appears to 
break the preview generating process and results in a debilitating 
stream of messages on the back-end:

2009-06-23 05:59:36.862 [libfaad @ 0x7ff6cc838460]faac: frame decoding 
failed: Gain control not yet implemented
2009-06-23 05:59:36.862 [libfaad @ 0x7ff6cc838460]faac: frame decoding 
failed: Bitstream value not allowed by specification

Anyone have any thoughts on how to fix this one?

The other thing I haven't got to grips with is how to cut commercials 
out of the H264 recording....  I don't want to have to manually skip 
over commercials using the PS3 (ffd/rwd are a bit touchy anyway when 
streaming) so I'd really like to find a solution to cutting them out 
automatically.  I've searched and it seems to simply be not supported 
for H264.

I'm on Rogers cable in Canada, and all channels are encrypted, so all of 
my recordings are through the HD-PVR and in H264.


> Yes, it is doing that for the 5.1 HD channels. The problem is with the
> analog channels that only have stereo sound to begin with. I don't 
> think the
> SA3250HD can be set to output bitstream rather than PCM for analog stereo
> sources.
> On Mon, Jun 15, 2009 at 6:11 PM, John P Poet <jppoet[at]gmail.com> wrote:
> > On Mon, Jun 15, 2009 at 3:56 PM, Christopher
> > Meredith<chmeredith[at]gmail.com> wrote:
> > > I don't seem to see a way to do that on my SA3250HD. My problem 
> was that
> > my
> > > commercial cutting script doesn't like AAC audio. Fortunately, 
> after lots
> > of
> > > trial and error, I got it to work by converting the audio track to 
> AC3
> > > first. Ugly, yes. But it works.
> >
> > I am not at home, so I can't check at the moment, but the choice of
> > AAC/AC3 for PCM input is probably under the recording profiles.
> >
> > It would be better to get the STB to pass-through Dolby Digital,
> > though. Having it downmix the AC3 to PCM, and then having the HD-PVR
> > turn it back into AC3 is likely to incur noticeable loss.
> >
> > There has to be a way to set the SA3250HD to output dolby digital. A
> > little googling, and I see people talking about a "Digital Out" option
> > somewhere in it's settings.
> >
> >
> > John 

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