[mythtv-users] does mythtranscode affect audio levels?

Chris Pinkham cpinkham at bc2va.org
Thu Apr 6 04:14:39 UTC 2006


* On Wed Apr 05, 2006 at 09:27:52PM +0100, Nick wrote:
> When you transcode HDTV material, does the process also down-mix the
> audio from digital 5.1 to analog 2.0 audio, or does it keep the 5.1
> soundtrack and just transcode the video? Although I don't have access
> to HDTV here in the UK, when I've played with DVD material in the past
> I've had similar some experiences with the converted audio being
> quieter than the source material.

Currently it downconverts the audio from 5.1 to stereo if you use
the MPEG2 -> MPEG4 transcoder.  I'm not sure about the lossless, but
am pretty sure it preserves the AC3 5.1 track.

I am working on some changes to NuppelVideoRecorder which will let
you software encode in MPEG2 format meaning you'll be able to do
MPEG2 -> MPEG2 (lossly) transcoding meaning you can resize, etc.
but still get a mpeg-ps/ts file out of the transcoder so you can
play the recording in an mvp box or via xvmc, etc..

Once I find more time to work on this and it makes it into SVN,
then adding the 5.1 preservation code will be easier.  In order to
do it now, we'd have to extend the nuppel file format again to save
the 5.1 and then copy over all the AC3 and DTS code from
avformatdecoder.cpp to nuppeldecoder and that would be a big
mess.  I think it's much easier to just put the 5.1 ACE/DTS info
in a mpeg-ps/ts container and let the existing avformatdecoder
code handle it during playback.  In my testing, it appears that I
can put a MPEG4 video stream and an AC3 audio stream inside a
mpeg-ts file, which then can be played via avformatdecoder.cpp.
It doesn't appear that libavformat will let me put a MPEG4
stream inside a mpeg-ps file since it expects that to be
VCD/SVCD/DVD/VOB compatible.

--
Chris


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