[mythtv-users] Losing audio on "lossless" transcode

Ian Barton lists at manor-farm.org
Fri Oct 17 14:36:04 UTC 2008

>> How can I get MythTV to do a lossless transcode on transport streams
>> consisting of MPEG2 video and MP2 audio?  Neither MPEG2 nor MP2 show in
>> the transcoder options.
> Without wishing to dishearten you I haven't managed to find a single
> transcoding profile that will work with all UK Freeview DVB-T
> recordings.  What works for some recordings fails on others.  I
> believe that it's down to the variety of audio and video encoding
> times that are used.
> Having said that I don't recall the lossless transcode producing files
> without audio.  For me it errors just gives up on about 50% of the
> programems I try, without producing a file.  It's not a random error
> because some shows will always work while others always fail, so it's
> something to do with the encoding.  Are you testing using recordings
> from the same show or channel? If so you could try a different channel
> to see if you have more luck.
> From time to time I do manual transcodes, using ffmpeg and this is
> where I have to fiddle with the audio parameters sometimes to get
> sound.

I use mythtranscode and ffmpeg called from a custom python script. This 
works for most UK DVB stuff. However, mythtranscode fails miserably on 
some recordings, usually from UK TV History. It complains that it can 
find any video frames, even thought the recording plays fine.

My court of last resort, which hasn't failed yet, is to demux them using 
ProjectX and the remux.

Note before doing any other transcoding on DVB recordings you need to 
rebuild the seektable and do a lossless mpeg->mpeg transcode using 
mythtranscode, or you will get audio/video sync problems.

The two commands I use are shown below. The %s is a parameter 
interpreted by the python script.

'/usr/local/bin/mythtranscode --showprogress --honorcutlist --mpeg2 -c 
%s -s "%s" -o /tmp/%s.tmp'

'/usr/local/bin/mythtranscode --mpeg2 --buildindex  --showprogress 
--chanid %s  --starttime "%s"'


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