[mythtvnz] Transcoding help?...

[Tony Sauri]

On Sun, 09 Aug 2009 20:14, Ross and Jemima Knudsen wrote:
> Hi all,
>
> Well I was hoping that I might be able to do this all by myself but it
> doesn't look like I will be able to.  I've been trying hard to figure
> out how to transcode recordings so i can store them and free up my
> recording drive.  

I have been trying to do something similar but not for the same reason.  I do 
not have a HD capable display system and wish to transcode the DVB-T 
broadcasts to SD so I can watch them.
>
> However one good thing did come from that is that I managed to use the
> patch (based on Paul Kendalls patch) Handbrake have for ffmpeg and apply
> it to the ffmpeg svn, compile and install it.  Now ffmpeg can correctly
> identify the streams in recordings but I can't get it to transcode
> successfully.  

Yes I have done that too.

The specific problems that I have run into are:

1)  mythtranscode seems to be a bit flakey and from time to time segfaults 
and/or returns a 134 or a 139 error code.  (mythcommflag also occassionaly 
gives a 139 error code).

2) There is no way to command mythtranscode to decimate the frame rate from 50 
to 25 on appropriate streams (TV1 and TV2).

3) When using ffmpeg, for a long time I found it difficult to maintain AV 
sync.

My current preferred ffmpeg command line is:

ffmpeg -i inputfilename -target pal-dvd -f vob -async 4 outputfilename.mpg

I have not come to the end of my journey yet ... it is still necessary to 
rebuild indexes and do the commflagging.

I am currently writing a userjob that will analyse each stream and transcode 
it appropriately and then reindex and commflag.  Also do the necessary bit 
fiddling in the database.

> Initially I'm trying to transcode the audio only (copy the video stream)
> so that I can further process the video like in Avidemux, DeVeDe, etc.
> This should be a relatively quick process as it doesn't have to
> re-encode the video.

Try
ffmpeg -i inputfilename -vcodec copy -acodec libmp3lame -ar 48000 -ab 128k -ac 
2 -f mp4 -async 4 outputfilename.mpg

This example cuts audio to stereo mp3 if you wish higher spec audio I cant 
help you at the moment as I can't test better than stereo on my system.

It also creates an MP4 file rather than a mpegts.  My experiments indicate 
that you get a worthwhile file size reduction just by this simple step.  
(When using ffmpeg to produce a mpegts it reports a 60% muxing overhead!)

Hope this helps a little.

Note:  All invocations of FFMPEG are the version with Paul Kendall's patch. 

Regards

Tony

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