[mythtv-users] Transcoding and aspect ratio (DVB-T)

[Osma Ahvenlampi]

On ma, 2006-02-27 at 04:57 -0500, Chris Pinkham wrote:
> I don't know about the individual frames, but the nuppel container we
> are using does support storing the aspect ratio.  I changed
> mythtranscode
> and NuppelVideoRecorder a long time to preserve the aspect ratio of
> the
> source material instead of using 1.0 as the aspect (which really meant
> 4:3).  I also changed the mplayer patch to use the true aspect ratio
> from the .nuv file.
> 
> I've seen others report this, but it's not occurring for me and my
> 16:9 transcodes from MPEG-2 to MPEG-4 so I can't debug it.

I think the issue is that since MPEG-2 contains the aspect ratio on a
per-frame level or at least embedded into the stream multiplex in any
point, the aspect may change during recording. It routinely does in the
Finnish DVB-T transmissions, where it can be for example that a movie is
broadcasted in 16:9, but the commercial breaks are in 4:3 (which
incidentally makes it really easy for the commercial detection code to
work). 

As I guess the nuv container only contains the aspect information in the
beginning of the stream (which technically makes it not a stream, btw),
mythtranscode picks the first available aspect information from the
stream and uses that. What may easily happen is that since the recording
usually begins a couple of minutes prior to program change, the first
data is incorrect - and during transcoding the later correct data is
thrown away.

I've found that when I make a cutlist before transcoding and ensure that
the post-cutlist program starts from the point where the aspect ratio
matches the real recording content, whether 4:3 or 16:9, the transcoded
MPEG4 .nuv is correct. If I just let the transcoder work on the whole
recording, I'll have to override aspect ratio during playback.

-- 
Osma Ahvenlampi   <oa at iki.fi>    http://www.fishpool.org