[mythtv] MPEG2-MPEG2 transcoding: Illegal video stream

Geoffrey Hausheer mythtv0368 at phracturedblue.com
Thu Mar 2 17:02:04 UTC 2006


On 3/2/06, Thomas Börkel wrote:
> Can you prevent that from happening? Is this less likely, if I set
> cutpoints only on I frames?
No.  As the audio and video don't use the same frame size, you will
lose some time at every cutpoint.

>
> Until Myth 0.19, I used avidemux do cut. OK, it can only cut reliably on
> I-frames, but I never got sync problems.
well by itself, it can do the same thing I do in mythtranscode, but
any tool that operates on the resulting file will have the same
problem.  The fact that you haven't noticed it just means that your
audio frame size matches your video frame rate pretty well.  you would
get different vesults depending on the audio bitrate, but I'm not sure
what settings would be best/worst

>
> > (so you could be skewing the audio by 80ms per cutpoint).
>
> Usually, I have a cutpoint at the beginning and end and then 2 for each
> comm break (normally 2 for a series). So, it's 6 cutpoints and this
> could result in up to 6 x 80ms = 480ms. Oh, oh...
sorry, I meant 2 frames per cutpoint pair so it is half that.  That is
also a theoretical number.  In practice it will never be this bad, and
could be in the 1-10ms range depending on your encoding frame-rates

> > For you to use these streams to burn to DVD, you need to specify the
> > '--ostream dvd' flag, as ths will create NAV packets, which should
>
> Where would I specify this? In the GUI? Or do I have to use
> mythtranscode from the command line?
You need to use the command line until I add a GUI item.

>
> I once tried mythtranscode 0.19 from the command line, but it ignored my
> cutlist. I tried by specifying the filename and by specifying
> channel/start time. I used options --mpeg2 --honorcutlist --showprogress
those look fine.  you need to use --starttime and --chanid instead of
--infile if you want to pull the cutlist from the database.

> But that is not the only reason I demux/remux. I often want to use
> tcrequant on the video stream.
Interesting.  One option here is to allow myththrancode to call
external applications on the elementary streams as it runs, and to let
mythtranscode handle stitching the streams back together.  I'll have
to think about that.  Of course changing the format (like doing an
mp2->ac3 conversion) would make things more complicated, so maybe that
is something best handled by mythtranscode itself.  It has always been
my goal to have mythtranscode be able to spit out DVD compliant
streams, though that goal may be a long way off, as there are some big
hurdles in the way.

I need more input about your specifci issues, since I can't reproduce
them so far.

.Geoff


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