[mythtv-users] How do you transcode?

Scott Alfter mythtv at salfter.dyndns.org
Wed Sep 6 18:44:20 UTC 2006

Hash: SHA1

Graeme Wilford wrote:
> I'm trying to resize and transcode PAL mpeg2 to mpeg4 but the results
> show terrible combing artifacts. I've tried various combinations of
> DCT/MV options in the mythtranscode config screen to no avail. If you
> have this working, can you give me your recipe? Perhaps you do this
> with a user job instead?

If your source is interlaced (and it probably is), you might get better results
encoding interlaced video.  I use mencoder at the command line; among the
options available for lavcopts are ildct and ilme.

(Most of the stuff I transcode can be restored to progressive-scan by inverse
3:2 pulldown, so that's what I usually do.)

> Input:
> ~10Mbit 720x576 MPEG2 (PAL, 50Hz, interlaced) via PVR350 svideo-in
> from cable STB.
> Desired Output:
> ~2.2Mbit 600x480 MPEG4, preferrably de-interlaced before scaling

You're scaling vertically by a non-integer multiple.  Unless you have a really
good deinterlacer, this is often going to look nasty.  I'd leave the vertical
resolution alone.  (For that matter, I would be tempted to leave the horizontal
resolution alone and use autoaspect to encode the aspect ratio in the AVI.)

Assuming that the source file has already been edited (and that inverse 3:2
pulldown is appropriate), I'd use something like this to convert to MPEG-4:

mencoder foo.mpg -vf pullup,softskip -ofps 24000/1001 -ovc lavc -lavcopts
vcodec=mpeg4:vbitrate=1200:autoaspect=1:trell=1:mbd=2:v4mv=1:turbo:vpass=1 -oac
copy -o /dev/null && mencoder foo.mpg -vf pullup,softskip -ofps 24000/1001 -ovc
lavc -lavcopts
codec=mpeg4:vbitrate=1200:autoaspect=1:trell=1:mbd=2:v4mv=1:vpass=2 -oac copy
- -o foo.avi

That leaves the audio alone; if you want to transcode it to (for instance)
160-kbps MP3, the second "-oac copy" needs to become something like "-oac
mp3lame -lameopts cbr:br=160:mode=1".  (Leave the first one alone; there's no
sense transcoding audio in the first pass.)

For more hints, you might want to look at this:


 / v \ Scott Alfter
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