[mythtv-users] Transcoding and deinterlacing, and other transcode issues

[Brad Templeton]

On Wed, Jun 14, 2006 at 09:15:11AM +0200, Niels Dybdahl wrote:
> >
> >As for the number of profiles an HD user needs, here are the ones I
> >think could be handy:
> >
> >
> >        a) Transocde 1080i to 720 lines, for people with 720 line TVs
> >        for whom storing the extra megapixel is of limited value.
> >            Goal: Reduce size, but lose no quality you would see on
> >            your TV.   De-interlace the 1080i of course before
> >            resizing.
> 
> 
> How do you save size with that? 1080x1920x25 is 51.84 Megapixel/second while
> 720x1280x50 is 46.08 Megapixel/second. There is hardly any difference
> between those numbers.

Well, my understanding is that since compressors all rely heavily on
elimintating redundant information frame-to-frame, frame rate increases
do not add to the byte count as much as raw pixel resoution increases.
(Size does not increase linearly with pixels or frame rate, but it
increases more with pixels than it does with frame-rate.)

I have done more of my experiments in long term stuff, going to 1280x540,
because there one can do perfect deinterlacing.  This looks very good
even at about 2gb/hour, in fact it's decent at far less.

Other experiments that might make sense would be 1280x1080i, so you
retain the interlace lines but lose the information horizontally that
you will be throwing away at the last second during display anyway.
> 
> 
>        d) "Streaming" quality transcodes of SDTV that fit within
> >            your upstream bandwidth (384K or 512K for example.)

And I should note that long term, a cool tool would be one to
act like the slingbox and do this in real time.   I was given a slingbox
but it seems like a real kludge to have a mythtv box output the
recorded video to be hardware recompressed and sent out.  (I would need
tv-out too, which I don't have set up.)

I have a 2nd house I stay at and want to watch my Daily Shows.  So
right now I sometimes transcode them down to about 500kbps which is
what my DSL upstream can handle tolerably, but I haven't got it quite
right.      I think vlc has some live streaming transcode options.

The slingbox though does a bunch of clever things, tracking the outgoing
bandwidth and adjusting the transcode appropriately.   It does
surprisingly well.
> 
> 
> One more possible target: DVD resolution, but that is taken care of by
> MythArchive.
> 
> Of course it would be nice to select whether the output should be
> deinterlaced or not. The transcoding system does even allow adaptive
> deinterlacers, which are prohibited while viewing because of
> CPU-requirements. Are there any adaptive deinterlacers available that could
> be added as a user job?

Actually, I now see that filters have been added to the transcode
profiles.  Presumably mencoder filters?  Anyway, it should be possible to
put in cubic deinterlace filters here.  I am not sure if you should
resize first with interlace-aware resize, then deint, or deint first and
then resize.    Is mythtranscode using interlace aware resize all the
time?  Is this what caused my problems on transcoding 720p video?