[mythtv-users] HDTV to DVD w/o decode and encode

Scott Alfter mythtv at salfter.dyndns.org
Wed May 11 04:44:29 UTC 2005


Joe Barnhart wrote:
> I'm not alone in my search for a decent tool to archive shows
> originally broadcast on HDTV onto DVD media for keeping or sharing with
> others.  For example, I'm not an Elvis fan, but my friend is and I want
> to give her a DVD of the recent broadcast she missed.
> 
> The show is about 16G bytes in its original 1080i presentation.  There
> is a simple scaling relationship between the original format (1920x1080
> MPEG2-TS) and the desired format (720x480 MPEG2-PS).  Why is there no
> program that takes one MPEG2 stream and scales it by a simple fraction,
> without decoding and re-encoding the MPEG structure?

What you're asking is impossible; MPEG doesn't work that way.  The only
lossless manipulation of MPEG data that might be possible would be to crop the
frame to a smaller size (you would need to crop by multiples of 16 if you were
to do this) or rotate the entire frame a multiple of 90 degrees.  MPEG video
gets coded in macroblocks of (typically) 16x16 pixels.  To scale the video, you
would have to decode the macroblock, scale it, and create a new macroblock.
For shrinking from HDTV to DVD, each new macroblock would pull data from
several macroblocks in the original video.

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