[mythtv-users] SVN mythtranscode

Steve Adeff adeffs at gmail.com
Thu Jan 12 13:49:11 UTC 2006


On Thursday 12 January 2006 06:17, Adam Egger wrote:
> On 1/12/06, Phill Edwards <philledwards at gmail.com> wrote:
> > > Haven't seen anyone reply to this yet so I will.  Notice he said
> > > 'lossless'. The MPEG2->MPEG2 transcoding in SVN is for cutting only, it
> > > does not re-encode the frames (except around the cutpoints I believe). 
> > > So, the space savings from using it are because you get to cut out
> > > those 20 minutes of commercials out of every hour of show and if you're
> > > using TS files, you may save space since the output format is PS.
> >
> > Thanks Chris. What prompted me to think that lossy might be an option
> > in SVN was that he said "...using the -m switch (which should turn ON
> > lossless) creates a file the same size as without it". That implies
> > that if you don't use the -m switch you get a _lossy_ conversion which
> > is what I actually want! If there is no lossy trancoding, why is there
> > a -m flag to specify lossless if that's all it does anyway?
>
> -m doesn't mean lossless, it just means "Perform MPEG2 to MPEG2
> transcode". You can change the transcode settings for this profile to
> lossless in mythfrontend.
>
> Adam

to add to Adam's remarks... you CAN'T do a lossless transcode from a lossy 
format to a lossy format. it's impossible. you can do what may seem an 
imperceivable loss in quality, but inherent to lossy formats, any transcoding 
done will loose quality. If you do any decrease in resolution, filtering, 
etc, even more so.

what the new SVN MPEG2->MPEG2 transcode can do when just doing commercial 
cutting is to copy frames where possible, and do a reencode of frames that 
require it around cut points only. I believe the quality level of this 
reencode is set similar to the rest of the mpeg, maybe slightly better to 
help preserve quality, Geoff would be better to explain what his work did.

-- 
Steve


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