[mythtv-users] Transcoding 101

Mark Fraser mythtv at mfraz74.orangehome.co.uk
Sun Aug 10 19:45:53 UTC 2008


On Saturday 02 August 2008 02:58:13 C.T. Paterson wrote:
> 2008/8/1 C.T. Paterson <i.adore.my.64 at gmail.com>:
> > 2008/8/1 moodyjunk at frontiernet.net <moodyjunk at frontiernet.net>:
> >> Before going through the bother of setting up transcoding I need some
> >> basic information comparing MPEG-2 to MPEG-4 and can't seem to find it
> >> on the web.  I'm all for saving space, but I want a loss-less recording.
> >
> > To let you know where I'm coming from; I've got some shows that I
> > record regularly, commercial strip, and re-encode as mpeg-4s.  At the
> > end, I have an mp4 with no commercials (not commercial skipped - no
> > commercials).
> >
> >> 1) Does transcoding from MPEG-2 (HD cable) to MPEG-4 reduce the
> >> quality of the recording?
> >
> > Yes.  In my case, though, still perfectly watchable.  I transcode all
> > recordings that I plan to keep around.
> >
> > Ogg Vorbis' sister encoder for video is Theora.  I've now told you
> > everything I know about it - but it might be worth some investigation
> > for you if *further* loss (remember - mpeg-2 is lossy) is a big
> > problem.
> >
> >> 2) How much space does it save?  10%, 50%?
> >
> > Using archived Simpsons episodes as examples; the mpeg-2 recordings
> > (with commercials) are 1.2 gigs.  The commercial-free mpeg-4s clock in
> > at around 160 megs, give or take.
> >
> > If we extrapolate the 160 megs to be 22 of the 30 minutes we get
> > around 220 megs...so about a 80% savings from the re-encoding.
>
> People have rightly pointed out that the savings depends on your
> settings.  My script is below to help inform my numbers;
>
> mencoder ${1} -ovc lavc -vf scale=640:480 -oac mp3lame -o ${2}
>
> Cheers.

Sorry to get to this a bit late, but what would I use for scale from DVB-T in 
the UK? As far as I can tell, 640x480 is for square pixels on ATSC.

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