[mythtv-users] Transcoding 101

[C.T. Paterson]

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.

-- 
"My country is the world, and my religion is to do good."
 -- Thomas Paine