[mythtv-users] Transcoding 101

Nick Rout nick.rout at gmail.com
Sun Aug 10 21:38:52 UTC 2008


On Sat, Aug 2, 2008 at 1:58 PM, C.T. Paterson <i.adore.my.64 at gmail.com> wrote:
> 2008/8/1 C.T. Paterson <i.adore.my.64 at gmail.com>:
>>
>>
>> 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.
>>

Be specific. mpeg4 covers many different encoding possibilities.
xvid/divx is mpeg4, so is h.264, but you will end up with very
different quality and file size.

Although I am not advocating another bt discussion, the file sizes of
commonly downloadable TV programmes is illustrative. They commonly
come in varying quality and filesize, but commonly there are 3 broad
groupings used by the "scene":

1hr minus ads  = 45 mins

- at SD/xvid/mp3/avi commonly 350 MB (1/2 of a CD)
- at SD/xvid/mp3/avi higher quality, 700 MB (1 CD)
- at 720p h.264/ac3/mkv, 1.1GB (1/4 of a DVD)

You should be able to acheive similar on your myth box. What "target"
you want to aim for depends on how much disk space you want to use,
and what quality you want to watch in.

>> 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}

Well in answer to the OP's question about quality, he is starting from
HD so reducing to 640x480 certainly will reduce quality. Of course
there are a lot of variables in "quality" - if he is watching via an
SD tv, it won't matter a lot.


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