[mythtv-users] transcoding HDTV increases the file size

Patrick Wagstrom wagspat at iit.edu
Sun Aug 13 16:59:33 UTC 2006


I'm trying to wrap my head around some of the transcoding stuff I've
been seeing on my system -- particularly regarding HDTV.  I currently
use a firewire connection off a DCT-6200 to record HDTV and a WinTV
PVR-250 to record standard def.  My system runs MythTV 0.19.  Usually I
get fairly good compression on stuff after cutting commercials and
transcoding for SD.  Something like this:

Battlestar Galactica "Pegasus": 1.6 GB => 889.9 MB (Autodetect)
Family Guy "Fifteen Minutes of Shame": 782.8 MB => 462.5 MB (Autodetect)

Not bad at all, takes the file down to 60% of it's original size.

However, for HDTV, the performance is not so great, when recording
Battlestar Galactica in HD off Universal HD, there are a lot of extra
commercials at the end of the program.  I take an hour or so of
recording and crop it down to the 37 minutes of actual show.  I would
think I should get at least similar compression, approximately 60% of
the original size, but I actually end up with 80%.  Maybe that's an
outlier or something, but I'd think that cutting out 40% of the program
would give me about a 40% space improvement.

Battlestar Galactica "Colonial Day": 5.2 GB => 4.2 GB 

Here's where it get's really interesting.  Many HD programs actually
become larger after transcoding.  For example, UHD ran Kindergarten Cop
without commercials the other night.  I trimmed it to remove the little
bit before the recording and the little bit after the recording.  In
all, I probably cut 6 minutes out of 120 minutes (5%).  I wouldn't
expect much of a performance boost.  Imagine my surprise when I saw

Kindergarten Cop: 9.6 GB => 11.8 GB (Autodetect)

So, umm, anyone know whats going on?


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