[mythtv-users] transcoding HDTV increases the file size

Michael T. Dean mtdean at thirdcontact.com
Sun Aug 13 17:57:21 UTC 2006

On 08/13/2006 12:59 PM, Patrick Wagstrom wrote:
> 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
> this:
> Kindergarten Cop: 9.6 GB => 11.8 GB (Autodetect)
> So, umm, anyone know whats going on?

Bitrate.  Bitrate.  Bitrate.  (File size is determined 100% by bitrate.)

Are you using the option:

Scale bitrate for frame size
If set, the MPEG4 bitrate will be used for 640x480.  If other 
resolutions are used, the bitrate will be scaled appropriately.

If so, try it without and specify an appropriate bitrate (i.e try 
something like 6600kbps).


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