[mythtv-users] bitrate questions

[Maarten]

Something has me stumped for a while now.  I just have to ask...
(Note: I'm not complaining about anything, just curious...)

I've been running mythtv for 6 - 8 months now, and I use 480x480 res @ bitrate 
3000, which gives me reasonable quality (my feed isn't too good to begin with 
and I have a fairly crappy bttv 8x8 card, not a PVRx50 or similar).
I started at default settings (bitrate 2200) but that gave me more artifacts 
than I liked, so I slowly upped that to 3000.
This setting means mythtv takes a little more than 1 Gig per hour.  

Now to what I don't understand. Supposedly, the mpeg4 encoder which I use 
gives somewhat smaller files than what an mpeg2 stream coming from a PVRx50 
would give, right ?  But on the other hand, SVCDs are 480x480 res, always 
mpeg2, look about the same (or even just slightly better) as my mythtv 
recordings but they manage to squeeze 80-100 minutes on a 800 MB CD.
So, taking the (supposedly) more optimized mpeg4 encoding into account as 
well, where does this discrepancy in space come from?

Mythtv: 	480x480, mpeg4 yields		1.1GB/hour
SVCD	480x480, mpeg2 yields		0,6 GB/hour

Well, obviously the SVCD uses a lower bitrate, but how do they manage to use a 
lower bitrate while still getting good quality ?  They fit almost twice as 
much video in the same space...

TIA,
Maarten

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