[mythtv-users] Capture card luminance bandwidth (BT8xx and PVR-250)

[Cory Papenfuss]

 	OK folks... I've finally fell off the deep end.  I've recently 
been trying to figure out how to get more analog resolution out of my 
PVR-250.  Looking through the SAA7115 datasheets, I've come to a 
conclusion that I would like to run past those in the video know.  From 
what I can see, the fact that the PVR-250 gets 400 lines of resolution 
with my test pattern is actually fairly impressive.  From what I can tell 
looking through some datasheets, the luminance bandwidth is limited to 
about 240 lines unless a comb filter is used.  The notch filter employed 
by the BT8xx chips (and the SAA7115 if so configured) makes the luminance 
signal crap out at about 3MHz.  At about 80 lines/MHz, that's only 240 
lines (i.e. about 330x480 for 4:3 capture).  My experience with 
BT8xx-based cards has been exactly that... anything over about 352x480 is 
pointless.  Even with a Kell-factor correction (of, say, 0.7), that's 
about 440x480 for a BT8xx-based card.

 	The PVR-250 with its SAA7115 chip (2-line adaptive comb filter for 
NTSC) gets about 400 lines from my EIA1956 test pattern.  When I disable 
the comb filter (hack the ivtv driver a bit), I get very comparable 
performance (resolution-wise) to a BT8xx chip (about 352x480 max useful 
capture size).  Does this sound plausible?

 	If that's true, then there should be a page posted about the 
objective comparisons of BT8xx and PVR-[1235]50-based cards.  Basically, 
it means that if you are using a BT8xx-based card, capturing anything over 
about 440x480 is absolutely and completely pointless.  Similarly, 
capturing anything about 640x480 for an IVTV device is similarly 
pointless.

 	I always knew that NTSC was crap, but this is a new level of 
revelation.  Of course, for S-VID this whole discussion is somewhat moot, 
since it can have fairly high bandwidth. by comparison.

Supplying food for though,
-Cory


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* Cory Papenfuss							*
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University 			*
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