Interesting thing on the combfiler. From what I can tell (briefly looking through the code), it's mostly a post-processing hack. The bt87[89] chip does *not* have a hardware comb filter on the luma channel, and thus relies on either a lowpass or a chroma notch. At least that's what the datasheet says... :)I read on http://mythtv.org/pipermail/mythtv-users/2004-March/037866.html that it's a good idea to use "v4lctl -c /dev/video0 setattr 'chroma agc' on" to really improve colour quality.
Since that posting I finally got around to doing the obvious and set it as an option parameter when loading the module:
options bttv chroma_agc=1 combfilter=2
The combfilter is a very good thing also as it gets rid of the multicolored flickering around high contrast edges. I read somewhere about how the combfilter options work and that "2" did a better job. Seems to work well.
Basically that means that while you can post-process a phony up a "comb-filter" to get rid of some color artifacts, you still lose all luminance bandwidth at the chip level. That's why the BT8x8 chips have excruciatingly low resolution (352x480 at *best*). The best they do is lowpass the luma to keep out the chroma... and that knee is at best 3MHz. At 80 lines per MHz, that's 240 * 4/3 = 320, so 352x480 is being a bit generous. With S-vid you can do better since it bypasses the Y/C demod.
Just thought some people might light to know... :)
-Cory
************************************************************************* * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * *************************************************************************
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