[mythtv-users] Using v4lctl to optimise recording quality

Cory Papenfuss papenfuss at juneau.me.vt.edu
Wed Dec 15 14:27:20 UTC 2004

>> 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.
 	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... :)

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

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