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