[mythtv-users] Using v4lctl to optimise recording quality

Bruce Markey bjm at lvcm.com
Wed Dec 15 10:36:42 UTC 2004

Phill Edwards wrote:
> 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.

> I can run this no problem from the command line as root, but when it 
> runs from rc.local at boot up it segmentation faults. Any suggestions on 
> how I can work around this?

Above. Maybe it was running before bttv was loaded?

> Also, do people generally agree that it's a good idea to use this or 
> should I not really bother?

Well, I do of course. I have different brands of cards that
have different chroma characteristics and this normalizes
them so I get pretty much the same color now for all cards.
One potential problem is that if there is distortion in the
analog signal before it is digitized, there is nothing you
can do to fix it after the fact so correcting the chroma
before recording is a good thing.

> Finally, when I run "v4lctl -c /dev/video0 list" I've noticed that the 
> audio mode is mono. Does this mean my recordings will all be in mono. 
> Should I use v4lctl to support stereo recordings? I've tried to do this 
> but it doesn't seem to change the setting from mono to stereo.

Not all cards have dbx-TV stereo decoders. I have two that do
and three that don't. If the card doesn't do stereo you can't
set it. However, I believe the card detects the signal and
sets the appropriate audio mode so I think the current value
is more informational than something to be set.

--  bjm

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