Date: Tue, 1 Dec 1998 17:01:31 +0100 (MET) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: mmt-ref@tu-dresden.de Cc: roger@cs.strath.ac.uk, multimedia@FreeBSD.ORG Subject: Re: BT848 adaptive AGC Message-ID: <199812011601.RAA02548@labinfo.iet.unipi.it> In-Reply-To: <Pine.GSO.3.95.981201124934.3754A-100000@rncmm2.urz.tu-dresden.de> from "Christoph Fleck" at Dec 1, 98 03:38:23 pm
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> Thanks a lot for your fast answer.
>
> > > Can the BT848 driver enable the adaptive AGC mode?
>
> > > The adaptive AGC could avoid this.
> >
> > I doubt it!
...
> > The "CRUSH" bit is disabled by default (at least in my copy of the
> > sources) because I found out that the adaptation process is one-way:
> > it reduces the gain, but never brings it up again. So when you have
> > bad signal (usually from the tuner) and the capture chip looks for
> > sync pulses in the wrong place, the image turns rapidly black.
> > I think you don't want this feature, anyways.
>
> I could not play with this option in the past. Is it even not useful on
> very smooth videosignals?
no it is not, because it is unidirectional and it only serves to avoid
saturation on the ADC.
> > yuv422_prog(), yuv12_prog() -- i am not sure if this is a mistake
> > or it is intentional. You can easily try to disable it yourself by
> > commenting out the three pairs of lines in the above functions.
>
> I am not yet looked at the code. A small prog to set the registers
> at runtime could help me very much.
invest 10 minutes, look at /sys/pci/brooktree848.c, find the 3 places
where {e|o}_scloop are set and change the value to 0 (or comment out
those lines). This is really the quickest way to do (after all you have
only two possible values so it does not pay to modify/add some ioctl to
do the same job).
> If there is no solution by the software to freeze or even limit the restless
> AGC, I probably try to build an external referencevoltage for manual adjusting
> of gain control. But this should be the very last.
nor very effective as if the problem is agc-related you'd have to track
the input signal and understand how to defeat the agc...
luigi
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