Peter Gregson wrote:
> I'm looking for a simple GOOD black level clamp circuit for clamping a
> video signal.  Currently, I use the black level timing indicator out of
> an LM1881 to indicate when the video signal is displaying the black
> level. I then use a diode-switched sample and hold to capture the DC
> black level, save this on a holding capacitor, and input this signal
> into an op-amp subtractor (the other input is a desired clamp level).
> The output of this is fed back to the bias circuit at the video
> input so as to achieve the desired clamped video signal.
> Peter H. Gregson, Ph.D, P.Eng,
> Computer Vision and Image Processing Lab
> Department of Electrical Engineering
> Technical University of Nova Scotia
> Halifax, Nova Scotia
> B3J-2X4
> telephone:  (902) 420-7783
> FAX:        (902) 422-7535
> e-mail:

The way I have seen this done is to clamp the actual video to the level 
you want. (Or maybe I made this up?) If I remember right...

Video In -----|(--+-------- Clamped video
     FET switch  [ ]--- Signal from LM1881
                 GND (Or what ever voltage you want)

If I understand what you are trying to do, I think this should work.

BTW, the LM1881 data sheet says if you have noisy video, use the RC 
filter they show for the video. I think it was a 510K ressistor and .1 uf 
or something like that. I ALWAYS use the filter.

Jeff Sampson

Date: Wed, 22 May 1996 18:33:32 -0700

Original Subject: Re: a good video black level clamp

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