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: gregson@tuns.ca
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...
Coupling
Cap
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 jeffs39@skypoint.com
Date: Wed, 22 May 1996 18:33:32 -0700
Original Subject: Re: a good video black level clamp