Quote:
Originally Posted by miniman82
why not use the LM1881 to trigger the comparator, thereby stabilizing the black level before the beginning of each horizontally scanned line?
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This may work, however you might need to experiment with the value of R set on the LM 1881.
Its nominal value is 680K for NTSC signals, but I have had problems with the resulting pulse being slightly too wide to properly drive a clamp. I've reduced R set to 470K or 560K with better results.
The pulse must start and end during the back porch time and must not extend past the end of the H back porch.
If it does, the clamp will be clamping to a voltage other than ground, the usual voltage level of the back porch portion of the waveform.
Hope you are successful.
Cliff