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Clamping DC Biased Composite Video Signal Back to ground
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xavier60:
Sync separation is easy so long as not too much low frequency content has been lost from the video signal.
The sync tips are clamped at 1V. It cant be set to 0V.
xavier60:
Maybe a small MOSFET could be used for clamping. Something like a 2n7002 or lower rated to keep the capacitance low.
IconicPCB:
A simple one transistor clamp.

One side of capacitor is clamped to ground the other ( input ) floats at an offset.
xavier60:
I tried unidirectional clamping years ago. I found that it didn't cope well with scenes that contained large brightness difference  between top and bottom.

Extra: A good test of the performance of sync and clamp circuits is to put an undersized capacitor in series with the input video cable like 47uF. This causes large fluctuations in sync tip level with various video content.
syntax333:
I manage to solve the problem. It was broken decoupling cap at sync separator LM1881. After I replaced it, LM1881 triggers as it suppose to.

Which results clamping the CVBS to ground.
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