Thursday 24 July 2014

DC restoration of a video signal


I have a composite PAL/NTSC video signal. One problem I am having is getting a reliable sync signal from it; picture brightness causes the sync pulse (normally 0.3V) to vary; so much that my scope won't trigger on it reliably (with the video trigger.) I've heard I need a DC restoration or DC clamp circuit, but I can't find any information on this, except occasional references in books and on the internet, but no detailed information. Does anyone know what circuit I need?



I have previously used an LM1881 or LMH1980 to extract sync because they build the DC clamp/restoree parts in. However, the problem with these chips is that they are expensive and often bigger than a discrete solution.




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