Many sources say that composite video input should be AC-coupled. Voltage levels for blank, black and white signal are also specified. How is it possible to detect these voltage levels after AC coupling (aka dc-blocking capacitor)?
Doesn't the DC-component (average) of the signal vary wildly depending if the content is all black (0V-0.3V) or all white (0V-1V)?
Answer
Yes, it does. That's why the next step is usually DC restoration, which clamps the sync peaks to a specific voltage. (The component following the blocking capacitor is a diode, rather than a resistor.) This then establishes the correct relative voltages for "black" and "white".
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