# AC-Coupling on Composite video

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)?

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".

In this case AC coupling does not mean that the signal DC bias is zero or floating. The receiver can and will bias any DC voltage it wants, for example using sync tip clamping. The voltages for blank, black, white etc are referenced as AC peak-to-peak voltages referenced to sync tip, so DC bias is really not defined on cable anyway.

AC coupled means the receiver will take care of DC clamp to avoid ground errors.

DC Offset will cause a shift in bright/darkness. Gain affects Contrast relative to 50%.

simulate this circuit – Schematic created using CircuitLab

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)?

Yes it does. So the technique is basically to use a negative peak detector to clamp the sync tip to some known value, then derive the sync tip timing from the clamped tip.

With the timing known, a sample-hold grabs the clamped signal value during the blank interval, and uses this to offset the video portion. The resulting signal has a known value for the blank video, which is itself less than zero intensity (blank is "blacker than black"). Now the clamped video can be offset so that black is at zero, and the rest follows.

The key is to realize that the signal processing chain uses non-linear processing to restore voltage levels on a line-by-line basis.

• Actually, sampling the blanking level and comparing it to the sync level is a way of establishing the correct signal path gain, not really what the OP is asking about. – Dave Tweed May 30 '19 at 13:30
• @DaveTweed He's correct . Actually It's *offset or bright/darkness. Not Gain is which is Contrast. DC offset is avoided using AC coupling. – Tony Stewart EE75 May 30 '19 at 13:34