Fading a video signal is not as simple as attenuating the whole signal, since there will be a point where the sync amplitude becomes too small. Not only the fading effect, but you’ll get also a rolling and tearing picture if you do this. One solution for this can be as complex as running a separate sync signal into the monitor, but this is not a low cost solution.
Here we present a simple video “volume control”, that fade the picture to black while maintaining a good video fidelity. LT1360 (U1) and its associated components form an elementary sync separator. R1, D1 and C1 clamp the composite video signal. To compensate for the drop across D1, D2 is employed to bias the input of U1. The sync information that occupy the most negative portion is amplified by U1 when D1 conducts. In the feedback network of U1, the clamp circuit (D4 to D8) prevents the amplifier from saturating. The shape of the sync waveform is improved by D3 and CMOS inverter U4.
Most of video signals will works with this circuit, except for very noisy or distorted signals. The rest circuit is an LT1251 video fader (U2) configured to fade the signal ranging from the original level to black level. [Circuit schematic diagram source: Linear Technology Application Notes]
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