Design Ideas: July 6, 1995

Video fader preserves synchronization

Frank Cox,
Linear Technology Corp, Milpitas, CA

The common video effect "fade to black" is usually accomplished by increasing video signal attenuation to the point where the picture disappears, leaving a black screen. As the composite signal is attenuated, the signal's sync amplitude becomes too small to synchronize the picture properly, and the picture rolls and tears. One solution is to run a separate sync line to the monitor, but this is not a viable choice in composite systems.

Fig 1 shows a simple video "volume control" that operates on the picture but leaves the sync unchanged, allowing a smooth fade to black while maintaining video fidelity. High-speed op-amp IC₁ and its associated components form a basic sync separator. C₁, R₁, and D₁ clamp and clip composite video. D₂ biases the input of IC₁ to compensate for the voltage drop across D₁. When D₁ conducts, IC₁ amplifies the most negative portion of the waveform (containing the sync information). Clamp-network D₄ through D₇ in the feedback of IC₁ prevents the amplifier from saturating and, thus, from slowing the response. D₃ and a CMOS inverter complete the shaping of the sync waveform. The sync separator works with most video signals that have a minimum sync amplitude of 0.3V and are not excessively noisy or distorted.

Video-fader IC₂ is configured to fade between the original video and the sync stripped from that video. A voltage reference and a potentiometer generate a control voltage. This node (pin 3 of IC₂) should be bypassed if the control is mounted a significant distance from the circuit or if the control generates any noise when adjusted. As the control voltage nears the lower 2% of its range, IC₂ automatically shuts off the channel with the active video (pin 1) and fully turns on the channel with the sync (pin 14). This feature makes the circuit more tolerant of offset or gain errors in the control signal. (DI ##1724)