Circuit facilitates video fading

JM Terrade, Clermont-Ferrand, France -- 3/15/2001

When you're copying videotapes, it's sometimes desirable to suppress some passages. Using the pause control of the recorder does not yield satisfactory results. Another method produces better results (Figure 1). The video source connects to the video-in plug, and the recorder connects to the video-out plug. Turning potentiometer P₁ adjusts the image brightness from normal video to a black image. With the P₂ potentiometer ganged to P₁, the sound also varies accordingly. The objectives in building this circuit are to use inexpensive, readily available components and to obtain batteryless operation. The video signal follows two paths (Figure 2). In the first path, the signal undergoes amplification by a factor of two and connects to one end of a potentiometer. In the second path, the synchronization pulse, separated from the input signal, connects to the other end of the potentiometer. The wiper of the potentiometer connects to the second video amplifier, which provides the video output.

When you adjust P₁'s wiper from one end to the other, the video image disappears and fades to a black screen. Because P₁ and P₂ are ganged, the sound follows the image brightness. The circuit could have used triple integrated video amplifiers, such as an AD813, and a video sync separator, such as an LM1881. However, these ICs are expensive (approximately $25) compared with the six standard transistors shown in Figure 2. R₁ sets the input impedance at 75Ω. Q₁, Q₂, and associated components form a video amplifier with an approximate gain of two. R₂, R₃, and D₁ set the dc voltage, and C₁ blocks any dc voltage from the source. The amplified video signal connects to P₁ through R₄ and the C₁ dc-blocking capacitor. R₅, Q₃, Q₄, and associated components form a sync separator. The sync pulse connects to the bottom of P₁ through R₆ and P₃, an adjustable voltage divider. The wiper of P₁ connects to the second video amplifier comprising Q₅, Q₆, and associated components. You can adjust the amplification with P₄. R₇ sets the output impedance at 75?.

P₂, ganged to P₁, is a simple voltage divider, using C₂ to block dc voltages. The sound input uses the left channel, and the output goes to both the left and right channels. With a video source connected to video in, a dc voltage of 9 to 12V appears at Pin 8 of the video plug. IC₁ and the C₂ through C₆ capacitors derive power from this pin and provide a stable 5V for the circuit. D₂ is a high-brightness LED that indicates that a video source is present. R₇, C₇, R₈, and C₈ provide decoupled supplies for the amplifiers. Video cables are often of poor quality. For that reason, the circuit in Figure 2 provides for amplification of the video signal. Also, compensation of the first amplifier provides amplification of color burst with a concomitant improvement of video quality. To adjust the circuit, first turn P₁ fully clockwise and then adjust P₄ for a good video image. Then, turn P₁ fully counterclockwise and adjust P₃ to obtain a stable black image.

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