VIDEO_FADER

Published:2009/7/14 21:34:00 Author:May | From:SeekIC

The circuit shown here does an adequate job of allowing you to fade a video signal virtually down to the black level without losing sync on the monitor. The sync pulses are extracted from the composite video (CVBS signal) by op amp IC2 and its surrounding components. Components C2, R11, and D2 form a video-clamping circuit. Diode D3 provides a bias at the + input of IC2. Because of the rectifying action of D2, the op amp amplifies only the negative part of the CVBS signal. The clamping circuit in the feedback path of the AD847 (D5 to D8) prevents the op amp from going into saturation. The amplifier sync signal is digitized by a diode, D9, and a Schmitt-trigger gate, IC3d, before it is appied to the + input of one of the two fast op amps contained in the video fader IC, an LT1251.The sync level is set to the optimum level with the aid of preset P1. The LT1251 uses preset P2 to determine the level ratio between the sync channel and the video channel. The control voltage for the fader is derived from a reference voltage created by zener diode D1. The mixed output signal is available at pin 8 (dc-coupled), at an impedance of about 75Ω. Current consumption of the circuit is less than 30 mA. The indicated test voltages are applicable with no input signal applied to the circuit. The fader also reduces the level of the color burst. Consequently, the picture can go black and white just before the black level is reached.



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