Design Ideas: September 2, 1996

The increased use of video for data and image display has expanded the need to selectively view portions of a video signal. The circuit in Figure 1 produces an oscilloscope trigger that synchronizes the horizontal sweep to a video signal that allows you to view any horizontal line by adjusting a potentiometer. The circuit also incorporates a scan counter that you can use as a video gate for the display of a specific number of video lines.

IC₁ separates horizontal (pin 1) and vertical (pin 3) sync signals from a composite video input. Monostable vibrator IC_2A establishes the vertical start position that IC₃'s Q_A output synchronizes to the horizontal sync signal. Decoded counter IC₄, using a "number of lines" selector switch (S₁), returns IC₃'s Q_A output to its original state. You can use the Q_A output to drive an analog gate to present video data only during the preset time. (DI #1910)