The circuit in Fig 1 applies a symmetrical dc control voltage to the contrast inputs of video amplifiers IC1 and IC2 to fade from one video source to the other. IC1’s and IC2’s bandwidths handle SVGA or Super Mac (up to 70-MHz) video signals without degradation. For effective fading, both video sources must be locked to each other. The circuit operates from a single +12V supply.
The circuit also has three additional op amps, IC3, IC4, and IC5, to drive coaxial cable. The circuit requires only three ICs if its outputs drive high impedances.
Both IC1 and IC2 are in their standard configurations except for their contrast-voltage circuits. R1 and R2 set the maximum gain for IC1 and IC2, respectively.
A 0 to 4V signal controls contrast, with the maximum gain at 4V. The two 200-k(ohm) resistors, in series with the contrast potentiometers, limit the maximum contrast-control voltage to 4V. A 0V contrast-control voltage gives a video output 60 dB below the maximum output level. Quad CMOS op amp IC6 generates the required control voltage for the video fading.
IC6A accepts a TTL input. Its output goes to 12V for a low logic input and to 0V for a high logic input. The output of IC6A goes to IC1 via R3 and C1. This RC network has a time constant of 1 sec, giving about a 3- or 4-sec period for fading. IC6C inverts the output swing of IC6B and is the contrast-control voltage for IC2. The sum of the outputs from IC6B and IC6C always add to 12V. If you eliminate the RC network, this circuit operates as a video switch.
Referring to IC1, R4, R5, and R6 balance the gain between the three channels. All control voltages for the IC1 are 0 to 4V, thus R7 is in series with the nine trimming potentiometers, limiting the maximum adjustment voltages for IC1 and IC2 to 4V. R8, R9, and R10 set the black level of the video signal to both video amplifiers. You can set IC1’s black level below 0.5V, but if you use a single supply for IC3, then you should keep the black level above 1V.
For proper dc restoration, you must apply a negative-going clamp pulse to IC1’s pin 14 during the "back-porch" portion of the video signal. Tie IC1’s blank-gate input high because this application does not require the blanking function of IC1. You can substitute other video amplifiers in this circuit if they have a dc control voltage for contrast.
The output of IC1 should go to a 390V impedance. Looking at the red channel of IC1, R11, R12, and R13 combine to give the correct impedance for IC1’s output. In a monitor, IC1’s and IC2’s outputs can go directly to the cathode-driver amplifiers. (DI #1401)