Cadet VI Fader is a 2-input, 1-output voltage controlled crossfader module. The A and B input signal paths have a bandwidth of 40 MHz, and the voltage control signal path has a bandwidth of 10 MHz, making it more than adequate for NTSC 480i/PAL 576i video signals. This function has many uses in a video synthesis system, such as proportional mixing, high speed keying, and linear multiplication.
The circuit revolves around U2, the LT1251 Video Fader IC. It is one of our favorite chips to work with due to its trimmerless precision and lack of external components. A and B inputs are tied to ground via R6 & R15 and connected to the U2 input pins via current limiting resistors R5 & R12.
U4 and U1.1 form a buffered 2.5V precision voltage reference. This reference sets the voltage control scale of the fader IC at U2 pin 12. This sets the voltage control scale to 0V (A 100%, B 0%) to 2.5V (A 0%, B 100%). The 2.5V reference also feeds R21 bias control, which is inverted and buffered by U1.2. U1 is a standard TL07X series op-amp, which is insufficient for video bandwidths but acceptable in this context.
U3.1 buffers and amplifies the voltage control input by a gain of 2.5. This scales a 0V to 1V input signal to match the precision voltage reference. R6's value may be modified to adjust the gain of the input buffer, or omitted entirely if a unity gain buffer is desired. U3 is an LM6172 high bandwidth dual op-amp capable of processing video signal paths with ease. The circuit around U3.2 provides two functions. (1) Creating a buffered "attenuverter" circuit with potentiometer R4, allowing adjustment of gains from -1 to 1, with a null point in the center of this range. (2) Summing the bias control's output with the scaled and/or inverted voltage control input signal. The resulting sum is fed into the U2's voltage control input pin 3.
Feedback resistors R15 and R16 connect to output pin 8, which is driven to the output jack via R19. R13 and R15 may be populated with resistor values matching R15 and R16 if driving the output at a gain of 2 is desired.
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