Lesson 1/Learning Event 3
5. Clamp. When the signal arrives at the clamping circuit it is forced to
maintain a constant DC reference level which is produced by the clamp circuitry.
Suppose you were responsible for maintaining a level of exactly one inch of water
leaving a dam and you use a flood gate to regulate the water flow over the dam. A
clamping voltage would accomplish this same type of regulation in a circuit.
6. Blanking. The signal has been clamped at a constant DC reference, before it
enters the blanker. The signals blanking level which is 75 percent of peak carrier
amplitude to which the horizontal and vertical blanking pulses rise are cleaned up
of any spurious voltages and sent to the white clipping stage.
7. Clamped and blanked video arrives at the white clipper stage. This circuit
effectively removes large high light information, thereby easing the dynamic range
requirements for the camera control circuitry and the viewfinder. The function of
this circuit can easily be understood if you compare the video to hedge rows and
the white clipper to hedge clippers. Hedges grow unevenly at the top. The hedge
clipper and white clipper both chop off unwanted peaks. This makes the video, or
bush, flat across the top.
8. Outputs. The first output of the video amplifier is sent to a noninverting
amplifier which sets the video signal voltage to the manufacturer's specifications
and feeds the signal to the viewfinder circuitry (fig 14 foldout located at the
end of Lesson 3).
a. The second video output is sent to a preequalization network, which
preemphasizes and boosts the video signal to counteract for signal loss through the
camera cable over long distances.
b. If the camera cable length is short, 50 feet or less, preemphasis is not
necessary. The camera cable driver outputs the video signal through the camera
cable to the camera control unit.
8
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