a. The color camera system utilizes a filter input FET preamplifier
with an improved signal-to-noise ratio, and a 7 mHz bandwidth in the green
channel.
The red and blue channels have an input stage with a 3.5 MHz
bandwidth and approximately twice the gain of the green channel. The optics
of the camera are so arranged that the signal current from the green pickup
tube is approximately twice that of the red and blue.
Consequently, the
video signals out of the preamplifiers are approximately equal.
b. This arrangement is used because the green signal makes up a
majority of the encoded luminance signal and therefore, requires the best
signal-to-noise ratio.
In addition, the green channel supplies the high
frequency information for the red and blue channels. Limiting resolution in
the luminance output is thus determined by the green signal.
(1) In the stages that follow, the FET input section of the video
preamps is first amplified, clamped and then blanked before it is sent
through the camera cable to the camera compensation module in the camera
control unit (CCU).
(2) The cable comp module (A29) consists of three similar circuits
that serve to equalize the high-frequency losses that occur in long cable
lengths. Its also adds the individual black level setup that is maintained
through most of the video path.
(3) From the cable comp module, green video goes to the video sync
stripper board (A14), where the internal sync pulse is added to the video in
the green video I board (A13). Red and blue video signals are sent directly
from the cable comp board to their respective video I boards (A11, A12).
(4) On the video I boards, the video signals are first amplified and
then attenuated by the switch master gain resistor.
The signal is then
white clipped and applied to the automatic gain control (AGC) amplifier,
which maintains a fixed video gain throughout the video chain system.
(5)
The shading amp that follows the AGC circuit multiplies the video
by shading
signal from the shading generator board (A3). This is used to
compensate
for fixed variation in sensitivity that occur in the optics or in
the pickup
tube.
(6) There are two outputs from the video I board.
One is a full
bandwidth signal that is only used in the green channel.
The other is
filtered by a 2.2 MHz, lowpass filter and these outputs feed the masking
module (A31).
(7) The masking module consists of three similar circuits that permit
the addition or subtraction of one color from another without affecting
white balance.
6.
The full bandwidth green signal and the low pass green signal also go
to the contours separator board (A12), where they are subtracted from one
another. This forms a high pass or contours signal which is then applied to
a coring amplifier.
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