Service manual

STP 11-25R13-SM-TG
S - 28
e. The Q Signal
. Applying the green signal to the base and the red and blue to the emitter of the
Q amplifier forms the Q signal. Here, the green is 180 degrees out of phase with the red and blue. The
resistor network sets the percentages at 21 percent red, 52 percent green, and 31 percent blue. The
resultant output of the transistor is 21% R + 31% B - 52% G (see Figure S-27).
Figure S-27. Q Matrix
(1) In the I matrix and amplifier, on the base there is we have +28 green +32 blue = -60. Blue 60
when added to the base voltage of -60 = 0. This means that if the camera is picking up a black and white
scene, the three signals cancel; there is no I signal.
(2) The Q matrix works similarly. This produces brightness with no color. The red and green
inputs were inverted in phase in order to produce the I and Q signals. Therefore the amplifier associated
with the matrix for the I and Q signals is usually referred to as an inverter.
f. Band Limiting and Delay
. Refer back to Figure S-24. The outputs of the individual matrix
sections feed the filter section. The filter section establishes a bandpass of each of the video signals, 4.1
MHz for Y, 1.5 MHz for I, and 0.5 MHz for Q. The I channel is broad-banded but has vestigial sideband
characteristics because of the upper-frequency cutoff of the transmitter (see Figure S-29). This channel
is single sideband for frequencies higher than 0.5 MHz.
(1) Transmission of frequencies up to 0.5 MHz is double sideband on both the I and Q channels.
Such operation allows two types of receiver action:
(a) The receiver may use the extra color information in the wideband I channel.
(b) Receivers may ignore this extra information and reproduce only chrominance detail supplied up
to 0.5 MHz.
(2) Since the Y, I, and Q signals are each a different bandwidth, and since their envelopes must
have a specific placement in time with regard to each other, the I and Y signals are sent through delay
networks so that all three signals have the proper time placement when applied to the mixer (adder)
section.