In the color operating mode, the luminance signal output from the AGC amplifier contains the
color signal information at 3.58 MHz. This must be removed since the color and luminance signals are
processed separately. The signal is applied to IC-3 (2/4), where the luminance and chrominance signals
are separated by means of a comb filter. This technique allows the two signals to be separated without
loss of the high frequency video components which occupy the same band as the chroma signal.
DL1 is a tuned delay line which has a bandwidth limited to about 1 MHz, centered on the 3.58
MHz chroma frequency. The output signal from the delay line is of course delayed by one horizontal
line (1H)(about 63.5 microseconds).
In the NTSC system, the phase of the color subcarrier is shifted by 180 degrees during each
horizontal interval. The chroma signal in any given line of video is therefore 180 degrees out of phase
with the chroma signal in the line immediately preceding or following it. For this reason, the chroma
components are cancelled by mixing the 1H delayed chroma signal and the nondelayed full NTSC color
video signal. It is this fact which forms the principle of operation of the comb filter.
Learning Event 5:
FUNDAMENTALS OF Y HIGH FREQUENCY PREEMPHASIS AND FM MODULATION
The output from the Y-AMP stage in IC-4 (2/4) which appears at TP-2 is rerouted to two places.
One path to the sync separator in IC-4 (3/4) has already been described. The other path is through a Y
HIGH frequency pre-emphasis circuit. From TP-2, the main signal goes directly to mixer Q3 while high
frequency components of the signal go through high pass filter C22. The high frequency components
are amplified by Q2 with low and high excursions of the signal being clipped by diodes D7 and D8. The
effect of this stage is to sharpen rise and fall time (edge enhancement) of the high frequency component.
The high frequency components rejoin the signal at mixer Q3. The output of limiter Q2, D7, and
D8 is cut off during the back porch of the horizontal interval by means of a delayed sync signal applied
to Q6. This is done to remove any color burst information remaining on the signal amplified by Q2. Q6
will also mute the limiter output when VTR is placed in the DUB mode. The output of mixer Q3 is
passed on to Y-BUFFER, Q4, and Q5, and appears at TP-3.
The FM modulator, preemphasis circuit (4/4) and clamp circuit (3/4) are all contained within IC-
4 (1/4) along with the Y-AGC and sync separator. The clamp stage is also within a part of the AGC
loop, as described previously. The clamped luminance signal passes through the preemphasis stage and
is then supplied to the white and dark clipper. A 3.58 MHz trap circuit is included as part of the
preemphasis stage. This trap is used only when the machine is operating in the color mode and is
switched in and out by Q7. The trap is needed to remove 3.58 MHz stray pickup occurring within IC-4
(4/4). Transistor Q8 and diode D12 perform these functions and R63 is the white clip adjust control.
The output of the white and dark clipper stage re-enters IC-4 (4/4) at pin 6, the input to the FM
modulator. In the FM modulator, the luminance signal is converted into an FM signal deviating between
3.8 and 5.4 MHz.