of L1, C11, and C12. In the color mode, the signal goes through low pass filter FL1 and through a comb
filter circuit contained in IC-3 (2/4). The path taken is selected by an electronic switch inside IC-4 (2/4).
The monochrome mode signal goes to pin 19 and the color mode signal is applied to pin 17. The
electronic switch, actuated by a DC control signal applied to pin 18, selects between them.
The output signal from the electronic switch passes through a Y-AMP stage, whose output
appears at TP-2. This signal takes two paths, one path reenters IC-4 (3/4) at pin 22 and the other path is
through the Y high frequency preemphasis circuit whose output appears at TP-3. The pin 22 signal path
is through a sync separator and separated composite sync is supplied from pin 24. The H sync
component passes through a delay network composed of R54 and C32 whose output is reapplied to IC-4
(3/4) at pin 1, the Y/SYNC MIX input.
The luminance signal from TP-3 which enters IC-4 (3/4) at pin 4 goes through a clamper stage
and is then applied to the Y/SYNC MIX stage inside the IC. The delayed sync pulse added to the
luminance signal is the AGC reference pulse. The AGC reference pulse which is formed by delaying
the horizontal sync pulse is phase inverted and added-to the clamped luminance signal on the back porch
of the horizontal interval. The added pulse is at an amplitude equal to 100 percent peak white in the
luminance signal. The luminance signal, with reference pulse added, is supplied to a detector stage
inside IC-4. The detector detects amplitude variations in signal level by comparing the H sync pulse
amplitude against the fixed reference pulse amplitude. Any change in the input signal level is reflected
by a change in sync pulse amplitude relationship of the sync to the reference pulse.
The DC output signal from the detector, filtered by C34, is the AGC control signal applied across
deviation control R23. It is also supplied to the RS2 board as the video level meter drive. The DC
control signal from the deviation control is reapplied to IC-4 (1/4) at pin 16 where it goes through a DC
amplifier and is used to control the gain of the AGC amplifier stage.
The DC control signal from the detector in the Y-AGC circuit is supplied to the VIDEO LEVEL
meter circuit on the RS-2 board. Since the DC control signal varies with the sync level of the input
signal, it can be applied to the meter circuit as in indication of input sync signal level. R-4 on the RS-2
board is the calibration control for the meter. This type of Y-AGC system is insensitive to change in the
brightness level of the scene, so the VIDEO LEVEL meter is conveniently effective in monitoring the
record level. Since the AGC system can operate only in the record and E-E modes, the video meter has
no deflection in the playback mode.
Learning Event 4:
FUNCTIONS OF COMB FILTER CIRCUIT
As stated in the description of the Y-AGC system, the signal path taken by the luminance signal
at the output of the AGC amplifier is different, depending on whether the machine is in the color or
monochrome operating mode.