Toshiba D-VR3SU, Service Manual

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Circuit Operating Descriptions
7-29
2) Main Emphasis Circuit
The dynamically emphasized luminance signal is now supplied to the main emphasis circuit where all the high
frequency components of the signal are boosted more than the low frequency components. The boosting action is
required for the high frequency components because in the FM recording method, the noise of the playback signal
increases in proportion to the modulated signal frequency or low level signal. By using the nonlinear emphasis
and main emphasis system, the total S/N ratio is increased. The output of the main emphasis circuit is then
supplied to the white and dark clip circuit.
3) White and Dark Clip Circuit
After emphasis is performed, large overshoots and undershoots in the luminance signal are limited to a specified
level. This is done to avoid FM over modulation. The output of the main emphasis circuit is then supplied to the
FM modulator circuit.
4) FM Modulator
A. The amplitude of the FM signal is limited, so the signal is recorded on tape near the maximum
record level which increases the S/N ratio.
B. The FM carrier is se to 3.4MHz (at the Sync tips) and the deviation to 4.4MHz by inside IC
circuit (for the white peak). The actual device which constitutes the FM modulator is a stable multivibrator.
This multivibrator generates a sine wave output of variable frequency.
The frequency of sine wave is governed by the level of the processed video signal at any given point. Therefore,
the processed video signal varies the frequency of the sine wave which is frequency modulation (FM). During
playback in SLP mode, the crosstalk of the adjacent track is more apparent than is standard mode. It appears as
jitter and noise on the monitor. To reduce this noise from the screen, the FM carrier frequency has to be 1/2fh
shifted up during recording. This is done by applying the head switching pulse to the FM modulator control
pin57 during SLP recording. The FM modulated luminance signal goes to record equalizer circuit and it is mixed
with chrominance signal at the record Amp circuit inside video IC.
5) Record Amp
The frequency modulated luminance signal and chroma signal are mixed in the record amp of pre-amp block
inside video IC. Then this mixed signal is amplified and supplied to the video heads via the rotary transformer
and recorded on the magnetic tape.
Tape speed selection determines which video heads will be used. That is, signal output from pin 66 (SLP) and 73
(SP) of pre-amp block are supplied to video heads.
Control signal of speed mode is applied to pin 53(clock), 54(data), 55(chip select) of video IC from Micom IC.