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Frequency
modulator
Q108 and Q109
Coupling
X' former
T101
Record amplifier
Q201 through
Q204
Video
heads
Head
amplifiers
and switchers
Q221 through
Q227
Video amplifier
Q301 and Q302
Limiters
Q303 through
Q310
Demodulator
D309, D310
Low-pass filter
FL301
&2
This
is
a
symmetrical
free-
running
multivibrator
acting as
a frequency
modulator.
The
oscillating frequency varies from
3.8 MHz to 5.2 MHz in AV-CE
Series Videocorders.
T101 couples the modulator out-
put
to
the
record
amplifiers.
These
are power
amplifiers
to
feed record current to the video
heads.
R201
adjusts
record
current. R209
and R211 adjust
the frequency
response
charac-
teristic.
A slip-ring and
brush
assembly
couples recording current to the
video heads.
The
playback
video
head
out-
puts
are
amplified
and
alter-
nately gated
on
by the 25PG
signal to provide a flat, noise-
free output.
The
PB video head
outputs are
mixed
in the
base
circuit
of
emitter follower Q227. The out-
put of Q227 feeds a two stage
video
amplifier
that
provides
middle-frequency peaking.
The
PB
signal
is
amplitude-
limited by four diode pairs with
transistors
supplying
interstage
amplification.
The
input
to the demodulator
from Q310
is differentiated by
R335
and
the
primary
wiring
of T301.
The push-pull output
of T301
is applied
to the pair
pulse
detectors
(demodulator)
D309 and D310.
These diodes
produce
two
negative
output
pulse per one input pulse.
Thus,
the
carrier
frequency
is effec-
tively doubled
and placed out-
side the video band.
The output of D309 and D310
is amplified by Q311 and fed to
a
low-pass
filter
to
obtain
a
video output that is proportional
to the pulse frequency.
Video amplifier
Q312 and Q313
Noise eliminator
Q314
Video amplifier
Q315
Video output from the low-pass
filter is amplified by Q312 and
Q313.
R349
and
C328
reduce
high frequency
gain to provide
de-emphasis.
Only
the
high frequencies
are
separated
at Q314
input,
am-
plified and sliced by D311/D312
diode pair. Noise eliminated high
frequencies are then mixed with
original
signal
at Q315
input.
8-2.
SERVO CIRCUIT
The AV-3420CE
has both a head-drum (scanner)
servo
and
a capstan
servo
system.
Section
8-2-1
describes
the camera-record
mode
where
the head
drum servo free-runs at 13,125 Hz with the capstan
servo disabled.
Section 8-2-2 describes the TV-record
mode
where
the
capstan
servo
is locked
to the
external
TV
signal.
Section
8-2-3
describes
the
playback mode where the capstan servo is locked to
the playback control-track signal and the head drum
servo runs at 13,125 Hz.
8-2-1.
Camera-Record
Mode Servo
In the
camera
record
mode
the servo
system
acts
as
a
simple
speed
regulator
to
make
the
scanner
turn
at 25rps.
The
reference
for
the
system
is a multivibrator
having
a constant
pulse
width.
Variations
in speed,
as detected
by variations
in the
output
frequency
of the tone
(feedback)
generator, alter the duty cycle of this multivibrator
to
produce
a dc correction
voltage
that
controls
motor
speed.
A belt linkage to the capstan drive
pulley
provides
tape
driving power.
The capstan
servo
is not
operative
in the camera
record mode.
When
using the AVC-3420CE,
the Videocorder
free-runs at a constant
speed, and supplies vertical
and
horizontal
drive
signals to the
camera.
The
head
drum
contains
a
dc
servo
motor
with
a
13,125 Hz frequency
generator beneath
it.
Faster
rotation
of the head
produces
a generator
output
higher than 13,125 Hz which lowers
the dc motor
drive current and thus slows the head rotational speed.
The head
drum
contains
a fre-
quency
generator coil that pro-
duces 525 Hz output
pulses per
single
revolution
of the video
13,125 Hz
Frequency
Generator FG
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