When
the
input
signal
is
lower than predetermined level, the
DC
output
obtained
is
small and
can
not
turn
on the H306, thus the H306
keeps
its
turn-off
stage
and this makes H307
turn
on,
decreasing the collector voltage and turning
off
H308. Thus no audio signals
can
pass
through
the F ET. This
is
the fundamental principle
of
the muting operation
but
for
more elaborate mut-
ing operation the second and the
third
inputs
are
necessary.
The second
input
is
used
to
protect the muting operation and MPX stereo beacon lamps
from
misoperation due
to
undesirable
noises.
The high frequency noises included in the detected
audio signals
are
separated
by
a small capacitor
C471
and amplified by the noise amplifier tran-
sistor H471
and
its
output
is
rectified
by
the
two
diodes. The rectified
DC
output
is
proportional
to
the
noise components in the audio signals.
When there are excessive
noises
in the audio signals
such
as
obtained
with
a station incorrectly
tuned in,
the
rectified
DC
output
turns on the transistor H472, decreasing the emitter-collector
resistance
to
zero. This
means
the collector
of
H307
is
short-circuited
to
the ground, therefore
the H308
is
turned
off
and any audio signals having
excessive
high frequency
noises
can
not
go
through
the
FET's source-drain path.
The transistor H303 connected in
series
with
the 19KHz
pilot
signal amplifier transistor H302
is
also turned
off
(when the transistor H472
or
H307
are
turned on.) and no current flows
in
the
H302, resulting
in
turning
off
the
stereo beacon lamps. Thus misoperation due
to
undesirable
noises
is
also avoided.
The
third
input
is
obtained
from
the
FM
discriminator circuit. The
DC
output
so
called
"S"
curve is applied
to
the gate
of
H478 through a resistor R273 and
dividing
network (R485 &
R486). The
DC
output
is
zero
with
a station correctly tuned in,
but
will
vary
from
negative
to
positive values
or
vice
versa
when the tuning
point
is
deviated toward either plus
or
minus
frequency
from
the correct tuning frequency.
When the
DC
output
is
increased
to
a greater level than
that
of
predetermined, the increased
source potential
of
H478
makes
the transistor H481
turn
on, and this makes the H306
turn
off,
... H307
turn
on, ... H308
turn
off,
... H303
turn
off
(this
means
rio 19KHz
pilot
signal
is
amplified and
no
stereo beacon
is
turned on.).
When
the
DC
output
is
increased
to
the negative
predetermined level,
the
decreased
source potential turns
off
the H479 which in
turn
makes
the
H480
turn
on and
the
H306 in turned
off.
The subsequent
changes
are
exactly the
same
as
that
just described above.
Thus when the tuning
is
shifted-or-deviated
to
the certain frequencies in which undesirable
noisy side-audio signals
are
produced, both muting and 19KHz switching transistors
are
operated
automatically and open
the
circuits.
With the station correctly tuned in, the
bias
current
of
the FET H478
is
adjusted
so
that
both
transistor H480 and H481
are
not
turned on, giving no effect on the transistor H306.
3.2 MPX Stereo Decoding
Circuit
The buffered and non-equalized audio signals
are
applied
to
the
first
amplifier H301 which
serve
as
a tuned amplifier
for
the
pilot
signal in the composite signals and
as
a
buffer
amplifier
for
the audio signals. The amplified 19KHz
pilot
signal
is
led
to
the second 19KHz amplifier H302
and
further
amplified
if
switching transistor H303
is
turned on
by
the controlling
DC
signal
as
described in the preceding chapter. The final 19KHz
pilot
signal
is
rectified
by
the doubler
circuit
consisting
of
the
H315 and H316
to
obtain synchronized
38KHz
amplifier driving signal.
The H304
is
the
38KHz
tuned amplifier
and
supplies its
output
to
the switching
matrix
circuit
consisting
of
four
diodes. While the composite signals
are
applied
to
the center tap
of
switching
transformer
1I2
L302. The right and
left
stereo signals decoded
by
the switching
circuit
are
led
to
the crosstalk cancelling amplifier which utilizes complementary configuration
with
NPN and
PNP
transistors through de-emphasis network consisting
of
C315
and
R335, and C316 and R336,
L305
is
a
low
-
pass
filter
networks having very sharp
cut
off
characteristics and eliminates unde-
sirable residual switching signals. Transistors H313 and H314
are
buffer
amplifiers and their
outputs
are
led
to
the function switch.
3
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