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2)
Center-Channel Recognition
The
outputs
of
the
left
and
right
bandpass
filters
are applied
to
the "+" inputs of
comparators
IC24a, IC24b,
whose
inputs are
connected
through voltage divider
R250,
R251
to the
peak-detected
L+R.
This offsets the
comparator
thresholds
in
proportion
to
the
peak
level of
the
L+R.
The
outputs
of the
comparators
are
processed by Exclusive-OR gate IC25a.
This
produces
a
LOW
(-15V)
signal
when
its
inputs are identical,
and
a
HIGH
(ground)
signal
otherwise.
The
duty cycle
of the
output
of
IC25a
thus
depends on
the
amount
of
time
that the
left
and
right
channel
comparator
outputs are
identical.
If
both channels were
absolutely
identical,
the output
of
IC25a would always
be
LOW.
As
the
channels
become
less
and
less
identical
(more and
more
"stereo"),
the
amount
of
time
in
which
the output of
IC25a
is
HIGH
becomes
ever greater.
R255
and
C219
time-average
the
output
of
IC25a.
The
voltage on
C219
thus
corresponds
to the
stereophonic content
of the
left
and
right signals: the
more
stereo, the
closer the voltage
approaches ground.
This voltage
is
detected
by
comparator
IC28a,
whose
trip
point
is
adjusted by
means
of
R263 (Mono
Threshold).
Hysteresis
is
supplied by positive
feedback
through
R26Q
(IC25d serves
as a buffer),
so that the
mono-to-stereo and
stereo-to-
mono
trip
points are
not
identical:
once
the circuit has
decided
that
program
material
is
"stereo"
or
"mono",
the
hysteresis
makes
it
more
difficult for
the
circuit to
"change
its
mind". This avoids "chattering"
between
mono
and
stereo on
marginal
program
material.
IC28b
is
a
silence-gating detector.
When
the
peak L
+
R
level
falls
below
approximately 68mV,
the output
of
IC23b goes
LOW
(-15V)
and
turns
on
tri-state
buffer IC41a. Ordinarily, the Disable
pin
(f/19)
is
HIGH
(ground)
and
the output
of
IC41a
has
a
high
impedance.
When
IC41a
is
turned
ON,
it
completes
a positive
feedback path through R257, R258,
CR212
to
make
sure that the state of
IC28a
does
not
change
during
silence.
(IC25c
is
an
inverter;
feedback
is
taken
to
the
"-"
input of
IC28a;
thus, overall positive
feedback
occurs.)
The
voltage on
C219
is
forced
to
be close
to
-15V
if
IC28a
was
indicating
"mono"
before
silence
occurred (IC28a output
at
ground), or -7.5V
if
IC28a
was
indicating
"stereo" (IC28a output
at -15V).
The
output
of
IC25c
is
HIGH
if
stereo
is
detected;
LOW
for
center-channel
mono.
When
the
RECOGNITION
switch S14
is
set to
1CH ONLY,
IC25c
is
completely
disconnected
from
the
main 275A
logic.
If
S14
is
set to
M/S+1CH,
IC28d
operates
the
same
as
IC28c
(described
above
in
Single
Channel
Recognition)
when
IC25c
indicates stereo
.
When
IC25c
indicates
center-channel
mono
,
it
pulls
R281
to
-15V.
This puts
approximately
-15V
on
pin 11 of
IC28d.
IC30b cannot
pull
pin
10
of
IC28d
down
this far.
Therefore, the output
of
IC28d
is
forced
LOW,
which
invokes Synth
From
L mode.
If
Auto
mode
is
not
active, the
Center-Channel
Mono
Recognition
circuit
is
suppressed by
pulling
C219
HIGH
through
CR211.
This
corresponds
to
the "stereo"
(do-nothing) detection condition.
Detection
of
center-channel
mono
can
also
be
suppressed by
pulling
C219
to
ground through User Control
Interface
connector
J2-
13.
6-6
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