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clocks D-flip-flop
lC14b on
zero-crossings,
permitting the output
of
IC14b
to
follow
the
output
of
IC14a
only
on
the next zero-crossing after
IC14a
has
changed
state.
5.
MONO/STEREO
RECOGNITION
CIRCUIT
NOTE:
To
better
understand
the
explanation
below, please note
that the
SYNTH
FROM
L,
SYNTH
FROM
R,
and
BYPASS
logic
operates
between -15V
(LOW) and OV
(HIGH). These
lines
are activated by
a
-15V
(LOW)
signal.
The
three outputs
from
the
AUTO
Recognition
circuit
activate
their
associated
mode when
they go
LOW,
and do
nothing
when
they
are
HIGH
(at
ground).
1)
Single
Channel Recognition
The
left
channel
is
amplified
30dB
by IC23a, and
is
then
AC-coupled
(through
C221)
to
comparator
IC31.
R276,
R278
apply
+50mV
of
DC
offset to IC31's
input.
The
DC
output
from
the
L+R
peak detector
is
applied
to lC31's "+"
input.
When
sufficient
left
channel
signal
is
present, the
output of
IC31 goes negative with
sufficient
duty cycle
to
hold
C223
close
to
-15VDC
through
CR217.
This forces the
output
of
comparator IC28c
to
ground (HIGH),
indicating "signal
present on
left
channel".
If
signal
is
absent
on
the
left
channel, but
right
channel
signal
produces
a
DC
output
from
the
L+R
peak
detector, the "+" input of
IC31
will
always be
more
positive
than
its
"-"
input
and
output
of
IC31
will
be held towards +15V, permitting
C223
to
discharge
towards ground
through R286.
When C223
has discharged
to
approximately -7.5VDC,
the output of
lC28c goes
to
-15V (LOW),
indicating "left
channel
absent, right
channel
present". This single-channel condition forces the
synthesizer
into
Synth
From
R
mode,
provided
that
Auto
mode
has
been
chosen.
If
Auto
mode
is
not chosen, the output
of
IC31
is
forced
towards -15V by
CR216
at
all
times and
the output of
IC28c can never go
LOW.
When
the
synthesizer goes
into
Synth
From
R, the output
of
IC34c
(Right
Auto
Offset
line)
goes
to
-15V
(it
is
otherwise
at ground).
This
changes
the offset
on
the
input of
IC31
from
+50mV
to
-432mV
and
is
a
"gating" function:
even
in
absence
of
signal,
the output of
IC31
will
now
go towards
+15V, and the
circuit
will
continue
to call for
Synth
From
R
mode. To
break
out of
this
gated
mode,
there
must
be enough
signal
present
in
the
left
channel
to
overcome
the
-432mV
offset
and once
again
charge
C223
towards
-15V.
Automatic
detection
of a "signal
present on
left,
absent on
right"
condition
is
performed
in
an
entirely
symmetrical
way
by
means
of
IC23b, IC30, IC34d.
This
automatically
calls
for
Synth
From
L.
When
signal
is
present on both
left
and
right
channels, the outputs of
both IC28c
and IC28d
are
HIGH
(ground). This forces the
output
of
IC29d
LOW
(-15V), calling
for
Bypass
mode
(unless
this
state
is
overridden
by the
Center-Channel
Mono
Detector's
calling for
Synth
From
L, or
unless the
output
of
IC29d
is
overridden by
CR210's
pulling
the junction of
R291
and
CR218
HIGH;
the
latter
override occurs
when Auto mode
is
defeated).
6-5
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