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Co-Operator
TECHNICAL DATA
6-5
A
gating detector monitors
the level
of
the
Co-Operator's
input
signal,
and
activates
the gate
if this
level
drops
below
a threshold
set
with
the
GATE THRESHold
control.
The GAIN
REDUCTION
meter
consists
of
ten
comparators arranged
to
produce
a
meter
with
a
linear scale (calibrated in
dB).
High-frequency
limiting
is
effected
by
applying
the output
of
the leveler/compressor
to
a
bandpass
filter.
When
summed
with
its
input,
the output
of
this
filter
provides
a
6dB/octave pre-emphasis
up
to
20kHz.
The +3dB
breakpoint frequency
for the
pre-emphasis
is
determined
by
the
amount
of bandpass
output
that
is
summed
with
the input signal
—
the greater the contribution
from
the
bandpass
output, the
lower
the
breakpoint frequency.
The
contribution
from
the
bandpass
output
is
determined
by
the
HF
LIMIT
PRE-
EMPHASIS
switch
and
by
circuitry
which
can dynamically
reduce
the
pre-emphasis
to effect
the
high-frequency
limiting function.
The
output
of
the
high-frequency
limiter
is
applied
to
a
clipper
which
provides
absolute
peak
control
at
the
Co-Operator's output
when
the
HF
LIMIT
button
is
set
to
operate.
(When
it
is
set to
defeat,
absolute
peak
control
is
effected
by
controlled
clipping in the leveler/compressor's
VCA.)
If
the
subsequent
de-emphasis
has
been jumpered
out,
the absolute
peak
ceiling
at
the
Co-Operator's output
will
be independent of
frequency;
if
de-emphasis
is
applied,
the
peak
ceiling will
be frequency-dependent,
falling at
6dB/octave
beyond
the
break
frequency determined
by
the setting
of
the
HF
LIMIT
PRE-EMPHASIS
switch.
The
peak-detecting
PEAK OUTPUT LEVEL
meter
is
an
LED
bargraph
that
monitors
the
output
level
of
the
Co-Operator
just prior to the
balanced output
stage.
The
meter
is
driven
by
a
peak
detector capable of reading
the
peak
level
of
a
10-microsecond
pulse
with an accuracy of 0.5dB
(typical)
when
compared
to
its
reading
on
a steady-
state
tone.
It
thus
provides a true
peak-reading
capability, rather
than a quasi-peak
capability
like
an EBU-standard
PPM
(peak
program
meter).
Unregulated
voltage
is
supplied
by two
pairs
of
full
wave
diode
rectifiers.
Regulated
voltages
are
supplied
by
a
pair
of
overrated
500mA
"three-terminal"
IC
regulators.
2.
Input Buffer
The
signal enters the
Co-Operator
in
balanced form,
receives
modest
RF
suppression,
then
is
applied
to
a
very low-noise
opamp
configured
as
a
differential
amplifier
with
a 0.5
gain.
When
both
non-inverting
and
inverting inputs are
driven
by
a
source
impedance which
is
small with
respect
to
100K
(as
600
ohms
or
less
would
be),
the
amplifier
is
essentially insensitive
to
signal
components
that
appear
equally
on
the
non-inverting
and
inverting inputs
(such
as
hum), and responds
with
full
gain
to the
difference
between
the non-inverting
and
inverting
inputs.
It
therefore serves as
an
"active
transformer".
Since
the
INPUT ATTENuator
control
is
located
after
the
opamp,
the
opamp
will
overload
if
its
differential
input
exceeds approximately
+26dBu
(OdBu = 0.775V
RMS;
for
this
application, the
dBm
@
60012
scale
on
voltmeters
can be
read
as
if
were
calibrated in
dBu).
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