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Co-Operator
TECHNICAL DATA
comparator
has
an open
collector
NPN
output stage
and
charges
the high-
frequency
limiter
timing
module
negative through
attack
time
resistor
R203.
C125,
R202
form
the
6dB/octave high-pass
filter
which
prevents
the
high-
frequency
limiter
from
being
activated
on
low-frequency
program
material.
In the
absence of high-frequency
gain reduction, the output
of
the high-
frequency
limiter
release
time
module
(at
pin 2)
is
biased
at
a positive voltage
determined by
L FET
BIAS trimmer
R204.
This pinches
off
IC3.
When
high-frequency gain reduction
occurs, the voltage
at
pins
2 and
7 of
the
high-frequency
limiter release
time
module
goes
more
negative than
the
quiescent
voltage, turning
on IC3 and
resulting in
less
and
less
pre-emphasis.
Pre-emphasis
is
dynamically decreased
until
comparator IC1 no
longer
fires,
indicating
that
the
high-frequency overload has
been removed.
IC101
drives the
HF
LIMIT LED.
The
FET
control
voltage
is
applied
to
IClOl's
pin
6;
the
quiescent
FET
bias
is
applied
to
pin
5.
In addition,
IClOl's
pin 5
is
offset
by
current
flowing through
R207, which
forces
IClOl's
pin
5
to
be
more
negative than
its
pin
6,
and
causes pin 7
of
IC101
to
go low
(close to
ground).
When
the voltage
on
pin
6
becomes more
negative than pin
5
due
to
high-frequency gain
reduction, pin
7 goes
high, lighting
HF
LIMIT
LED
DS102.
Q106
serves as
a
zener diode
to
ensure
that
DS102
is
off
when
IClOl's
pin
7
is
close
to
ground.
The
output of
the
high-frequency
limiter
is
applied
to clipper
R191,
CR113,
CR114. The
subsequent
de-emphasis
is
provided
by
C126
and
associated
switched
resistors.
The
clippers
are biased with temperature-compensated
±5.5VDC
source
IC2
and
associated
components.
Hie
clippers
can be
defeated
by
forcing the
±5.5VDC
supply
to the
power
supply
rails
by
placing
jumper
A
in
the
"DE-
FEAT"
position.
6.
PEAK OUTPUT
LEVEL Meter
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.
Component-level
description:
The
output
of
ICllla
(pin
1)
is
applied through
the
METER CAL
control
R218
to amplifier
IC113a.
The
output of
IC113a
is
rectified
by
an
inverting
half-wave
precision
rectifier
IC113b and
associated
components.
Double
the
output
of
the
rectifier
is
summed
(through
R217)
with
its
input
to
create
a
full-wave
rectified
signal
at
the
+
input
of
IC114b
(pin
5).
The
rectifier
has
a
voltage gain
of
0.89.
IC114b
operates as
a dual-time constant
peak
detector.
A DC
voltage
equal
to the
peak
value
of
the
rectified
signal at
pin 5
of IC1 14b
is
developed
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