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TECHNICAL DATA
Orban Model
464A
6-10
5.
High-Frequency
Limiter
The
output
of
the
leveler/compressor
is
applied
to a
bandpass
filter
with a
peak
frequency of 36.4kHz,
a
"Q"
of
0.77,
and
a
peak
gain
of OdB.
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.
Note
that
swept
sine
wave
tests
of
the
high-frequency
limiter will
not
yield the
exact
inverse
of
the
pre-emphasis
curves.
This
is
because
a
high-pass
filter
causes
the
comparators
to
see a
signal
which
is
slightly different
from
the signal
at
the high-
frequency
limiter output,
and because
the
threshold of high-frequency
limiting
is
set
above
the steady-state output
level
of
the
leveler/compressor.
The
threshold
is
set
this
way
to
keep
the
high-frequency
limiter
from
being
activated
by
peak
overshoots
resulting
from
the
slow
attack
time of
the leveler
when
operating
on program
material.
The
output
of
the
high-frequency
limiter
is
applied
to
a
clipper
which
provides
absolute
peak
control
at
the
Co-Operator's output provided
that
the
HF
LIMIT
button
is
set
to
OPERATE.
(If
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
high-
frequency
limiter
is flat
±0.1
dB
to
20kHz, and
falls
at
12dB/octave
thereafter
when
de-emphasis
is
applied.
Component-level
description:
The bandpass
filter
consists
of
IC102b and
associated
circuitry.
Bandpass
response can
be measured
at
pin
7
(or at
test
point
TP4).
The
contribution
from
the
bandpass
output
is
determined
by
the
gain
of a
volt-
.
age
divider, the gain
of
which
is
adjusted
with
HF
LIMIT PRE-EMPHASIS
switch S106.
The
contribution
of
the
bandpass
filter
to the
output
is
further
determined
by
the resistance
of
JFET
IC3,
which
can dynamically reduce
the
pre-emphasis
to effect the
high-frequency
limiting function.
The
loss in
the voltage divider
is
compensated
for
by
1004,
which
has a
gain
of 29dB.
The
output of
1004
(representing the
bandpassed
signal)
is
summed
with
the input signal
in
IO03a
to create
the
pre-emphasized
signal.
The
+3dB
breakpoints
that
correspond
to
the
time
constant
calibrations for
the
HF
LIMIT
PRE-EMPHASIS
switch
are:
1.06kHz
for 150ns,
1.59kHz
for
100ns,
2.12kHz
for
75ns,
3.18kHz
for
50ns,
4.24kHz
for 37.5ns,
and 6.37kHz
for 25ns.
The
positive
and
negative
peak
levels
of
the
pre-emphasized
signal are eval-
uated
by two
comparators
in
Id.
If
either
exceeds
the
±3.0V
threshold
voltages
established
by R198-R201,
the appropriate
comparator
fires.
Each
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