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2.
Dual-Band Compressor:
This
consists
of
two compressors
in
parallel:
"Bass"
which
processes audio
below below
200Hz
(12dB/octave
crossover),
and "Master"
which
processes
above 200Hz.
A
BASS
COUPLING
control adjustable
by
the user
determines
if
the
two bands
will
operate
discriminately ("independent"
mode),
or
if
the "Bass" band
will
be
forced
to
track the
"Master" band ("wideband" mode),
preserving
frequency
balances.
Intermediate
bass
coupling
settings
are
also
available.
Even
in
"wideband" mode,
the
bass control
loop
is
still
active.
Therefore,
heavy
bass
will
cause
a
momentary
reduction
in
the gain of the "Bass"
band
rather than
forcing gain reduction
of the entire signal
(as
in
a
true
wideband
system), thus
avoiding
pumping.
Time
constants
and other parameters
of
the
Dual-Band Compressor have been
adjusted
so that the
summed
and
preemphasized output
of
the
two
bands can
be
directly applied
to
the
peak
limiting
system.
No
further gain reduction
is
required
for distortion control,
and
maximum
naturalness
is
preserved.
The
release
time
of
the
"Master" band
only
is
adjusted with the
RELEASE
TIME
control,
thus
permitting
loudness/fatigue
tradeoffs
according
to
your
needs.
Gain
reduction
in
both
"Master"
and
"Bass"
compressors
is
metered
by
edgewise-reading meters
calibrated
with
a
dB-linear
scale.
To
indicate that the
normal
gain
reduction
is
lOdB,
this
point
has
been
calibrated
as
"OdB
gain
reduction" on
the
meters,
with
the
scale
extending
from +10
to
-15dB
gain
reduction.
No
attempt
has
been
made
to
make
these
meters extremely
accurate.
Their
readings
may
disagree with the actual gain reduction
by
as
much
as
+2dB.
This
accuracy
is
fully
adequate
for
the purpose, since the
amount
of
gain reduction
varies
widely
with
variations
in
program
material
and
operator
gain
riding.
A GATING
function
is
provided
which
prevents
noise
rush-up
during
program
pauses
(particularly
with
noisy
16mm
optical
sound
tracks)
and
makes
the
25dB
gain reduction range
usable.
The
GATING
function
is
designed such that the
gain
does not
get "stuck" forever
in
the 0
to
15dB
gain
reduction
region, so low-level
program
material
is
eventually increased
in
level.
Since gain recovery
is
slow
in
GATED
mode,
the
gradual
increase
in
level
is
essentially
imperceptible.
In
GATED
mode (GATE
lamp
illuminated)
it
is
normal
for
the
meters
to drift
slowly
to "0".
The
meters
will
recover towards "+10"
when
the
program
material
is
above
the
gate
threshold
but
of
a
level
insufficient
to
produce lOdB G/R.
The
output
level of the
compressor
is
determined by
the
CLIPPING
control. This
control
sets
the drive level
to
the
subsequent high-frequency
limiter
and
clipper,
thus
determining
the
amount
of
limiting
and
clipping.
3.
Preemphasis
And
High Frequency
Limiter:
The
summed
outputs
of
the
two
compressors
are applied
to
a
phase
corrector,
24dB/octave
15kHz
lowpass
filter,
preemphasis
network,
and
high-frequency
limiter.
The
purpose
of
the
lowpass
filter
is
to
prevent out-of-band
components from
affecting the operation
of the
high-frequency
limiter
and
to
avoid
intermodulation
between
out-of-band
frequency components and
in-band
frequency
components
in
the
clipper.
Phase
correction
reduces
the
peak
level
increase
caused
by
filter
ringing
and
preemphasis
to
the theoretical
minimum,
thus
reducing the
amount
of clipping.
The
high-frequency
limiter
is
controlled
by
high
frequencies
only (rather
than by
the peak
level
of
the
preemphasized
signal),
thus eliminating
any
possibility
of
modulation
of
high frequency content by
low frequency
material.
1-4
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