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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
10dB,
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
10dB
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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