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It
is
important
to understand that
this scheme results
in
sinewaves
not hitting
100%
modulation
in
OPERATE
mode.
This
occurs
for
two
reasons.
First,
the
dynamic
response
of
the
previous
multiband
compressor
section
is
such
that
steady-state
lkHz
sinewaves
reach
about
60%
modulation
if
the
CLIPPING
control
is adjusted to 12:00. This is a direct consequence of the natural loudness
balances produced by this processing. Because sinewavos have a very low peak- to-
average ratio compared with speech or music, their peak level must be reduced
to prevent
them ( or similar
program
material) from being unnaturally loud and
giving the processing an artificial, strained quality.
Second, a certain amount of headroom is left between the threshold of the first
clipper
and
the
threshold
of
the
overshoot
compensator
to
accomodate
the
distortion
corrector
signal
and
overshoots
in
the
15kHz
lowpass
filter
without
performing
excessive,
non- distortion- cancelled
clipping
in
the
overshoot
compensator.
Because of the previously mentioned characteristics of the multiband compressor,
sinewaves
below
about
2.5kHz
are
not
ordinarily
clipped;
thus,
no
distortion-
corrector
signal
is
produced.
However,
varying
the
clip
threshold
does
make
better
use
of
available headroom
than would be
the case if the clipper
were
always
left
at
the "- 1.0dB" threshold to which it
switches
in the presence
of
substantial high frequency energy.
It
is
important
to note that
despite ( in reality, because of) this behavior with
sinewaves, extremely high loudness is
obtainable with speech or music because
the processor's behavior is optimized for these signals.
5.b) Hilbert-Transform Clipper: ( on Cards # 0 and # 1)
The input to the Hilbert- Transform Clipper is applied to a main chain of allpass
filters ( IC13,
IC8A,
and
associated
components),
and
to
a 4kHz
lowpass
filter
(IC14 and associated components) plus a second chain of allpass filters ( IC10 and
associated components). These filters are designed so that comparing the signal at
pin # 1 of IC8A with the signal at
pin # 7 of IC1OB will
show
identical
levels
(+0.2dB) and a 90 degree phase difference (+ 2.5 degrees), 30-4000Hz. Above 4kHz,
the output of the second chain should fall at better than 3QdB/octave, and a 90
degree
phase
difference will no longer exist. ( The existence of the 90 degree
phase difference can be checked by means of a Lissajous pattern displayed on an
oscilloscope with X/Y display capability. If the oscilloscope X and Y inputs are
connected to the two points mentioned above, a circle should be produced on the
screen,
indicating
an
accurate
90
degree
phase
relationship
between
the
two
chains.)
The
output
of
the
main
chain
is
full- wave
rectified
by
means
of
precision
rectifier IC8B and associated components, while the output of the second chain is
full- wave
rectified by means of precision rectifier IC9A,
IC4A,
and
associated
components. These two rectified outputs are applied to a vector sum generator
using
a single
log-antilog
XY/Z
multiplier/divider.
The
vector
sum ( i.e.,
the
square root of the sum of the squares of the two rectified inputs) is computed
by
the " implicit" technique
using feedback ( W.J.
Wong and W.E.
Ott: Function
Circuits --
Design
And
Applications.
New
York,
McGraw-Hill,
1976,
p.
206.)
The
vector sum
is developed at pin 5 of IC4B. In addition, a threshold voltage
(-2
volts when no input signal is present) is added in through R68 and R69 ( the
RATIO trimmer). IC4B serves as a threshold amplifier. If the instantaneous input
level to the Hilbert-Transform Clipper is below 2.0V peak, the sum of the output
of
the vector sum generator and the threshold voltage will drive the output
of
B-9
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