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The
threshold of limiting of the high-frequency limiter
is
user-adjustable
over
a
3dB
range, permitting brightness and
high
frequency
distortion to be
traded
off
according
to
your
needs.
Because
the
peak
limiting
system
incorporates
IM
distortion cancellation,
substantially
more
clipping
can be accomplished without
objectionable
distortion
than
in
conventional systems,
and
significantly
improved
high
frequency
power
handling
capability
is
achieved.
4.
Loudness
Controller:
The
concept
of
Loudness
is
different
from
the
concept
of
Level. Loudness
is
subjective
sound
intensity.
It
has
no
physical
reality;
it
is
what
the
listener
perceives
in
his
mind
.
Level,
on
the
other hand,
can be
measured
in
many
objective ways:
a
VU
meter
and
a
PPM
are
two
common
level
indicators
in
broadcast.
No common
level
meter can
provide
a
reading
which
correlates well
with Loudness.
CBS
Technology
Center,
in
experiments over
the
course
of
some
20
years,
has
developed
a
technique
of
measuring Loudness by
means
of
complex
electronic
circuitry.
This technique provides
results
which
correlate quite well
to
how
loud a
panel
of listeners
judges
a
sound
to be.
Ordinarily,
gain
reduction
in
OPTIMOD-TV
is
determined by
the
compressor
control
circuitry.
However,
Loudness
can
be
controlled
by
using
the
CBS
measurement
technique:
If
the
loudness
exceeds
a
preset
threshold,
then
the
loudness
controller
will
activate a
feedback
loop
to
further
reduce
the gain
as
necessary.
This
is
the
technique
used
in
OPTIMOD-TV;
it
is
the
most
sophisticated
known
technique
for
measuring
and
controlling
the
loudness
of
broadcast
audio.
To
estimate
the perceived
loudness, the
preemphasized outputs
of
OPTIMOD-TV
are
summed,
deemphasized,
and
fed
into
seven
parallel
filters,
covering
the
frequency
range
of
approximately 250Hz
to
15kHz.
(Alternately,
the
unpreemphasized compressor
outputs can be used
to
drive
the
Loudness
Controller
when
the
main
processor
outputs
are
unavailable
--
in
a
split-chassis
configuration, for
example.)
The
region
below 250Hz
is
not treated
because
the ear
is
relatively insensitive
to
energy
in this
region, Also,
if
energy
below
250Hz were
dominant,
gain
would
almost
certainly
be
under
the
control
of
the
compressor
control
circuitry.
The
output
of
each
filter
is
rectified
and
the
DC
output
currents
of
the
rectifiers
are
summed,
applied
to
a
threshold
circuit
(to
determine where
the
loudness
controller
takes
over
from
the
compressor),
and
then
fed
into
the
master
compressor
control
voltage
integrator.
The
seven
filters
have
different
gains
which
are
chosen
so
that
if
the
filters'
inputs
were
all
driven
from
a
swept
sine
wave,
then
the
summation
of
the
filters'
rectified
outputs
would
closely
approximate
the
sensitivity
of the ear
as a
function
of
frequency.
In
addition,
the
fact that the
filters
are
summed
together simulates
a
property
of the ear called
"loudness
summation":
If
a
given
amount
of
energy
in
a
complex
sound
is
spread
over
several
frequency
bands,
it
will
sound louder than
if
it
is
concentrated
in
one band
only.
Because
certain
sounds
in
entertainment
programming
(pistol shots,
explosions, or
screeching
tires,
for
example)
are
supposed
to
be loud for
dramatic
impact,
we
have
made
the loudness
controller
defeatable
locally
or
by
remote
control.
It
may
be
activated
or
defeated manually
or
by
means
of
the
automation computer
as
desired.
1-5
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