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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.
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