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Bias
sources
are required for the diode clippers
in
the audio processing. There
are two such sources;
the first creates
approximately + 1.2
volts ( for Cards # 3
and # 4), while the second creates + 4.2 volts ( for Cards # 8 and # 9). Both sources
employ a pair of opamps. The first is a unity- gain voltage- follower whose input is
a temperature- compensated
voltage
created
by
a resistor/diode
network;
the
second
is
a unity- gain
inverter
which
creates
the
complementary
negative
voltage.
9) Loudness Controller: ( on Card # 2)
The subjective sensation of loudness is complex and is not readily measurable by
simple
electrical
means.
No
common
meter,
whether
PPM,
VU,
or
true
RMS,
provides a good correlation to subjective loudness because loudness perception is
a function of several factors:
1.
The sensitivity of the ear as a function of frequency;
2.
The
spectral
distribution
of
the
sound
energy ( a given
amount
of
energy
spread
out
over
a wide
frequency
range
sounds
louder
than
the
same
amount
of
energy
located
within
a third- octave
frequency
band);
and,
3.
The duration of the energy.
The Loudness Controller estimates the subjectively perceived loudness of a given
piece
of
program
material.
If
the
loudness
exceeds
a certain
threshold,
the
Loudness Controller produces an output current which is applied to the " Master"
timing module on card # 5, causing gain reduction to occur above and beyond that
ordinarily produced by the compressor control circuitry. In this way, the Loudness
Controller " takes
control"
of
the gain reduction circuitry until
the loudness is
reduced below a preset threshold.
This
gain reduction ordinarily
occurs within 50-100ms,
depending on how
much
extra
gain reduction is required in a given case. While this action effectively
reduces or
eliminates
viewer
annoyance
due to excessively
loud commercials
in
most
instances, it can also reduce the impact of certain dramatic sound effects
in entertainment programming. The controller is thus defeatable either manually
or remotely by means of optically- isolated terminals on the rear panel. Both the
local
Loudness
Controller
OPERATE/DEFEAT
switch
and
the
corresponding
terminals on
the rear
panel
interface with a CMOS
flip-flop which latches to
determine
the
state
of
the Loudness Controller.
An
internal
strap
determines
whether the Loudness Controller is initialized in OPERATE or in DEFEAT mode
upon power- up.
The Loudness Controller works by dividing the audio into seven frequency bands,
rectifying
their
outputs,
summing
the
rectified
outputs together
with
different
gains, applying the sum to a thresholding device ( to determine at what point the
Loudness Controller begins to act), and finally applying the weighted, thresholded
sum to the " Master" timing module.
Factor ( 1) ( above) is taken into account by choosing the weighting of the bands
such that the summed output from the bands ( after rectification) follows the 70
phon
equal
loudness
curve
as
determined
by
psychoacoustical
research
at
CBS
Technology Center.
A-11
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