3-14
OPERATION ORBAN MODEL 8685
In a peak limiter, each Fourier component of the input signal is a sepa-
rate “carrier” and the peak limiting process produces modulation side-
bands around each Fourier component.
Considered this way, a hard clipper has a wideband gain control signal
and thus introduces sidebands that are far removed in frequency from
their associated Fourier “carriers.” Hence, the “carriers” have little ability
to mask the resulting sidebands psychoacoustically. Conversely, a look-
ahead limiter’s gain control signal has a much lower bandwidth and pro-
duces modulation sidebands that are less likely to be audible.
Simple wideband look-ahead limiting can still produce audible inter-
modulation distortion between heavy bass and midrange material. The
look-ahead limiter in your Optimod uses sophisticated techniques to re-
duce such IM distortion without compromising loudness capability.
Loudness Control
The 8685’s third-generation CBS Loudness Controller follows the multiband com-
pressor in the 8685’s signal flow diagram. However, it takes into account the effect
of the following look-ahead limiter so that it effectively monitors the loudness at
the limiter’s output, not the loudness at the multiband compressor’s output. It does
so by scaling its sidechain drive by two gain factors: (1) the MB
LIMIT DRIVE control
and (2) the 858’s active D
IALNORM value, which is determined either globally or by
the active preset depending on the active preset’s D
IALNORM setting. (See Low-IM
Look-Ahead Limiter on page 3-13.)
The
loudness controller constrains the loudness of most commercials well enough to
eliminate viewer annoyance. It works by constantly monitoring the subjective loud-
ness of the 8685's output. The subjective loudness is a single value that represents
the listener’s impression of the loudness in the listening room. It takes into account
the contribution of all stereo and surround channels.
When subjective loudness would otherwise exceed the threshold set by the
L
OUDNESS THRESHOLD control, the loudness controller reduces the gain of material
above 200 Hz, preventing loudness from exceeding the threshold. To prevent the
loudness controller from causing too much dynamic bass boost, you can use the
L
OUDNESS CONTROLLER BASS COUPLE control to limit the maximum difference be-
tween the gain of the band below 200 Hz and the band above 200Hz. For example,
when this control is set to 3
DB and the loudness controller’s gain reduction is 10 dB,
the gain reduction below 200 Hz will be 7 dB. However, if the loudness controller’s
gain reduction is 2 dB, the gain reduction below 200 Hz will be 0 dB because the dif-
ference is now less than 3 dB.
The loudness controller is triggered mainly by program material that has a lot of en-
ergy between 1 and 7 kHz, which is the ear’s most sensitive range. If you are using
the two-band structure and you find that the loudness controller is producing audi-
ble artifacts because the program forces it to do a great deal of gain reduction, we
suggest that you use the five-band structure instead. This will automatically re-
equalize such program material and will de-ess extremely sibilant program material
before the loudness controller receives it.
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