Orban OPTIMOD-FM 8600, Operating Manual

Page: 184 / 397

3-4
OPERATION ORBAN MODEL 8600
portant to be aware of the many negative subjective side effects of excessive density
when setting controls that affect the density of the processed sound.
Clipping
sharp peaks does not produce any audible side effects when done moder-
ately. Excessive clipping will be perceived as audible distortion.
Look-ahead limiting is limiting that prevents overshoots by examining a few milli-
seconds of the unprocessed sound before it is limited. This way the limiter can antic-
ipate peaks that are coming up.
The 8600 uses look-ahead techniques in several parts of the processing to minimize
overshoot for a given level of processing artifacts, among other things.
It is important to minimize audible peak-limiter-induced distortion when
one is driving a low bitrate codec because one does not want to waste
precious bits encoding the distortion. Look-ahead limiting can achieve
this goal; hard clipping cannot.
One can model any peak limiter as a multiplier that multiplies its input
signal by a gain control signal. This is a form of amplitude modulation.
Amplitude modulation produces sidebands around the “carrier” signal. In
a peak limiter, each Fourier component of the input signal is a separate
“carrier” and the peak limiting process produces modulation sidebands
around each Fourier component.
Considered from this perspective, a hard clipper has a wideband gain
control signal and thus introduces sidebands that are far removed in fre-
quency 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 produces modulation sidebands that are less likely to be
audible.
Simple wideband look-ahead limiting can still produce audible intermod-
ulation distortion between heavy bass and midrange material. The look-
ahead limiter in your Optimod uses sophisticated techniques to reduce
such IM distortion without compromising loudness capability.
Distortion in Processing
In a competently designed processor, distortion occurs only when the processor is
controlling peaks to prevent the audio from exceeding the peak modulation limits
of the transmission channel. The less peak control that occurs, the less likely that the
listener will hear distortion. However, to reduce the amount of peak control, you
must decrease the drive level to the peak limiter, which causes the average level
(and thus, the loudness) to decrease proportionally.
Loudness and Distortion
In FM processing, there is a direct trade-off between loudness, brightness, and dis-
tortion. You can improve one only at the expense of one or both of the others.
Thanks to Orban’s psychoacoustically optimized designs, this is less true of Orban
processors than of others.