Orban OPTIMOD-FM 8600Si, Руководство по эксплуатации

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OPERATION ORBAN MODEL 8600Si
Introduction to Processing
Some Audio Processing Concepts
Reducing the peak-to-average ratio of the audio increases loudness. If peaks are re-
duced, the average level can be increased within the permitted modulation limits.
The effectiveness with which this can be accomplished without introducing objec-
tionable side effects (such as pumping or intermodulation distortion) is the single
best measure of audio processing effectiveness.
Compression reduces the difference in level between the soft and loud sounds to
make more efficient use of permitted peak level limits, resulting in a subjective in-
crease in the loudness of soft sounds. It cannot make loud sounds seem louder. Com-
pression reduces dynamic range relatively slowly in a manner similar to riding the
gain: Limiting and clipping, on the other hand, reduce the short-term peak-to-
average ratio of the audio.
Limiting increases audio density. Increasing density can make loud sounds seem
louder, but can also result in an unattractive busier, flatter, or denser sound. It is im-
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 an-
ticipate peaks that are coming up.
The 8600Si 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 sepa-
rate “carrier” and the peak limiting process produces modulation side-
bands 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.