Optimod-AM 9400 V1.2 Operating Manual - Orban

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OPERATION ORBAN MODEL 9400
rate “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 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 produces modulation sidebands that are less likely to be
audible.
Simple wideband look-ahead limiting can still produce audible intermodulation
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.
Loudness and density
The amount of gain reduction determines how much the loudness of soft passages
will be increased (and, therefore, how consistent overall loudness will be). The
automatic gain control (AGC) and the multiband limiter both provide gain reduction,
although their effects are quite different.
In a competently-designed processor, audibly objectionable distortion occurs only
when the processor is clipping peaks to prevent the audio from exceeding the peak
modulation limits of the transmission channel. The less clipping that occurs, the less
likely that the listener will hear distortion. However, to reduce clipping, you must
decrease the drive level to the clipper, which causes the average level (and thus, the
loudness) to decrease proportionally.
Receiver high frequency rolloff introduces further complications. A typical
receiver’s severe HF rolloff reduces the headroom available at high frequencies and
makes it difficult to achieve a bright sound. This is because bright sound requires
considerable high frequency power to appear at the output of the receiver, thus
requiring a very large amount of high frequency power to be transmitted so that a
sufficient amount will survive the receiver’s rolloff.
To increase brightness and intelligibility at the receiver, the 9400’s NRSC pre-emphasis
boosts the treble at 6dB/octave starting at 2.1 kHz. HF CURVE settings from 0 to 10
produce more severe pre-emphasis, boosting at 18dB/octave with 2 kHz up about 3
dB. Without very artful processing, this pre-emphasis will radically increase the level of
the peaks and force you to decrease the average level proportionally. Orban's high
frequency limiting and distortion-canceling clipping systems greatly ease this trade-off,
but cannot eliminate it. Therefore, you can only increase brightness by reducing
average modulation (loudness) unless you accept the increased distortion caused by
driving the final clippers harder.
In processing, there is a direct trade-off between loudness, brightness, and distortion.
You can improve one only at the expense of one or both of the other two. Thanks to
Orban's psychoacoustically-optimized designs, this is less true of Orban processors than
of any others. Nevertheless, all intelligent processor designers must acknowledge and
work within the laws of physics and psychoacoustics as they apply to these trade-offs.