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3-14
OPERATION
Protection Limiting: Orban's Approach
Traditionally, protection limiters have used peak-sensing automatic gain control (AGC)
processors to control peak levels. Superficially, this approach seems reasonable. The
purpose of a protection limiter is to control the peak levels in a transmission channel.
This traditional approach ignores one crucial requirement for protection limiter perform-
ance: The limiter must provide natural-sounding control that is undetectable to the ear
except by an A/B comparison to the original source material. Because the human ear is
basically average-sensing, not peak-sensing, the simplistic peak-sensing AGC technique
causes highly unnatural variations in subjectively-perceived loudness. Audio material
with a high peak-to-average ratio emerges from such a limiter much quieter than audio
material with a low peak-to-average ratio. The ear perceives this as an unnatural, un-
pleasant pumping quality. Thus the traditional peak-sensing AGC limiter fails to provide
natural sound quality and we must use more sophisticated techniques.
To achieve natural sound quality, the gain control section of the limiter must respond
like the ear. This means that the gain control must respond approximately to the power
(not the peak level) in the signal. Further, because the sensitivity of the ear decreases
dramatically below 150Hz, the control must be frequency-weighted to compensate. Oth-
erwise, heavy bass would audibly modulate the loudness of midrange program material,
a problem called spectral gain intermodulation.
The dual-band limiter controls the level driving the following look-ahead limiter. Prior
to the HF limiter, a phase-coherent crossover divides the signal into frequency bands
above and below 150Hz. The above-150Hz material is connected to the master band,
which determines the overall limiting. This prevents limiter-induced spectral gain inter-
modulation — audible modulation of the loudness of midrange and high frequency pro-
gram material by bass-generated limiting.
The below-150Hz material is connected to the bass band. The gain-control signal pro-
duced by the master band is cross-coupled into the bass band, so that the gain of the bass
band ordinarily tracks the gain of the master band exactly, preserving frequency bal-
ances. When the bass band encounters exceptionally heavy bass, it momentarily provides
extra limiting to preclude excessive level at the dual-band limiter's output.
The dual-band limiter has an attack time of approximately 2 milliseconds. This moderate
attack time prevents it from producing limiting on every transient spike. Such limiting
could otherwise create audible holes in the program.
The dual-band limiter is gated: When its input level drops below the factory-set thresh-
old of gating, the release rate is radically slowed to avoid audible noise breathing.
This compressor gate is not the same as a conventional noise gate because it
is not intended to reduce noise or other low-level undesired sounds to a
lower level than that occurring in the original program. Its only purpose is to
prevent the unnatural exaggeration of such material.
ORBAN Model 6200
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