Operating instructions

OPTIMOD-AM DIGITAL OPERATION
3-3
Some audio processing concepts
Loudness and coverage are increased by reducing the peak-to-average ratio of the
audio. If peaks are reduced, the average level can be increased within the permitted
modulation limits. The effectiveness with which this can be accomplished without
introducing objectionable side effects (like clipping distortion) is the single best
measure of audio processing effectiveness.
Density is the extent to which the short-term RMS amplitude of audio envelope peaks
is made uniform (at the expense of dynamic range). Programs with large amounts of
short-term dynamic range have low density; highly compressed programs have high
density.
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.
Compression 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 9400 uses look-ahead techniques in several parts of the analog processing chain
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-