Orban:
'While the physics of carrier pinch-off limit any AM modulation system to an absolute
negative modulation limit of 100%, it is possible to modulate positive peaks as high
as desired. In the United States, the FCC permits positive peaks of up to 125% modulation. Many countries have similar restrictions.
However, many transmitters cannot achieve such modulation without substantial
distortion, if they can achieve it at all. The transmitter's power supply can sometimes
be strengthened to correct this. Sometimes, RF drive capability to the final power
amplifier must be increased.
Voice, by its nature, is substantially asymmetrical. Therefore, asymmetrical modulation was popular at one time in an attempt to increase the loudness of voice. Traditionally, this was achieved by preserving the natural asymmetry of the voice signal.
An asymmetry detector reversed the polarity of the signal to maintain greater positive modulation. The peaks were then clipped to a level of -100%, +125%.
OPTIMOD-AM takes a different approach: OPTIMOD-AM's input conditioning filter
contains a time dispersion circuit (phase scrambler) that makes asymmetrical input
material, like voice, substantially symmetrical.
OPTIMOD-AM permits symmetrical or asymmetrical operation of both the safety
clipper and multiband distortion-canceling clipper. Asymmetrical clipping slightly increases loudness and brightness, and can produce dense positive peaks up to 125%.
However, such asymmetrical processing by its very nature produces both odd and
even-order harmonic and IM distortion. While even-order harmonic distortion may
sound pleasingly bright, IM distortion of any order sounds nasty.
There is really nothing lost by not modulating asymmetrically: Listening tests easily
demonstrate that modulating symmetrically, if time dispersion has been applied to
the audio, produces a considerably louder and cleaner sound than does asymmetrical modulation that retains the natural asymmetry of its program material.
Some of the newer transmitters of the pulse-width modulation type have circuitry
for holding the carrier shift constant with modulation. Since artificial asymmetry can
introduce short-term DC components (corresponding to dynamic upward carrier
shift), such carrier shift cancellation circuitry can become confused, resulting in further distortion."
Bob Orban wrote that about 35 years ago. His first AM Optimod had a phase flipper to maintain positive peaks. The problem with phase flippers is that they are annoying to on air talent if they are monitoring themselves off air in real time. The stuff about transmitter capability went away decades ago. Most modern transmitters can easily reach 150 to 160% positive modulation.
To my knowledge, nobody has ever manufactured a processor for assymmetrical modulation. To properly accomplish asymmetry one must start at the individual microphone inputs and assure the peaks are going in the correct direction. Then every compression and limiting section in the processing chain that employs peak detection must either have positive peak detection disabled or at least be adjusted to a higher threshold. This can be done if attention is paid to every detail. I have done it at numerous stations and it does in fact sound better and more open than phase scrambled audio.
All that said, the usual method is to just clip the negative peaks until the positives reach the desired level.