Lexicon CP-1 PLUS V2.0 Owner's Manual page 56

Theory
and
Design
CP-1 Digital Audio Environment Processor
Music Logic
Conventional stereo generally relies on the acoustics of the playback room
to regenerate the important side, or lateral, energy which is present in nearly
all music performance spaces. Speakers which are arranged for film view-
ing, however, often have the front loudspeakers too close together for
optimal reproduction of the spaciousness of the original soundfield. With
the CP-1, additional speakers located near the sides of the listeners can add
to the playback spaciousness. In the Concert Hall mode, the CP-1 calculates
and generates the ambient sound from a larger room and presents this
sound through the additional speakers. This works well, but the ambience
is generated by the processor — not contained in the recording. In the
Surround modes, surround channel information is extracted from signals
encoded on the recording by the sound mixer — the CP-1 does not add
anything to the sound.
On films, this surround information is intended to be reproduced in
monaural from an array of speakers all around the rear of the listener.
Unless there is enough energy accidentally encoded into the surround
channel to give significant sideways energy, surround speakers won't
contribute very much to the listening experience of ordinary stereo music.
In addition, with all speakers except the fronts reproducing the same
monaural signal, no directional effects are possible. There may be some
sense of the music coming from all around you, but the violins and cellos are
equally loud from both sides.
Music Logic solves this problem in a novel way. The side speakers repro-
duce the left and right front loudspeaker signals with two additions: delay
and inverse steering.
The delay serves a simple purpose. Sounds are kept from being localized to
the sides by an approximately 20ms delay inserted between the front and
side speakers.
Inverse steering acts to remove a strongly steered signal from certain
directions. As an example, assume you have a strong signal in the left
channel of a film. With normal steering the processor enhances the level of
that signal in the left speaker, and actively removes it from the other
speakers. Inverse steering actively removes this signal from the left side
speaker, while keeping its level strong in the left front speaker. This signal
removal is done cleverly, so the level of any unsteered signal which might
be present at the same time (such as music in a film, or the rest of the
orchestra in a music recording) is not significantly reduced. The sonic result
is a much wider soundstage and a very spacious sound. When music is
played, any loud instrument or sound effect is reproduced from the front
speakers, not distractingly located off to the side.
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