Lexicon CP-3 PLUS - THEORY AND DESIGN REV 2 Manual

CP-3PLUS
Theory
and
Design

Lexicon
Lexicon Part #070-08806 Rev 2 Printed in the United States of America
Lexicon, Inc.• 3 Oak Park • Bedford, MA 01730 USA • Tel [phone] • Fax [phone]
Copyright ©1991, 1995 Lexicon. Inc.
All Rights Reserved.
Lexicon Patent: U.S. no. 4, 862, 502; other patents pending on the CP-3.
Acknowledgements
The CP-3PLUS is manufactured under license from Dolby Laboratories Licensing Corporation. Additionally licensed under one or more of the
followingpatents:U.S.numbers3,632,886,3,746,792and3,959,590;Canadiannumbers1,004,603and1,037,877."Dolby"andthedouble-
D symbol are trademarks of Dolby Laboratories Licensing Corporation.
The CP-3PLUS is manufactured under license from LucasArts Entertainment Company. "THX" and "Home THX Cinema" are trademarks of
LucasArts Entertainment Company.
"Auto-Azimuth" and the A-Z logo
are trademarks of Lexicon, Inc.
AUTO AZIMUTH

Lexicon
Expanded Remote Standard Remote
Rear Panel
Front Panel
The CP-3PLUS Digital Surround Processor

1
CP-3 PLUS Theory
and Design
CP-3PLUS Digital Surround Processor
Introduction
to the CP-3PLUS
All of the operating modes of the Lexicon CP-3PLUS Digital Surround
Processor have a common goal: to draw you, the listener, more deeply into
a musical performance or a film. For music the CP-3PLUS uses true stereo
digital processing to recreate the original recording space or to create a new
one of your choosing. For films it offers an extremely accurate version of
Dolby®Pro Logic Surround decoding with all of the enhancements of the
LucasFilm Home THX®Cinema system and Lexicon's own decoding for
monaural film soundtracks. The increase in impact of a musical perfor-
mance or film when heard with the CP-3PLUS is incredible.
A great deal of effort went into designing an instrument which would be
flexible enough to satisfy the most critical listeners and yet be simple to
operate. Essentially, the CP-3PLUS is a signal processing computer that can be
custom-programmed for any specific system. Once installed, it can be
operated by either of the two remote controls supplied with the unit. The
Standard Remote features simple, intuitive controls for most day-to-day
operations; the Expanded Remote, which is used to customize the CP-3PLUS,
allows complete control of every aspect of operation.
To re-create the experience of being at a live musical performance the
CP-3PLUS draws on recent studies of concert-hall acoustics, and applies this
research to home listening rooms. Our auditory sense is quite adept at
interpreting clues about our physical environment. Even with your eyes
closed, it is possible to get a good mental picture of the room or hall you are
in by listening to the ambience, or reflected sound energy, in the room. We
are not aware of our auditory sense in everyday life because it confirms
what our eyes identify as the environment. When we listen to recorded
music, however, there are no visual clues and we rely completely on our
sense of hearing. The introduction of two-speaker stereo systems over
thirty years ago brought dramatic improvement to high fidelity music
reproduction. With a carefully-designed system, and good recording, it
became possible to produce a good sonic picture of the original event.
Unfortunately, our listening rooms do not approximate the acoustics of a
good concert hall, an intimate jazz club, or a magnificent cathedral — our
ears tell us where we really are. The Lexicon CP-3PLUS is designed to
overcome this fundamental limitation to two-speaker reproduction and
bring us closer to the ultimate goal of transporting ourselves to the original
musical event. The object is to increase the sideways-moving sound in a
room, thus increasing Spatial Impression, orSI. The CP-3PLUS increases SI by
either extracting it from the original recording, using the Panorama or
Surround modes, or by generating a new acoustic environment with
Ambience or Reverb.
When a listener is in the correct spot the Panorama mode provides an almost
ideal re-creation of the original recording space. It works by using digital
signal processing to cancel the crosstalk between the listener’s ears, effec-
tively spreading the sound from the two front loudspeakers in a wide arc in
front of the listener. With the optional addition of rear speakers, Panorama
can be startling in its realism.

2
CP-3 PLUS Theory
and Design Lexicon
The Ambience and Reverberation modes transform the listening room into
a new acoustic space, letting you choose an environment which matches
your music or your mood. Unlike most ambience processors, the CP-3PLUS
provides full stereo processing, preserving the critical SI information in the
recording and expanding upon it. The Ambience mode generates the side
and rear reflection patterns of idealized rooms and concert halls. The larger
spaces add the true depth and realism of a concert hall to classical and
popular music, while the smaller spaces are ideal for jazz and rock. The
Reverberation mode is similar, but places more emphasis on rich, dense
reverberant decay than on early reflections. It is especially good for simu-
lating large, highly reverberant spaces such as churches, stadiums, and
cathedrals.
The requirements for processing sound for home theater are quite different
than those for music. Lexicon invented the technology that permits the most
accurate reproduction of film sound in the same system that is used for
music listening, and the software-based CP-3PLUS is optimized for each of
these unique tasks. The Music Surround mode is specifically designed to
optimally play conventional stereo music through any system which in-
cludes side or side-located rear speakers. Additionally, the CP-3PLUS is able
to perform automatic analysis and error correction to compensate for
problems in the source material.
For films encoded with Dolby Surround, Lexicon has incorporated the
Lucasfilm Home THX Cinema processing into the CP-3PLUS. This utilizes a
patented, completely digital Dolby Pro Logic Surround decoder, and is the
only one with automatic correction of inter-channel phase and channel-
balance errors (the most common audio problems in currently available
video releases of films).
A new Stereo Surround feature introduces a technique for extracting five
channels of surround information from a standard 4-2-4 matrix-encoded
soundtrack (Dolby Surround, Ultra*Stereo, etc.) This feature, available in
the THX and TV modes, allows the stereo music in soundtracks to be
reproduced with a full stereo spread, unencumbered by the relatively
narrow separation of the front speakers. Left-to-surround and right-to-
surround pans are also enhanced. Instead of sound jumping from a front
channel to both surrounds, the left and right side speakers can function
independently to facilitate smooth and fluid pans. Sounds directed to hard
left and hard right (the main left and right speakers) will not appear in the
side surrounds.
This technique overcomes the limitation of the monaural sound channel
inherent in these formats, while remaining true to the front hemisphere
directional cues. The audio imaging tracks the picture image in a way which
fulfills the intent of the sound mixer. The drama of this effect is most
noticeable with strong stereo music soundtracks.
The CP-3PLUS also provides modes for expanding monaural film sound
tracks (Mono Logic), general TV viewing (Television) and, of course, direct
two-channel stereo playback (Effects Mute ON).

3
CP-3 PLUS Theory
and Design
CP-3PLUS Digital Surround Processor
Lateral Sound
Concert Hall Acoustics
For decades the study of concert-hall acoustics relied on certain basic
measurements to characterize halls, the main one being the time it takes a
sound to drop in level (decay) by 60 decibels. This is called the reverberation
time or RT60 and is approximately the same as the time it takes a hand clap
to subside to inaudibility. The RT60 is measured as a function of frequency,
usually in bands one octave wide, over the range of audible frequencies. The
resulting curve forms a frequency contour for the hall.
While it was found that most good halls have comparable RT60 curves,
different halls with similar RT60 measurements can sound very different
from each other and listener reactions to them can vary widely. Clearly,
other important factors in the sound field were not being measured.
Through the work of many people (including Manfred Schroeder, A. H.
Marshall, Michael Barron and others) some of these factors were identified.
In an effort to answer the question of why some halls sound so much better
than others, Schroeder devised a method for comparing them without
transporting his subjects from hall to hall. Using a dummy head with
microphone diaphragms in place of ear drums, Schroeder made binaural
recordings in many halls. These recordings, played back through ear-
phones, gave excellent reproduction of spatial qualities. Unfortunately, the
stereo image tended to appear entirely inside the head, spoiling the accu-
racy of such recordings as a test of concert halls.
To overcome this problem, Schroeder played his recordings through a pair
of loudspeakers in an anechoic chamber, using a special electro-acoustic
technique (developed by Atal, Schroeder, Damaske and Mellert) to elimi-
nate crosstalk between the listener’s ears. Normally each speaker is heard
by both ears but Schroeder’s system canceled the sound reaching the right
ear from the left speaker and vice-versa. Provided that the listener held his
head in exactly the right spot, the sound had all the excellent localization
properties of earphones but was properly located outside the listener’s
head.
This technique allowed the first direct comparisons of specific halls.1 From
these studies it was found that the best halls were all characterized by
having large differences in the sound between the two ears in the dummy
head. Very simply, the best halls gave the most stereo.
Michael Barron defined this characteristic in halls as Spatial Impression (SI)
and found that it was created by sideways-moving reflected sound.2 Only
reflections which move from side to side produce SI, because only they give
rise to sound differences between the two ears. Reflections from the front,
ceiling, floor or back wall add loudness and muddiness to the sound but it
is the lateral reflections that draw the listener into the music.
Halls with similar RT60 sound very dif-
ferent. The best have large amounts of
sideways moving reflected sound.

4
CP-3 PLUS Theory
and Design Lexicon
Understanding the importance of lateral reflections enabled more accurate
evaluation of architectural acoustics. The best halls were usually found to
have high lateral energy for obvious architectural reasons, such as a long
narrow shape with high ceilings. Fan-shaped halls, although they have
better sight lines, are more adaptible to multiple uses and hold a greater
number of seats for their total volume than a shoe-box hall, tend to have
fewer attractive, sideways reflections and more of the unpleasant, monau-
ral overhead and rear-wall kind. These differences can be overcome by
design. A notable example is the Boston Symphony’s fan-shaped Tangle-
wood music shed. Tanglewood sounds good because the reflectors above
and in front of the orchestra (known as clouds) have angled sides that reflect
energy sideways onto the audience instead of just downward.
Lateral Sound in your Listening Environment
In an ordinary listening room, conventional stereo set-ups (with loudspeak-
ers separated by 60 degrees or less) do not excite enough SI to sound
pleasant without some help from the room. (A similar speaker arrangement
in the artificial environment of an anechoic chamber is exceedingly detailed
and precise, but unpleasant.) Some lateral reflections are needed to make
the sound musical.
Ordinary two-speaker stereo works as well as it does because sideways-
moving reflections can be excited at low frequencies by two loudspeakers
if they are placed asymmetrically in the room or if they are driven with out-
of-phase low-frequency information.3,4 (Out-of-phase bass is intentionally
provided in the best stereo recordings.) Another reason is that most listen-
ing rooms have reflective surfaces to the sides of the listener. A popular
listening room treatment puts absorptive material at the front of the room,
leaving the walls by the listener reflective. This improves the clarity by
removing the front reflections, while retaining those from the side. This also
explains the appeal of loudspeakers that produce lots of sideways-reflected
energy.
Unfortunately, in most two-speaker set-ups the mid- and high-frequency
lateral sound is reduced unless the speakers are unusually widely placed.
The listener can hear a little of the original hall, stretched between the stereo
loudspeakers, but never really becomes a part of it. What is worse, the lateral
sound that exists in most playback rooms has so little delay that the ear can
not separate it from the direct sound. The reflections generate some SI but
they also cause coloration and muddiness. Small rooms usually sound
better if these reflections are broken up (with wall hangings, furniture or
bookcases) or absorbed (with curtains or sound-absorbent panels). When
this is done the room becomes quieter and clearer but not in any way like the
original hall.
The Lexicon CP-3PLUS resolves this deficiency by supplying appropriate
signals to loudspeakers at the sides of the listener or by modifying signals
to the main loudspeakers to fool the ear into thinking there are loudspeakers
In an ordinary room, the room supplies
these directions, and the sound is
cramped, but tolerable. The overall im-
pression, however, may be muddy due to
unwanted frontal reflections.
In an anechoic chamber, this sideways
sound is missing . . . and music sounds
unpleasant.
Sound from the side is vital to listener
comfort and involvement . . . It must
really be from the side!

5
CP-3 PLUS Theory
and Design
CP-3PLUS Digital Surround Processor
at the sides. Both methods depend on having the added sound come from
a different direction than the original music, and each method has some
advantages. With either method of generating SI there is an additional
choice the CP-3PLUS allows the user to make: the sideways energy cues of the
original recording can be extracted by the CP-3PLUS, or a different hall sound
can be generated and supplied from the correct directions.
Ambience Extraction
The Panorama and Surround modes in the CP-3PLUS extract the original
acoustic cues from the recording and present them to the listener from the
correct directions. We call this ambience extraction. The three Panorama
modes can do this even with only two loudspeakers. Panorama uses
crosstalk elimination to fool the ear into thinking there is a continuous band
of loudspeakers extending all the way to the sides of the listener and also
supplies a delayed and filtered L-R signal to rear speakers. On a good
recording this successfully recreates the original recording space, although
the ideal listening area may be small.
In the Surround modes, Television, Music Logic, Pro logic and Home THX
Cinema are also forms of ambience extraction. They divide the front energy
among three loudspeakers and supply a delayed and filtered difference
signal to surround loudspeakers. Because a center speaker is provided, the
left and right speakers can be placed far enough to the sides of the listener
to directly excite significant SI and the resulting image is wide but seamless.
This setup can sound similar to Panorama, and it works over a large
listening area.
Pro Logic, Televison and THX also provide steering. Steering works by
enhancing the directionality of the loudest sound in a mix; it steers the
sound out of loudspeakers where it is not needed and into the ones closest
to its direction in the image. In popular music the loudest sound is usually
the vocals, which will be preferentially steered toward the center loud-
speaker. Some steering is frequently beneficial to music recordings played
with a surround speaker arrangement, and it is essential for films.
Ambience Generation
The Reverb and Ambience modes in the CP-3PLUS synthesize the side and
rear sound of several different acoustic spaces. Rather than extracting the
acoustic cues of the original hall from the recording, they generate a new
environment, effectively enlarging and improving the acoustics of your
listening room. In these modes the original stereo channels are presented
unaltered to the main loudspeakers, and new signals are generated for the
side and rear loudspeakers. Both Ambience and Reverb can be configured
to use a version of Panorama to generate the correct side sound even when
side loudspeakers are absent. In this mode the side signals are first passed
through Panorama before being mixed into the main loudspeakers, so the
resulting hall sound is perceived as coming from the side and is well
separated from the original sound.
With the CP-3PLUS, confusing short re-
flections can be absorbed; the CP-3PLUS
will supply the essential lateral sound —
which can simulate a much larger space.
The more absorbent the playback room,
the better it will sound, and the closer it
will sound to a real hall, or larger envi-
ronment.
The confusing frontal reflections can be
absorbed, leaving the essential lateral
ones. This is better, but not ideal.

6
CP-3 PLUS Theory
and Design Lexicon
Loudspeakers placed on either side of the listening position are the most
effective and foolproof way to produce added Spatial Impression. Since it
is not always possible to have side loudspeakers, the CP-3PLUS uses crosstalk
elimination to simulate them when they can’t physically be there. In
Panorama the front speakers are driven entirely by the front digital outputs
of the CP-3PLUS.
Versions of the Atal/Schroeder/Damaske/Mellert technique mentioned
earlier have appeared in several consumer signal processors under various
trade names, as well as in a line of loudspeakers that achieved a similar effect
acoustically. These have all been what we call “first-order” devices. To see
what this means, imagine there is a sound coming from the left channel only.
This sound will travel to the left ear of the listener, then diffract around the
listener’s head and be heard by the right ear. If we take the left-channel
sound, delay it just the right amount, invert it in phase and feed it to the right
speaker, it will arrive at the right ear just in time to cancel the crosstalk from
the left speaker.
The Panorama
Mode
The main problem with a first-order device is that the subtracting signal is
also heard by the opposite ear. In our example, the canceling signal from the
right loudspeaker will diffract around the head to the left ear, interfering
with the left-speaker sound and producing a “comb filter” which colors the
sound in an obvious and unpleasant way. Furthermore, the listener’s head
is not well represented by a simple delay line. Both the delay and the
amplitude of the opposite-ear sound vary in complicated ways with fre-
quency.
Imagine a click in the left speaker . . .
Sound from speaker L travels to the left
ear and also to the right ear, a time ∆t
later.
If we supply a negative delayed signal to
the right speaker, this crosstalk can be
canceled.

7
CP-3 PLUS Theory
and Design
CP-3PLUS Digital Surround Processor
Lexicon’s implementation, called thePanorama mode, was designed using
measured data on sound diffraction around the head to shape the frequency
spectrum of the canceling signal. This signal is then itself canceled by a
second signal, and so on, so that both the crosstalk and the signal that is
canceling it are eliminated.
This works extremely well when the room is well damped and the listener’s
head is correctly positioned. The first order devices described earlier
required the listener to sit on the center line between the loudspeakers, and
to arrange the angle between the speakers to correspond to the modemed
delay. Although the CP-3PLUS provides adjustments to compensate for off-
center listening and for varying speaker angles, maintaining a consistent
listening position is still important and becomes more so with increasing
frequency. With wide speaker angles, a movement of as little as 1 inch can
make a perceptible difference. Fortunately, the effect is usually fairly good
everywhere within a zone about one foot wide.
To achieve the fullest Panorama effect, your main loudspeakers should
have good imaging. The smaller speakers that tend to be used with video
systems may have an inherent advantage here but the most important
requirement is that the two speakers have identical frequency response and
symmetrical dispersion. It is not necessary, or desirable, to turn your
listening room into an anechoic chamber but moving the speakers away
from the walls can be helpful, as can adding absorption (as provided by
carpets, curtains and/or sound-absorbent panels) to reduce the reflectivity
of the floor, walls and ceiling.
First-order correction travels to left ear,
where it will be heard unless canceled by
an additional correction. When these
higher-order corrections are supplied,
accurate cancellation is possible.