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

CP-3

PLUS

Theory and Design

Lexicon

AUTO AZIMUTH

Acknowledgements

The CP-3 following patents: U.S. numbers 3,632,886, 3,746,792 and 3,959,590; Canadian numbers 1,004,603 and 1,037,877. "Dolby" and the doubleD symbol are trademarks of Dolby Laboratories Licensing Corporation.

The CP-3 LucasArts Entertainment Company.

"Auto-Azimuth" and the A-Z logo are trademarks of Lexicon, Inc.

Lexicon Part #070-08806 Rev 2

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is manufactured under license from Dolby Laboratories Licensing Corporation. Additionally licensed under one or more of the

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is manufactured under license from LucasArts Entertainment Company. "THX" and "Home THX Cinema" are trademarks of

Lexicon, Inc.• 3 Oak Park • Bedford, MA 01730 USA • Tel 781-280-0390 • Fax 781-280-0490

Printed in the United States of America

Lexicon

The CP-3

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Digital Surround Processor

Front Panel

Rear Panel

Expanded Remote Standard Remote

CP-3

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Digital Surround Processor

CP-3

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and Design

Theory

All of the operating modes of the Lexicon CP-3

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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-3

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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 performance or film when heard with the CP-3

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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-3

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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-3

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allows complete control of every aspect of operation.

To re-create the experience of being at a live musical performance the

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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-3

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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, or SI. The CP-3

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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.

Introduction to the CP-3

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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, effectively 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.

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Theory

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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-3

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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 simulating 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-3

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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 includes side or side-located rear speakers. Additionally, the CP-3

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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-3

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. This utilizes a patented, completely digital Dolby Pro Logic Surround decoder, and is the only one with automatic correction of inter-channel phase and channelbalance 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-tosurround 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-3

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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).

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Digital Surround Processor

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.

CP-3

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Lateral Sound

Theory

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 earphones, gave excellent reproduction of spatial qualities. Unfortunately, the stereo image tended to appear entirely inside the head, spoiling the accuracy 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 eliminate 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 different. The best have large amounts of sideways moving reflected sound.

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Sound from the side is vital to listener comfort and involvement . . . It must really be from the side!

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, monaural overhead and rear-wall kind. These differences can be overcome by design. A notable example is the Boston Symphony’s fan-shaped Tanglewood 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 loudspeakers 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.

In an anechoic chamber, this sideways sound is missing . . . and music sounds unpleasant.

In an ordinary room, the room supplies these directions, and the sound is cramped, but tolerable. The overall impression, however, may be muddy due to unwanted frontal reflections.

Ordinary two-speaker stereo works as well as it does because sidewaysmoving reflections can be excited at low frequencies by two loudspeakers if they are placed asymmetrically in the room or if they are driven with outof-phase low-frequency information.

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(Out-of-phase bass is intentionally provided in the best stereo recordings.) Another reason is that most listening 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-3

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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

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CP-3

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-3 original recording can be extracted by the CP-3

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allows the user to make: the sideways energy cues of the

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, or a different hall sound

can be generated and supplied from the correct directions.

Ambience Extraction

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and Design

Theory

The Panorama and Surround modes in the CP-3

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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 loudspeaker. Some steering is frequently beneficial to music recordings played with a surround speaker arrangement, and it is essential for films.

The confusing frontal reflections can be absorbed, leaving the essential lateral ones. This is better, but not ideal.

Ambience Generation

The Reverb and Ambience modes in the CP-3

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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-3 flections can be absorbed; the CP-3 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 environment.

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, confusing short re-

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Theory

and Design

Lexicon

The Panorama

Mode

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-3

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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-3

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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.

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.

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 frequency.

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Lexicon’s implementation, called the Panorama 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.

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Theory

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-3

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provides adjustments to compensate for offcenter 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.

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Theory

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In a well-damped room with loudspeakers mounted on stands away from the walls, the Panorama effect can be very exciting, giving the closest possible approximation to the actual hall used for the recording. With true binaural recordings (made with a modern dummy head with accurate external ears and proper equalization) the playback can be uncannily realistic. And, unlike previous versions of this technique, the Panorama mode adds virtually no coloration to the original signal.

Panorama can be used with music, films, or from within the Reverb and Ambience programs to simulate side speakers if the listener is inside the effective area between loudspeakers.

Panorama is used in two ways in the CP-3

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. First, there are the three Panorama modes, designed to reproduce as closely as possible the sound actually recorded by the engineer. If the recording has good natural ambience, Panorama will spread that ambience around the listener, giving a true impression of the original hall.

The Normal and Wide versions of the Panorama mode differ primarily in their handling of low-frequency signals. Normal is designed for recordings whose bass energy is evenly distributed across the stereo stage, atechnique usually found in classical recordings. Wide is designed for recordings with centered bass, as is typical with most jazz, rock and popular music. The only other difference between these two modes is in their initial Effect Level. If you need more bass from Panorama Normal, use Wide and reduce the Effect Level. Conversely, if Panorama Wide is too bass-heavy, use Normal and increase the Effect Level.

The Low Frequency Width control provides another important adjustment to the bass in Panorama. This control is a simple implementation of a Spatial Equalizer (a function which Alan Blumlein referred to as a “shuffler”). One of the ways ordinary stereo excites SI is through the out-of-phase low frequency energy in the recording. The Low Frequency Width control allows the amount of out-of-phase bass in a recording to be adjusted. Even when the Effect level of the Panorama control is all the way down, the Low Frequency Width control is active, allowing the user to experiment with this property of sound.

When the front speakers are close together, the Panorama Effect is less precise but more dramatic, and it works over a larger area.

Recording engineers have only recently become aware of Spatial Equaliza-

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and many older recordings are greatly improved by increasing the low frequency width a little. When the rest of the Panorama mode is not used (by turning down the Effect control) just turning LF Width up a bit can make ordinary recordings quite spacious. The user should exercise caution, however, since some recordings (such as those on Telarc) use microphone techniques which already contain sufficient out-of-phase low frequency energy.

The crosstalk cancellation in Panorama increases the low frequency width as well as the high frequency width of a recording. Recordings in which the engineer deliberately added large amounts of low frequency width will sound too wide and phasey when played with either Panorama Normal or Panorama Wide. Negative values of the Low Frequency Width parameter can bring the low frequencies back in line with the higher frequencies and make the playback with Panorama more effective. The Binaural setting of

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