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

Download

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

PLUS

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

PLUS

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

PLUS

Digital Surround Processor

Front Panel

Rear Panel

Expanded Remote Standard Remote

CP-3

PLUS

Digital Surround Processor

CP-3

PLUS

and Design

Theory

All of the operating modes of the Lexicon CP-3

PLUS

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

PLUS

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

PLUS

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

PLUS

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

PLUS

allows complete control of every aspect of operation.

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

PLUS

CP-3

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

PLUS

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

PLUS

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

PLUS

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.

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-3

PLUS

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

PLUS

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

PLUS

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

PLUS

. 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

PLUS

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).

PLUS

CP-3

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

PLUS

and Design

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.

CP-3

PLUS

Theory

and Design

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.

3,4

(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

PLUS

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

PLUS

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

PLUS

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

PLUS

, or a different hall sound

can be generated and supplied from the correct directions.

Ambience Extraction

CP-3

PLUS

and Design

Theory

The Panorama and Surround modes in the CP-3

PLUS

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

PLUS

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.

PLUS

, confusing short re-

PLUS

CP-3

PLUS

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

PLUS

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

PLUS

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.

PLUS

CP-3

Digital Surround Processor

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.

CP-3

PLUS

and Design

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

PLUS

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.

CP-3

PLUS

Theory

and Design

Lexicon

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

PLUS

. 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-

3,4

tion

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

PLUS

CP-3

Digital Surround Processor

Panorama greatly increases low frequency width and should only be used with true binaural recordings, which have very little out-of-phase low frequency energy. A few compatible binaural recordings are becoming available, in which the low frequency width has been increased to match the requirements of loudspeaker playback. These recordings may sound best when played with the Normal setting.

Panorama is capable of simulating side loudspeakers effectively, but cannot mimic sound sources to the rear of the listener. So we have added a delayed Left minus Right signal which can be sent to the side and rear loudspeakers. The delay is adjustable, as is the treble rolloff.

For a listener in the ideal position, Panorama, with surround (side or rear) speakers, gives a nearly ideal re-creation of the original recording area.

Panorama is also used as an element in the Ambience and Reverberation modes, where it can synthesize side loudspeakers which are not present in the installation. In this mode the stereo inputs to the CP-3 to the front loudspeakers, with the digital outputs of the CP-3

PLUS

are fed directly

PLUS

mixed in according to the setting of the Effect Level control. The Ambience or Reverb side outputs are sent through Panorama before being mixed into the front loudspeakers, so the added sound spreads beyond them and does not interfere with the original material. When side speakers are present, the Panorama Effect parameter is automatically turned down and no mixing occurs into the front speakers.

CP-3

PLUS

Theory

and Design

Speaker alignment is important. A 6" difference in the distance to the rear wall can greatly change the effective area, unless compensated for by the LISTENER POS parameter.

While the Panorama mode re-creates the space that already exists in the recording, the Ambience mode actually generates the side and rear reflection patterns of a number of ideal concert halls. The reflections were determined by computer ray-tracing using architectural data, augmented by Lexicon’s decades of experience with digital concert-hall simulation.

The Ambience simulation is done in stereo. Instead of feeding a monaural signal derived from the combined left and right channel inputs of the processor, the CP-3

PLUS

has two input points corresponding to instruments placed on the left or right side of the stage. From these the computer calculates the loudness and delay of the reflections for the side and rear loudspeakers. By using full stereo for the inputs to the simulation programs, the spread or separation of the soundstage is automatically preserved in the simulation process. This obviates the need for adjustment on every different recording, a chore required for decent results on some other systems.

Ambience generates primarily the strong reflections which appear early in the reverberation process (in the first few hundred milliseconds). Although some reverberant decay can be added with the Liveness parameter, the early reflections constitute the primary audible effect, giving you the im-

The Ambience Mode

CP-3

PLUS

Theory

and Design

The best way to generate spatial impression (SI) is with appropriate signals from loudspeakers at the side. Reverb, Ambience and the Surround programs can all be used to generate these signals.

Lexicon

pression of the hall surrounding you while the music is playing. As in actual concert halls, the most important contribution to spatial realism will be the sound that comes from the sides.

Early research with quadraphonics involved extensive experimentation with speaker placement, and confirmed that additional speakers beside the listener sounded better than the conventional approach of putting pairs of speakers in front and behind. Our research into speaker placement with Ambience confirmed the previous results of others: The side speakers should be directly to the side of the main listening position, plus or minus about 20 degrees. Beyond this critical angle the spatial impression is greatly reduced.

The sides are the most important additional speakers, much more so than the rears. Keep in mind that, although you can use the left and right main speakers to simulate a phantom center speaker, you cannot produce SI with one speaker in the front and one at the rear . (You can perform this experiment for yourself with the CP-3

PLUS

using the Ambience mode: Try the side speakers both in their usual positions and in the front corners. If your listening room has enough absorption to damp its side-wall reflections, the side location will provide much more spatial impression, and will sound substantially better.) Speakers in the front are useful - if you want the best possible sound from Ambience you may want to try an additional pair in the front corners, wired in parallel with the rear. This additional pair, as well as the rear speakers, may sound best if placed above the listener. Be aware that you will need to shut off the extra front speakers when using the Panorama and Surround modes.

Placement of side loudspeakers is critical! Placement B sounds much better than A, especially when the room is well damped.

The effectiveness of the CP-3

PLUS

’s Ambience simulation is heavily dependent on the source material and the playback room. If the playback room is large and reverberant, its reflections may dominate those generated by the

PLUS

CP-3

. Carpet, drapes and furniture can all be used to break up or absorb undesirable reflections, making it easier to hear the processor’s output. The balance between the side, rear and front speakers is also very important. If the channels are set up properly, no single speaker will be audible by itself.

There are two basic hall shapes in Ambience: Rectangular and Fan. In a listening room with sufficient acoustical absorption, using source material without too much reverberation of its own, the two shapes are clearly distinguishable. For most classical recordings the rectangular hall sounds somewhat better because of its stronger side reflections. In our fan-shaped hall (unlike actual physical designs) you can increase the proportion of side to rear sounds, by increasing the Effect Level and shifting the F/B Balance controls toward the front. When this is done, another different but also pleasant sound results.

Although the Ambience mode can provide some recirculation (adjustable with the Liveness parameter), for long decay times it is better to use Reverb. The Large Hall ambiences are not intended to be used on material which is more appropriate to a smaller ambience, such as a small hall or a club.

PLUS

CP-3

Highly percussive material is almost always better in the Small mode which is quite successful in livening and expanding popular music.

If you have no side loudspeakers, both Ambience and Reverb contain versions of the Panorama mode that will simulate them within a narrow area between the speakers.

Digital Surround Processor

CP-3

PLUS

and Design

Theory

While the Ambience mode simulates the early reflections of real halls, Reverberation is more concerned with what happens to the sound after the first hundred milliseconds or so. The first reflections are not intended to simulate any particular hall and no real shape will be audible.

The Reverb mode produces a rapidly increasing echo density that smooths out impulsive sounds. The decay in this mode is unusually smooth and natural and can create the effect of a church or a very reverberant hall. The early sideways reflections, which produce the most SI, are weaker than they are in the Ambience mode. In Reverb, as in Ambience, the stereo input is fed directly to the front loudspeakers. Some of the side energy can also be fed to the front speakers; side and rear outputs are generated from the stereo input.

Direction is critical to maintaining clarity in Reverb and Ambience. The recording engineer has probably put as much reverberation in the recording as the music can withstand. Adding more through speakers located in front of the listener is generally not a good idea, since these effects combine with the sound from the front speakers, making the music muddy. Delay and reverb in the rear can occasionally be helpful but the ear is not particularly good at distinguishing between front and rear sounds and, as with Ambience, it is at the sides that Reverb is most needed. However, if you are interested in producing the largest possible effect from the Reverb modes, consider placing speakers in the front corners of the room and driving them in parallel with the rear speakers. Note, however, that this configuration does not compensate for the absence of side speakers, which continue to be the most important. Also, these additional speakers should be shut off for correct operation of the Panorama and Surround modes.

The Reverb Mode

If you have no side loudspeakers, both Ambience and Reverb contain versions of the Panorama mode that will simulate them within a narrow area between the speakers . Choosing a speaker configuration with no side speakers will automatically turn the Panorama Effect on and mix the side outputs into the front outputs.

Reverb is very good for simulating a large reverberant space.

CP-3

PLUS

Theory

and Design

Lexicon

The Surround

Modes

The requirements for processing sound for film viewing are quite different than those for music listening. Music demands re-creation of the original performance venue, or the evocation of an appropriate setting, as well as the creation of a believable soundstage. When visual images are introduced, not only is strong audio imaging necessary to reinforce the illusion that dialog originates at the screen image, but a diffuse soundfield must be created which envelops the viewer without distracting attention from the screen.

In addition, films are designed to provide an enveloping experience in large, reverberant auditoriums where background noise is a significant consideration, where the screen dominates the field of vision, and where the sound must be as uniform as possible over a large seating area — in short, quite a different environment from your living room.

When all of these factors are taken into consideration, it becomes obvious that no single speaker system, or single method of processing, is optimal, or even adequate for all types of music as well as for audio/video material.

The CP-3 precisely because it provides such a wide range of processing options. This flexibility is readily apparent in the variety of Surround modes available to the user, each optimized for maximizing listener involvement in different types of material played on a system whose speakers are laid out primarily for films.

PLUS

is able to optimize the listening experience of any material

TELEVISION provides surround effects to enhance television viewing of monaural, stereo and stereo-synthesized programs. This program has the most flexibility in adjustment of any of the Surround modes.

MUSIC uses a unique ambience extraction method which can provide spectacular results with music, as well as providing seven channels.

FULL (Full Range) allows unprocessed music to be played over all the speakers for background music, or for maximum acoustical output of the system.

MONO expands the music and effects on monaural films into the surround channels while leaving dialog in the front center.

PRO LOGIC provides the same decoding used in Dolby Stereo theater systems, using as many as eight speakers for front, center, side, rear and subwoofer channels.

THX CINEMA combines Dolby Pro Logic decoding and the spectral enhancements of the LucasArts Home THX Cinema system to recreate film sound in your home the way it was heard in the final production stages. In the CP-3

PLUS

a further enhancement, Stereo Surround, is also available.

CP-3

PLUS

Digital Surround Processor

CP-3

PLUS

and Design

Theory

Television sound playback presents a challenge simply because of the wide variety of recording and processing techniques used in program production and transmission. Programs and commercials are recorded in mono, stereo, and surround sound, then subjected to compressors, erratic stereo synthesizers, sleepy production interns, and other electronic tortures. Often all of this can be experienced on one channel within five minutes.

The Television mode makes use of directional steering to enhance dialog in the center channel and remove it from the left and right channels, while maintaining as much stereo separation as possible. In addition, it contains several interesting variable parameters. In the Preset version of Television these parameters are set for pleasing results with a wide variety of programming, but they can be changed to produce a versatile mode for film sound — and some music as well.

The Center Effect parameter, which controls the steering among the front three channels, is preset at 12, causing centrally located sounds to be diminished by 3dB in the side speakers and vice-versa. The setting for Dolby Surround encoded films is 16 (up all the way).

The rear noise reduction parameter (Rear Dolby B) is preset to ON, meaning that the high-frequency sound in the rear channels is being decoded by the Dolby B circuit. This control may be turned off for non-surround-encoded material.

Television

The Side Speaker Assign parameter can be used to connect the side speakers in parallel with the front speakers. The resulting wide frontal image is very effective on a wide variety of films and music.

The Re-equalizer applies the THX main equalization to all the channels.

Stereo surround is also available in this mode.

CP-3

PLUS

Theory

and Design

Lexicon

Music Surround

The Music Surround mode in the CP-3

PLUS

is designed to optimally play conventional stereo music through any system which includes side or sidelocated rear speakers.

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 viewing, however, often have the front loudspeakers too close together for optimal reproduction of the spaciousness of the original soundfield. With the CP-3

PLUS

, additional speakers located near the sides of the listeners can

add to the playback spaciousness. In the Reverb and Ambience modes, the

PLUS

CP-3

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-3

PLUS

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 Surround solves this problem in a novel way. The side speakers reproduce 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. The center speaker is steered, and can be delayed relative to the fronts. The center delay allows the center speaker to appear to be at exactly the same distance from the listener as the front speakers, even if it is placed (as usual) on a line between the front 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.

PLUS

CP-3

Rear speakers in Music Surround (as in the other Surround modes) are driven by the THX enhanced surround signal. The front left and right outputs are attenuated by the volume and balance controls, but are otherwise unaltered by the processor. This absence of main front steering is ideal for playing stereo music, where the original stereo signals are reproduced from the front speakers with absolutely no alterations. The Music Surround mode really shines on this material — the center speaker adds a little stability to the front image without being at all obvious, while the side and rear speakers add a tremendous amount of ambience.

For film use the lack of steering of the main speakers is noticeable on dialog, and any of the normal film surround modes may give better results. If the system has no center loudspeaker main steering is not needed, and Music Surround may well be preferred over the other modes.

Digital Surround Processor

CP-3

PLUS

and Design

Theory

The Full Range mode is provided simply to allow music to be played over your entire system. All speaker outputs in this mode are simply fed a stereo signal.

The left front and left side speakers are driven by the left input signal. The right front and right side speakers are driven by the right input signal. The center and rear channels are both driven by the sum of the left and right inputs. This mode includes a High Pass parameter which allows you to remove bass from side and rear speakers which might not be able to handle it. Center Level and Sub Bass Boost controls are also provided.

This mode is primarily useful for large audiences, or background music when entertaining.

Full Range

CP-3

PLUS

Theory

and Design

Lexicon

The Film Surround

Modes

The goal of the CP-3

PLUS

film surround modes is to maximize viewer involvement and to faithfully reproduce the director's intentions for the soundtrack in your own listening environment. Before explaining the way each of the film surround modes (Mono Logic, Pro Logic and Home THX Cinema) accomplish this goal, it is important to understand something about the way film soundtracks are made and presented in the theater.

In the early 1940’s, large movie studios owned their own theaters and took responsibility for their own quality standards. During this period, movie theaters had the best sound reproduction heard anywhere — each major studio had a master sound engineer to ensure that the sound systems in that studio’s theaters performed properly. A decade later, the studios were forced to sell off their theater holdings in an anti-trust action, and quality became the responsibility of independent theater owners. Since each theater could choose films from any studio, it was no longer practical, or feasible, for the studios to monitor the quality of each theater. At the same time, the impact of television caused a decline in theater attendance which left little money for individual theater owners to reinvest in their facilities. As a result, sound technology in theaters froze. Despite the advances being made in recording and in home music systems during this period, film sound remained essentially unchanged through the 50’s and 60’s — in fact, by the end of the 1960’s the average teenager had a music system at home which was considerably superior to theater systems.

This situation began to change in the 1970’s with the introduction of Dolby Stereo. This technological breakthrough, which allowed four channels of sound to be recorded onto the two available optical soundtracks of a 35mm movie print, yielded spectacular results — and created a demand for improved film sound tracks, and for better-sounding theaters. Theaters that upgraded their sound systems were rewarded with larger audiences. Over the next decade, Dolby Stereo became an established standard for film sound recording — but theater sound systems, although improved, varied in their ultimate accuracy.

In the 1980’s a new movie theater sound system was created under the name THX. This system addressed, for the first time, the design of a theater speaker system which could faithfully reproduce the film director’s intentions. The THX Sound System complemented and enhanced the playback of Dolby Stereo, and encompassed standards for power amplifiers, speakers, and the acoustics of theaters themselves, to assure the best possible reproduction of movie soundtracks. By 1990, this system was available in over 500 theaters across the country. The recent availability of film on video tape has led to a demand for home audio/video systems which equal the best theater sound. The Home THX Cinema system was designed to address this need, and the CP-3

The variety of CP-3

PLUS

surround film modes allows you to have the highest

PLUS

incorporates it into the THX mode.

possible sound quality for video material — whether you select classic monaural movies or the latest releases with all of their special effects — simply by changing modes.

CP-3

PLUS

Digital Surround Processor

CP-3

PLUS

and Design

Theory

Mono Logic is a surround conversion mode for monaural film sound tracks. A quick look through any video rental selection will prove the usefulness of such a mode; a large percentage of titles are mono.

The problem of mono-to-stereo conversion is an old one. One time-honored solution is to break the incoming signal into frequency bands, sending some to one channel and the rest to the other. When the filters are complementary (when the sum of the two output channels equals the original input channel) this solution can give stereo spread without ruining the tonal balance. When the filters are non-complementary, they can produce an unpleasant fake stereo effect.

Some effort has been made to design filter pairs for film sound which leave voice frequencies unchanged while spreading out the music. More recent designs have gone in another direction, using digital or analog delay lines to produce a comb filter effect. So far, these attempts have not been very successful.

The principal element of film sound is dialog and the principal rule in reproducing it is to assure that it appears exclusively in the center channel. Broadcasters, who have an interest in converting mixtures of dialog and music to synthesized stereo, have built circuits designed to turn off the stereo synthesizer when voice appears. Unfortunately, the switch from mono to stereo is often abrupt and the chances of dropping into mono by mistake during music are high. One basic problem with films, especially modern ones, is that music or background effects which should be spread out into the side speakers frequently appear at a low level beneath the dialog.

Mono Logic

The Mono Logic mode electronically identifies certain properties of film speech and removes it from the stereo synthesis. This allows music and effects in the dialog to be spread out while leaving the dialog centered. The remaining music and effects are directed to the input of a room simulation mode that creates a space the size of a large room or small theater. The room simulator has outputs for left, right, side and rear surround speakers.

The monaural input sound from the film is unchanged in the center speaker, so that all the dialog and music that the director expected to come from the screen still does — with no modification or reverb. Partly because of the acoustical character of the room synthesizer, the result is often so successful that switching from Mono Logic to Pro Logic or THX may make a surprisingly small difference.

The most critical adjustment in Mono Logic is the Effect Level. Ideally the film’s music and effects should appear to come from the front but with the added sense of a large space surrounding you. The side and rear speakers should not be individually audible.

During the early days of film stereo, dialog was sometimes mixed (by "panning" the monaural dialog track) to come from the same part of the screen as the image of the actor. Subjective reactions to this technique were varied, and technical problems with some magnetic sound tracks helped to discourage the practice, so modern movies are seldom mixed with panned dialog. In a home system with a good Pro Logic decoder, however, the effect can work quite well; recent releases with panned dialog include "Yellow Submarine" and "Superman I." In most films though, all dialog comes from the center channel.

CP-3

PLUS

Theory

and Design

Lexicon

High frequency attenuation is provided via the variable Treble Rolloff parameter and the Academy Filter On/Off parameter, which recreates the proper tonal balance of older, monaural films which were recorded with a much narrower and brighter frequency response than current films.

Pro Logic

A Dolby Stereo film sound track has four basic components: Left and right channels, a center front channel and a surround channel. The first three are fed to speakers arrayed behind the movie screen, while the surround sound goes to speakers on the side and rear walls of the theater. The four channels are recorded on separate magnetic tracks and are combined by the Dolby Stereo matrix encoder into two stereo channels called Lt and Rt (left total and right total) during the final mixing process. The original left and right channels go directly onto the left and right channels of the Dolby Stereo mix. The center channel is fed equally to both channels, in phase, and the surround track is fed equally to both channels, but 180 degrees out of phase. (It’s actually a little more complex than that, but the the end result is essentially the same for this discussion).

The center channel contains the most energy, including the dialog; music is normally mixed so that it appears to come from the front, with reverberation or ambience coming from the surrounds. The surround channel ambient information is a powerful tool for the film sound mixer. It is this information that helps us believe that the scene in the film is real. For special effects, music can be encoded to come from all around the listener or even from behind. In any case, with music and ambient effects there is almost always a substantial spread across the front of the loudspeaker array.

Sound effects can come from any direction around the listener and it is the job of the decoder to duplicate as closely as possible the film mixer’s placements.

Films originally have four channels: one for dialog and three for music and effects. To make a Dolby Stereo film, these are combined to two.

Dolby Surround Decoding

When the movie is shown the two Dolby Stereo tracks must be decoded and separated into the original four. The basic Dolby Surround decoder does this in a rather rudimentary way: it supplies a signal to the center channel which is just the sum of the two input channels. This signal contains the dialog. However, the left and right signals still contain dialog too, so the dialog is spread out among the three front speakers. Similarly, the Dolby Surround decoder takes all out-of-phase signals and sends them to the surround speakers, while leaving the original out-of-phase components in the left and right front speakers.

PLUS

CP-3

Digital Surround Processor

The basic Dolby Surround decoder has high channel separation between left and right decoded audio, and between center and surround. The separation between left or right and center, or between left or right and surround, however, is only a few dB. The simple Dolby Surround decoder does pretty well with music (although sometimes the center channel is too loud) but, because any sound will be reproduced in at least three loudspeakers, effects are smeared and often unconvincing .

Pro Logic Decoding

A Pro Logic decoder, like the professional Dolby Stereo cinema processor, both enhances the dialog in the center and removes it from the left and right, while maintaining as much stereo separation as possible. This is a form of directional steering. Properly done, steering prevents the dialog from appearing in the other channels and enhances its plausibility.

CP-3

PLUS

and Design

Theory

The requirements for film sound are quite different from those for the playback of music. The most important track in most films is the dialog (assuming you aren’t watching 10,000 Years BC or Quest For Fire...). When the two stereo channels are played back through two speakers with no decoder, dialog will appear to come more or less from the center, but only for those listeners on the center line of the main stereo pair.

The situation is similar with music and sound effects. For example, if the sound was intended to be in the left, the decoder will remove it from the center and surround channels. If it was intended to be halfway between left and center, the Pro Logic decoder presents it equally to the left and center speakers and removes it from the right and surround channels.

The Pro Logic decoder can give good stereo spread and precise control over front-to-back perspective. But the real strength of Pro Logic decoding emerges when music and dialog occur at the same time. When dialog is present, the center channel information must be removed from the left and right channels without reducing the spread or loudness of the music.

Pro Logic decoders sense both the direction of the loudest sound and the difference in level between it and any ambient information. They then use this information to direct the steering. The accuracy with which this is done is even more important in a home decoder than in a professional model, because the small size of the playback room makes decoding errors more audible than they are in a theater. The level detection must be very fast, and the matrix must adapt very quickly or there will be a time lag between the audibility of a sound and its correct steering. Since phase relationships determine how the sound is steered, Pro Logic decoding puts unusual demands on the accuracy of the phase and balance of the input channels. Other Pro-Logic decoders have a front panel control for adjusting input balance and for best results a user should carefully adjust this for each mode. But what if the channel balance varies during playback?

With conventional surround any sound comes from at least three directions.

Simple logic decoders turn down the left and right speakers during dialog. This seriously affects music and effects.

CP-3

PLUS

Theory

and Design

Pro Logic decoders remove dialog from the left and right channels, while maintaining stereo as much as possible.

Lexicon

The manual balancing procedure does nothing to correct azimuth errors. During the preparation of the master for a video tape or disc, misalignment of the playback heads or skewing of the film produce small time differences between the two channels. Azimuth is poorly controlled in both professional video recorders and optical film chains. In the final product, which has been through many generations, it can easily be wrong by 50 microseconds or more, and may vary as the tape or disk is played. At middle and high frequencies it doesn’t take much misalignment to generate large interchannel differences in phase, which are just what the decoder uses to do its steering.

The Dolby encode/decode system deals with this problem by reducing the treble in the surround, so the out-of-phase sibilants in the film do not splatter annoyingly from the rear. This does not, however, reduce the sibilants in the side speakers. Some non-Pro Logic decoders reduce these side-channel sibilants by narrowing the spread of the front channels in the presence of dialog; this compromise is unnecessary in the CP-3

The CP-3

PLUS

Decoder

PLUS

decoder is unusual in a number of ways. First of all, it is entirely

PLUS

digital. (Most surround decoders advertise that they are digital because there is a digital delay line for the surround channel but the matrix and the logic decoding are done in analog.)

Pro Logic requires phase accuracy. Common azimuth errors cause ghost dialog in all channels unless the azimuth error is corrected.

Because the CP-3

PLUS

is all digital, we can use some of the digital memory to delay all the output channels by 20 milliseconds — about the same as the acoustic delay you get in the front row of a theater. (The surround channel is delayed by an additional 15 to 30 milliseconds.) This delay allows plenty of time for the CP-3

PLUS

to determine the direction of sounds and adjust the matrix before the sounds are sent to the amplifiers. This substantially improves dialog and effects cancellation, as is immediately apparent from the spread of ambient material or music, even in the presence of dialog. The

PLUS

CP-3 errors in the incoming material. All the time the film is playing, the CP-3

can also sense and continuously correct both balance and azimuth

PLUS

is checking balance and azimuth, keeping the dialog perfectly centered. The result is superior steering. An added benefit is that the CP-3

PLUS

needs no front panel input balance control; the user need not bother with this adjustment. You can check the quality of the balance and azimuth in a tape or disc if you wish by turning the Auto Azimuth/Balance parameter off and observing any changes in the location of dialog and effects.

Stereo Surround in the CP-3

PLUS

As of this writing, the film industry is moving from a 4 channel audio standard to a 5.1 channel standard. The new standard is similar to the old, but has two surround channels instead of one, and an additonal low bandwidth channel for a subwoofer. Once one has heard some good examples of stereo surround channels the mono surround channel of Dolby Surround is no longer satisfying.

As most of the information in the surrounds is ambient information and

PLUS

CP-3

Digital Surround Processor

music, stereo significantly increases listening enjoyment. As listeners become accustomed to placing the surround speakers out to the sides instead of behind them, stereo surrounds become even more important.

The decorrelation circuit required in previous THX decoders overcomes some of the limitations of the mono surround channel. While it does not give directionality to the signal, it at least increases the sense of spaciousness and reduces the tendency of the surround speakers.toward localization. Artifacts from the decorrelation itself are usually mild. However, when an event is specifically steered to the surrounds the decorrelation can diffuse the signal too much. For example, if a jet flies from the front to the rear, the sound should not sound sharp and well localized in the front, and then disappear into a diffused mush in the rear.

Lexicon has developed a new technique for decoding a stereo surround signal from a standard matrix encoding. This yields a 4-2-5 encoding/ decoding process. When the source film was originally a 5.1 channel film the resulting 4-2-5 decoding is much closer to the original.

CP-3

PLUS

and Design

Theory

The decoding works by combining very high quality standard Pro Logic decoding with the technology of the Music Logic program. The front channels (left, center, and right) are decoded with standard Pro Logic, but with the addition of an adjustable delay for the center channel. The two rear channels operate in a dual mode. When the film surround content is primarily music or ambience, the rear channels maintain full stereo, reproducing the left and right front channels with the addition of frequency contouring and delay. (In the TV mode the contouring is adjustable — allowing full bandwidth surround if desired.) When the front hemisphere contains steered information, such as dialog or sound effects, these signals are cleanly removed from the stereo surrounds. Thus, dialog and all onscreen sound effects remain in the front where they belong. The new decoding depends critically on our ability to remove unwanted sound from the stereo surrounds, and this would not be possible without the accuracy afforded by our Auto Balance and Auto-Azimuth circuits.

As sound effects move toward the rear, the processor decodes differently. Effects which move from left to rear pan smoothly from the left front loudspeaker to the left rear, and then from left rear to both left and right rear loudspeakers. Effects which pan from right to rear behave similarly. Once full rear is reached, the decoder outputs are identical to a standard Pro Logic decoder, so full compatibility is maintained. The action of the decoder has been extensively tested both at Lexicon and at Lucasfilm to ensure the intent of the film director is accurately reproduced.

Although films have not yet been deliberately mixed to take advantage of the capabilities of the new decoding, the improvement on standard films can be dramatic. The improvement in spaciousness with music and ambience is obvious. On films with agressive sound effects the combination of surround steering and localization can convince listeners they are listening to a true discrete surround mix.

Auto Azimuth and Auto Balance features in the CP-3 coding of the new 5.1 channel film standard.

PLUS

allow accurate de-

CP-3

PLUS

Theory

and Design

Lexicon

Home THX Cinema

Home THX Cinema is designed to present film viewers with the same audio impressions perceived by the film director on the "dubbing stage" — the studio in which the final mix of the soundtrack is established. These stages feature a large screen for viewing the picture and a massive audio console for controlling every aspect of the soundtrack. THX attempts to precisely reproduce the experience intended by the director in a home environment. In general, this requires not only considerations of the characteristics of a small room (as opposed to a large dubbing stage), but many specific system performance requirements. A complete THX system for accurate translation of dynamic motion picture soundtracks requires a specific array of speakers which are designed to produce the soundtrack’s full frequency response and dynamics without distortion, amplifiers which meet exacting specifications for distortion, noise, stability and dynamic power, and a Dolby Pro Logic controller with electronic enhancement.

As a complete discussion of the theory of THX design is beyond the scope of this booklet, we refer you to LucasArts for detailed information on all aspects of the Home THX Cinema system. (LucasArts Entertainment Company, THX Division, P.O. Box 2009, San Rafael, California 94912 Telephone: 415-662-1900) Here, we will limit our discussion to the advantages of the

PLUS

CP-3

as a THX controller.

The main requirements of any THX controller are:

Dolby Pro-Logic Surround decoding Subwoofer Crossover Re-equalization Surround Decorrelation Surround Timbre-Matching

THX plus Stereo Surround 5-channel decoding combines full Pro Logic decoding with stereo surrounds.

Dolby Pro-Logic decoding is not only necessary to decode soundtracks which are encoded in a matrixed surround sound format such as Dolby Stereo or Ultra*Stereo, but enables all motion pictures (including the more than 5000 made in matrix stereo) to be reproduced as they were heard in the film studio dubbing stage — no matter what delivery system is used. The

PLUS

CP-3

is a completely digital, Dolby Pro Logic decoder with patented automatic correction of inter-channel phase and channel-balance errors. With this technology, home theater speakers not only reproduce the appropriate audio information, but correct errors which occur in the multiple rerecording processes of most video software releases.

The Subwoofer Crossover feature enables the subwoofer speaker to reproduce only frequencies in the low bass range, leaving the front, center, sides, and rears to reproduce frequencies above 80 Hz. This facilitates optimum placement for imaging and smoothness, protects smaller speakers, and uses amplifiers more efficiently.

PLUS

CP-3

Digital Surround Processor

Re-Equalization is provided as a control to compensate for the fact that the soundtracks in films sound "bright" when listened to in a home environment. This results from a combination of the way we perceive sound in large halls vs. small rooms, and the theater equalization which has become standardized throughout the movie industry. The re-equalization control restores the sound to its natural balance, and reduces listening fatigue dramatically by reducing excessive high frequencies.

As the surround channel in a Dolby stereo film is monaural, the signal is neither spacious nor enveloping. In a theater, the quantity and placement of surround speakers compensates for this; in the home, arrays of 12-18 speakers are generally impractical. To diffuse the perceived sound, the

PLUS

CP-3

(in the decorrelated mode) provides a decorrelation algorithm which, after the monaural signal is split in two, alters the phase/time relationships between channels slightly. This eliminates the mono effect from the rear channel and creates an enveloping soundfield. When the "Stereo Surround" parameter is selected, the rear channels become stereophonic, and the decorrelation is unnecessary. This setting is recommended.

Timbre matching compensates for the difference in characteristics of sounds coming from different locations. In real life, timbre differences help us differentiate sounds which originate from the sides and rear from those originating from the front . In a film, however, when sound is panned from front to rear, or from rear to front, these changes, which are present even with perfectly matched speakers, are undesireable. Timbre-Matching provides equalization to the surround channels, reducing the perceived change in character of sounds which are panned from the screen into the surrounds.

CP-3

PLUS

and Design

Theory

CP-3

PLUS

Theory

and Design

Lexicon

Speaker Placement

CAUTION: Speaker magnets can distort the TV image. If you see any picture discoloration in the area closest to the speaker, the speaker is too close to the screen.

To some degree, speaker placement will depend on whether your priorities lean more toward music listening or film viewing. Fortunately, the goals are the same for both listening situations: stable localization (imaging) in the front, and diffuse, enveloping surround sound from the sides and rear.

The placement of speakers will be somewhat restricted in your home by the need to have the sound closely associated with both the screen and your seating area. Within this limitation, however, there are many possible variations, for example in the height, angle and distance from the walls. These choices, again, will depend on the type and number of speakers you are using, as well as the physical characteristics of your room. Some of the general effects caused by room reflections are explained in the following section — “The Listening Room”. Additional information relevant to your particular system should be available from your speaker manuals. THX speakers, in particular, are designed for specific placement and the manufacturer’s recommendations should be followed for best results. With that in mind, here are some general guidelines:

If only two speakers are used, they should be placed relatively close to the sides of the screen. With this configuration and a single listener, the Panorama mode should be used for both music and films. For a wider sound area, try the Ambience mode and adjust the Panorama Effect.

With multiple speakers, the main front speakers should be close to the edge of the screen. If you have anything smaller than a 60" TV, this requires the front speakers to be quite close together. Although spreading the speakers further apart would provide a wider, more exciting soundstage, with some films a few sound effects will not correlate well with the visual action when the speakers are spread apart. Normally, the front speaker spread which is ideal for video is too narrow for music, but the stereo effects processing in the CP-3

PLUS

restores the lost spaciousness. The front speakers should be placed at equal heights and at equal distances from the front wall. Try angling the speakers in toward the listening position (toe in) and keep them away from the side walls.

A center speaker is so important for realistic dialog placement that if it is not possible to have a center speaker it is frequently better to plug the Center output of the CP-3

PLUS

into the audio input of your video monitor and use its built-in speaker (if it has one) than it is to run the dialog through the main loudspeakers. (The CP-3

PLUS

provides a control for this setup which splits

bass out of the center channel and directs it to the left and right speakers.)

The Center speaker should be located directly beneath or above the screen (shielded.) — Turning on its side a speaker which is designed to be used vertically will significantly alter the sound wave patterns it is intended to produce.

PLUS

CP-3

Digital Surround Processor

Right, Left and Center speakers should be positioned at equal heights and at equal distances from the front wall. Depending on their height, they should be tilted to aim vertically at the listening area — they should not necessarily be angled in from the side walls toward this area.

In most setups, although the left and right speakers are about the same distance from your listening position, the center speaker is often closer to you, causing sound from the center speaker to reach you earlier than sound from the left and right. Equalizing the path length (the distance from the speaker to your ear) of the three front speakers in your system will improve the image. A Center Delay parameter (along with other "Listener Position" controls) is provided in the CP-3

PLUS

setup menu to let you compensate

electronically for any physical misalignment .

Optimum side speaker placement depends on your room and listening position, the type of speakers used, whether you are primarily interested in film sound or music, and aesthetics (which we’ll leave to you). If you have THX-type surround speakers, follow the manufacturer’s recommendations for placement. If you are sitting within 6-7 feet of the main speakers, place the side speakers directly to the sides of the listening area. If you normally sit further away from the main speakers, the sides should be somewhat forward of the listening position, preferably angled back towards it. A bit of experimentation will prove very helpful. The goal is to blend the side speaker’s energy with the main speakers; you should not be distinctly aware of output of the side speakers.

CP-3

PLUS

and Design

Theory

Side speaker height should be at or above ear level. Placement near the ceiling/wall boundary may help disperse the sound, and will reinforce the bass response of the speaker, but very high placement can reduce the spaciousness that the true stereo processing of the CP-3

PLUS

provides. If pedestal or wall mounting proves impractical, ceiling-mounted speakers will suffice. Place them far apart and equidistant from the listening area.

Rear speaker placement is somewhat less critical. Here the main goal for both film and music is to produce a diffuse and enveloping soundfield. The height of rear speakers will depend on both room size and furnishing. In general, placement of the rear speaker(s) above the listening area can be very effective. Note, however, that placing these speakers too high in a narrow room, or at the ceiling, may reduce the spaciousness of the sound.

Subwoofer placement is less critical, but in some positions in your room may cause standing wave paterns and/or a rattling of furnishings. Refer to “The Listening Room” section, and specifically to your subwoofer manual for recommendations. The Subwoofer Output is a monaural signal created by summing the left and right inputs, then filtering out frequencies above 80 Hz at a rate of 24dB per octave. For the tragically technical, this is a Linkwitz-Riley LPF -6dB at 80Hz.

CP-3

PLUS

Theory

and Design

Lexicon

The Listening Room

If you are using THX speakers, be aware that they have significant design differences from standard (non-THX) speakers. Extensive information is available from the manufacturer on their design and on maximizing their effectiveness in a room. Although not contradictory to those recommendations, the following guidelines are intended as a generalized reference for those using standard speakers.

The physical characteristics and furnishings in your listening room will affect the way sound is reflected and dispersed through the room. A great deal of research has been done in the field of room acoustics and a superb environment can be constructed with this research in mind. In this discussion, however, we will limit ourselves by assuming that your room is already built, and that your goal is to get the best sound possible from your system in that room. Even within this limited scope, there are many factors which may affect the quality of sound. Fortunately, where there are problems, there are also some relatively simple solutions.

All rooms have acoustical characteristics determined by their dimensions and structural materials. The hard surfaces of the room reflect sounds back and forth in patterns which are likely to interfere with the sounds generated by your system. The nature of these reflections, and their effects, is dealt exhaustively in the wealth of material available on Room Acoustics — here we will simplify by stating that the first general goal is to dampen or diffuse these echoes — to create an acoustically neutral room which doesn’t interfere with the environment your system creates. The most obvious way to do this is to add absorbent material to those surfaces from which the most offensive reflections arise, and to break up undesireable reflective patterns with uneven surfaces. Fortunately, furnishing the room with carpets, drapery and furniture goes a long way toward accomplishing this. (Absorbent materials can be commercially-available acoustic panels, fiberglass, dense foam, drapes, or upholstered furniture. Diffusive materials can be commercially-available panels, or irregular furniture, such as bookcases.)

Now, what we really want to do is “tune” the room to maximize the clarity of dialog within a localized front soundstage, and to create a diffuse, nonlocalized soundfield for effects. This can often be done by adjusting speaker placement, by altering room furnishings or by some combination of both, but to do this effectively, we must look briefly at the kinds of interference you may encounter. Following are some basic problems and suggested solutions.

CP-3

PLUS

Digital Surround Processor

CP-3

PLUS

and Design

Theory

Room reflections cause blurring of the image of a localized front soundstage. Sounds which are ideally perceived as coming from the screen, reach the listener from other directions, spoiling the illusion. These unwanted reflections can be eliminated by placing absorbent or diffusive material at the point of reflection. To maximize the localization of the soundstage, an attempt should be made to deaden the area immediately behind and adjacent to the front speakers.

Speaker alignment is important. If front speakers are placed at unequal distances from a wall, the tonal balance between the two speakers will change. This is particularly important in placement of speakers in the front of the room where sounds should pan realistically. Care should be taken to place speakers the same distance from the front wall. An attempt should also be made to achieve approximate symmetry in the distance between each of these speakers and the side walls. In addition to being placed consistently with regard to room surfaces, the front speakers should be placed well away from side walls.

Room reflections

Room reflections interfere with the image of a localized front soundstage.

These unwanted reflections can be either absorbed …

…or diffused.

CP-3

PLUS

Theory

and Design

Lexicon

Room echoes

Irregular bass response

Room echoes cause interference which results in an unnaturally “bright” sound. These echoes can also be absorbed or diffused. Use heavy insulating drapes over large expanses of glass.

Bass frequencies have long wave lengths — these may be as long as one of the dimensions of your room. This can cause over-emphasis of some frequencies and cancelling out of others. This effect is most noticeable in rooms which have two or more dimensions which are equal (for example, length and width). This type of room will exhibit irregular bass response in different parts of the room. Keep in mind that this effect is worst in nearly empty rooms. Large pieces of furniture will break up the reflections between parallel surfaces quite nicely. Moving speakers further in from the walls can also make a noticeable improvement.

Structural resonance

Maximizing the effect of the

surround soundfield

Note that speakers placed against a solid wall can alter the bass and midbass response of your system — making it sound bass-heavy. Placing a speaker in a corner (where 3 surfaces meet) will make any speaker not designed for such placement sound muddy. Very thin walls will allow some of this amplified bass energy to escape, reducing the effect. Moving the speakers in from the walls also works nicely.

Structural resonance (pictures and windows rattling. etc.) can be caused even by desireable bass frequencies. If the bass response of your system is at the desired levels, these rattles can be eliminated by putting felt on picture frames, or around window cracks.

The listener should not be able to pinpoint the source of surround effects, therefore output from the surround speakers should not be aimed directly at the listening area. This is easily accomplished by raising the surround speakers. Although surround speakers require some reflected sound to be effective, these reflections should be diffusive — providing randomized reflections in many directions. Bookcases and other irregular surfaces provide this sort of diffusion, as do some commercially-available acoustic panels.

PLUS

CP-3

Digital Surround Processor

Creating a Custom Concert Hall Seat

The CP-3 virtually any acoustic space you desire. It is extremely simple to walk through the parameters of a program, vary them to your liking, name the space, and store it. For this example, let's do a fairly radical redesign — changing a church into a hall. We'll start with the biggest, most reverberant mode: Preset 9, CATHEDRAL.

Think about a nice concert hall where you would enjoy going out to see your favorite performer. Consider a hall of approximately 2,700-3,000 seats with a balcony (and, of course, valet parking). Get your performer's CD, cue it up in the CD player, but don't start it yet — We'll design the hall first.

First, let's decide how long the hall is, as this will influence the reverb time and the overall character. Press the PARAM button to select room size and set it to 50 meters (meaning the simulated hall will be 50 meters long).

How close to the stage do you want to sit? Might as well go to the good seats— how about row M in the center of the orchestra section? Predelay approximates the amount of time between the original sound of the performer and the sound of the hall. The closer you are to the stage, the longer it will take for the sound to bounce off the back walls and get back to you. Let's add a little extra. (Some predelay is inherent in the mode to keep the direct sound distinct.) Press PARAM ▲ to adjust PREDELAY to 24ms.

We'll assume you have side speakers and skip the Panorama Effect parameters.

The Speech Detection feature is actually a special dialog recognition program which reduces the reverberation on spoken voice. As we aren't using the radio (and, therefore, have no announcers or commercials), let's turn Speech Detection off by pressing PARAM ▼.

Think about what the walls look like in a good concert hall. Many irregularly shaped objects like elaborate columns, carved panels, draping and light fixtures come to mind.

PLUS

allows you to create

These objects absorb and diffuse the sound. A good sounding space never has a lot of high frequencies bouncing back at you. Press PARAM to select TREBLE ROLLOFF to manipulate this part of the hall. Change the value to 2.2kHz and then press PARAM again to go to the next parameter.

Skip the BASS BOOST parameter for now. You can set this to taste, after you've finished designing the hall.

Press PARAM to select MID RT. The reverb time of a space helps describe how large it is. For example, a cathedral has a much longer reverb time than a living room. Let's modify the reverb time of our factory preset to an appropriate value. A hall of the size we are creating will have a reverb time of approximately

2.00 seconds for the mid frequencies. Press PARAM until the display shows MID RT 3.2 SEC. (indicating it will take the midrange reverb 3.2 seconds to decay 60dB). Press PARAM ▼ to change the value to 2.16 SEC. and immediately press PARAM to go to the next parameter.

CP-3

PLUS

Theory

and Design

BASS RT describes how long the low frequencies of the hall stay around relative to the midrange. You have three choices: 1.25 (warm), 1.0 (neutral), and

0.7 (dry). Typically, a hall of this size will have a little longer bass reverb time. (Hence, the preset value of 1.25.) This one is your choice — use PARAM ▲ and ▼ to change the value.

Lastly, hold the Back BALANCE button down to move the balance all the way to the back. Now it's time to listen to your creation. Start your CD. It should sound like you are standing in the lobby of the hall with the doors closed. Slowly move the balance to the front of the hall one step at a time by repeatedly pressing Front BALANCE. (Don't hold it down.) You are now walking down the aisle to take your seat. When the sound feels right, stop pressing the balance key and "take your seat." If you want to make the effect even more dramatic, increase the EFFECT LEVEL (bottom left section of the remote.)

If you want to give your new hall a name and save it, press STORE, then press PARAM to select SET MODE NAME. Press PARAM ▲ or ▼ to display the current program name. Now, use PARAM to select any character in the current name and use PARAM ▲ and ▼ to select new characters. Once you've named your hall, press SETUP to return to the Store menu. Press PARAM to select STORE THIS MODE, then use PARAM ▲ and ▼ to step to the register where you want this mode stored. Once you have selected a location, press STORE.

It's fun, and pretty easy, to create almost any acoustic space you want. For the megalomaniacs among you try creating your own private Taj Mahal — in the Cathedral preset, increase MID RT to 11.2 and turn EFFECT LEVEL all the way up. Play a CD for a couple seconds, then hit pause and close your eyes ... Just remember, you can't hurt anything by experimenting, and the factory presets can't be permanently modified, so don't worry about losing them forever.

Enjoy!

CP-3

PLUS

Theory

and Design

Lexicon

References

1. Schroeder, M.R., Gottlob, D. and Siebrasse, K.F.,”Comparative Study of European Concert Halls: Correlation of Subjective Preference with Geometric and Acoustic Parameters”, J. Acoust. Soc. Am., vol. 56, pp. 1195-1204 (1974).

2. Barron, M., and A.H. Marshall, “Spatial Impression Due to Early Lateral Reflections in Concert Halls: The Derivation of a Physical Measure”, J. Sound Vibration, vol. 77, pp. 211, 232 (1981).

3. Griesinger, D., “Spaciousness and Localization in Listening Rooms and Their Effects on the Recording Technique”, J. of the Audio Eng. Soc., vol. 34 no. 4, pp. 255-268 (1986).

4. Griesinger, D., “New Perspectives on Coincident and Semi Coincident Microphone Arrays”, J. of the Audio Eng. Soc., 82nd Convention, London(1987) Preprint # 2464 (H-4).

5. Damaske and Mellert, “Ein Verfahren zur richtungstreuen Schallabbildung des oberen Halbraumes Hber zwei Lautsprecher”, Acustica, vol. 22, pp. 153-162 (1969/70)

6. Bishnu S. Atal and Manfred R. Schroeder, “Apparent Sound Source Translator” - U.S. Patent Disclosure, Patent No. 3,236,949, Feb. 22, 1966.

7. Borish, J., “An Auditorium Simulator for Domestic Use”, J. of the Audio Eng. Soc., 33 (5) p. 330 (1985).

8. Blumlein, A.D., British Patent 394,325, 14 June, 1933, reprinted inJ. of the Audio Eng. Soc., Vol. 6, pp. 91-98, 130 (April, 1958).

9. Schroeder, M.R., “Progress in Architectural Acoustics and Artificial Reverberation: Concert Hall Acoustics and Number Theory”, J. of the Audio Eng. Soc., 32(4) pp. 194-203 (1984).

10. Blauert, J. and Lindemann, W., “Auditory Spaciousness: Some further psychoacoustic analyses”, J. Acoustical Soc. Am. 80 (2), August, 1986.

11. Griesinger, D., “Theory and Design of a Digital Audio Signal Processor for Home Use”, . of the Audio Eng. Soc., 37 pp. 40-50.

12. Marimoto, and Pösselt, C., “Contribution of Reverberation to Auditory Spaciousness in Concert Halls”, J. Acoustical Soc. Japan (E)10, pp. 87-92, 2 (1984).

13. Bradley, J.S., “Contemporary Approaches to Evaluating Auditorium Acoustics”, Proc. of the AES 8th International Conference, 1990, pp. 59-69.

14. Griesinger, D., “Beyond Dolby Surround — a seven-channel decoder for two-channel music and standard surround films”, Audio Eng. Soc. Convention N.Y., Oct., 1991.

Lexicon

Lexicon Part #070-08806 Rev 2

Printed in the United States of America

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

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

Specifications and Main Features

Frequently Asked Questions

User Manual