1 General Concept
Miles Technology's patented Multisonic Imaging system is a method of creating stereo sound with several clear improvements over traditional approaches. It eliminates the need for a listener to stand in a small sweet spot which is equidistant from the left and right loudspeakers. With Multisonic Imaging, a symmetrical stereo image is clearly heard from practically anywhere in the listening area. Stereo coverage is wide enough that loudspeakers can even be placed across the long side of a rectangular room with excellent results.
Also, when compared to conventional two-channel systems, a Multisonic system provides much better focus and clarity across the front-line stereo soundstage.
Multisonic Imaging uses three loudspeakers--left, center, and right--to reproduce stereo signals. It is fully compatible with all existing audio and soundtrack formats, including CD, tape, video, broadcast, or live mixes, and requires no special recording or encoding process to operate.
It utilizes an adjustable electronic matrix circuit which achieves twice the separation of conventional systems in its output channels. The electronic Imager unit is combined with appropriate loudspeaker placement and loudspeaker-dispersion characteristics to provide Multisonic Imaging.
All that is needed is the Miles Technology M86 Multisonic Imager and three front loudspeaker systems. Multisonic Imaging will work together with the three loudspeakers to produce, in effect, a smooth wall of sound that maintains the same basic soundstage width as a high-quality two-channel system. However, it brings the stereo sound into much better focus and allows the listeners to be off-center. The system concept, design, and installation are simple.
In the past, attempts at multichannel sound from stereo sources have failed to supply the expected improvement in performance. The usual alternatives:
1) Complex, expensive left-center-right channel configurations which, if implemented with perfect loudspeaker coverage, can produce effective results.
2) Steered systems (often used for video soundtracks) that can achieve higher measured channel-separation performance, but create side effects including poor dynamics, unstable imaging, and muddy sound.
For live sound reinforcement, stereo and multichannel systems have frequently been avoided. Concerts have been presented with mono sound, possibly with special effects in stereo, due to the tremendous difficulties in supplying quality stereo to more than a relatively small percentage of the audience and listening area.
Multisonic Imaging is very effective in these applications. It solves or avoids the detriments of the other approaches. It is very easy to use and quite cost-effective.
Unlike other matrix systems, channel separation is really not a basic performance criterion, although it will be discussed later in this text from a technical standpoint. The reason for this is simple: Multisonic Imaging does not attempt to create extra directional channels or to isolate a sound to one loudspeaker.
Instead it utilizes all of the loudspeakers to reinforce a correct stereo wavefront, making it much closer to ideal. The goal is not to add more directions to the sound; rather, it is to stabilize and sharply focus the existing soundstage area between the left and right loudspeakers. With Multisonic Imaging, there is a very large area in front of the loudspeakers where this focusing effect occurs; often the entire room can be effectively covered with good stereo imaging.
In addition, compared to a two-loudspeaker setup, comb-filter and time-delay effects are reduced. This results in a significant improvement in sound clarity and intelligibility.
Multisonic Imaging provides the four-direction play-back imaging typically used in movies, but without a dynamic-distorting steering scheme. It also images in all directions simultaneously, and the center loudspeaker is no longer just for locking dialog to the screen. With Multisonic Imaging it is an integral part of the soundstage for all sources.
Surround outputs, which correctly reproduce ambient or surround information from any recording, are also included on the M86 Multisonic Imager. This high-fidelity ambience extraction accurately reproduces the program source and enhances the listening experience.
Even monophonic sources are improved by Multisonic Imaging and the M86. Mono sounds are clearly reproduced at the center loudspeaker. The M86 also includes Miles Technology=s SpreadSound function that spreads monophonic sources into a big, stereo-like soundstage. This unique spatial process maintains total clarity and uncolored sound.
Advanced versions of the Multisonic concept allow improved performance with all types of discrete multichannel systems. Wherever there are adjacent loudspeakers with discrete channels, the Multisonic concept allows the addition of a centered loudspeaker between them which will reinforce all phantom imaging as appropriate. This creates a much smoother and more accurate stereo wavefront which is especially beneficial when the listener is not equidistant from the loudspeakers.
(Multisonic is a trademark of Miles Technology Inc. referring to a special patented stereo imaging system based on an adjustable, separation-optimized matrix system.)
2 Multisonic Imaging: Professional Audio Applications
Multisonic imaging is suitable for a wide range of professional applications including high-power sound systems, monitoring systems, and small-room systems. It is extremely beneficial for both audio-only and audio-with-video systems; in short, any application where quality stereo is needed and loudspeakers are used. It is especially effective in covering large listening areas.
Here are some examples:
Auditoriums and Performing Arts Centers: Multisonic Imaging makes the often-desired left-center-right front-of-house system a convenient reality. Vocal intelligibility is greatly enhanced by the center channel, with overall music program subtleties brought out. Live mixes can be panned out for wide stereo, with results far superior to conventional two-channel systemsCcomb-filtering problems and hot spots are gone. Practically all of a venue's seats will receive excellent, balanced stereo sound.
Houses of Worship: Multisonic imaging provides a strong center channel that improves vocal intelligibility while also creating a much larger listening area receiving optimum, full-range sound. When recorded stereo music is played, the entire congregation hears true stereo sound; when one is speaking through a microphone, the clarity, focus and intelligibility are dramatically improved. This system is well-suited for all types of audio productions, and it works well in difficult acoustic environments.
Nightclubs: The feeling and motivation of dance music, based on the perception of Afeel-good@ sound, is much more clearly conveyed with both dynamic and spatial impact. Stereo sound is more evenly distributed over the entire dance floor and the entire audience area. Overall levels Asound@ more impressive with the same SPL; sound is distributed very evenly without hot or dead spots, and with low distortion. The entire audience will be involved in the stereo presentation of the music. Even with a loud, gut-thumping beat, patrons and employees will find it more practical to interact.
Foreground Music Systems: Music often is used to create an atmosphere at places like record stores and department stores. Yet it's difficult to provide sound at evenly distributed levels throughout the entire facility, especially in full stereo. The idea is for music to be heard clearly and yet not be overpowering. Multisonic Imaging achieves this, in addition to enhancing the drama and subtleties of music with the benefits of true stereo sound. This is a perfect applicationCfor motivating, for selling...even at an unobtrusive level, people will be touched by a sense of presence of the music.
Touring Systems: Multisonic Imaging can be used for large or small touring systems, not only for effects and stereo playback, but also as the primary imaging method for the entire production. The vast majority of the audience will hear excellent stereo localization. And those who are very close to a particular loudspeaker system will still hear the entire mix, with sounds panned to the opposite side slightly down in level (as appropriate) but still clearly audible in the mix.
Portable Systems: Multisonic Imaging enhances the performance of small portable systems, increasing the coverage area and providing a bigger sound. The low cost and small size of Multisonic Imaging truly increases the "bang for the buck" and overall performance of any portable or temporary audio setup.
Recording Studio Monitoring: Stereo recordings can be optimally engineered to take full advantage of the increased clarity provided by three-channel imaging. These optimized results can also improve performance with conventional systems. Also, the increased optimum listening area of the studio control room will be greatly appreciated by all those who need to "listen in" during the mixdown process. (The M86 has additional applications in recording and produc-tion, described in a separate application note.)
Television and Radio Production: Quite often, control and production rooms at broadcast facilities present a difficult environment for accurate stereo monitoring of source material. Multisonic Imaging solves this problem by increasing the optimum listening area so that several people in the control room can simultaneously hear the audio with correct stereo perspective. Also, any technical problems with the audio, such as loss of stereo or phase problems, will immediately be audible and obvious.
3 Multisonic Imaging: Small-Room and Home-Audio Applications
Multisonic Imaging is designed for reproducing high-quality stereo audio with optimum soundstage imaging and a larger listening area. It is perfectly suited for home systems, especially now that the use of a center loudspeaker is becoming common.
Multisonic Imaging is entirely appropriate for the audio purist because it does not add any electronic distortion to the audio path. Frequency and phase response are flat, no signal delay is added, and there is never any dynamic alteration to the signal. It is truly a high-fidelity process which adds nothing unnatural to the music. The Multisonic system will provide a more even, sym-metrical, and detailed soundstage, and it more clearly reveals subtle nuances in recorded music.
It is no longer necessary to put a listening chair in the middle of the room. The listener(s) can be at either side of the room, near or far from the loudspeakers, and still hear excellent stereo imagingCeveryone in the room can enjoy the high-quality sound presentation simultaneously. Multisonic Imaging solves the problem of hearing just the nearer loudspeaker, a flaw of two-loudspeaker stereo systems and other multichannel systems.
A Multisonic system will provide higher sound levels at lower distortion because it uses three loudspeakers rather than just two. In fact it's probably the only practical way to listen to two channels with three front loudspeakers while maintaining accurate stereo imaging.
In addition to better imaging and a much larger listening area, there is another benefit for the serious music listener. The Multisonic Balance control can be used to "mix" the center and side information to preference. For example, in a typical recording a soloist in the center can be made louder or softer relative to the rest of the music.
Surround loudspeakers have become popular in home theater systems and they can be equally rewarding for listening to music. All stereo recordings have a difference signal, which generally contains ambient information in the recording. When this difference signal is played through surround loudspeakers, it adds another dimension to the Multisonic Imaging experience. This approach to surround sound is clean and does not generate the distortion or artificial-delay effects associated with other types of surround systems.
Ideally the three front loudspeakers are placed on a slight arc so they are equidistant from a point centered and toward the back of the listening area. They can also be placed in line, or an any reasonable way according to user preference, with excellent results.
The M86 Multisonic Imager is patched into the system just ahead of the amplifier(s). It also can be connected into a tape-monitor loop on a stereo receiver. A single high-quality stereo amplifier can power all three front loudspeakersCthis is discussed later. A subwoofer system can be implemented with Multisonic Imaging and is recommended for high-power systems.
4 The Multisonic Home Theater System
Multisonic Imaging correctly places left, center, right, and surround sounds in any stereo movie or TV program. Although it works differently from conventional decoders, it is perfectly compatible with all stereo soundtrack sources.
Instead of steering the sound direction by sensing the inputs and riding the gain and cancellation of the outputs, it uses the inherent channel correla-tion for each signal component to achieve natural directionality. This results in a continuous left-to-right soundstage, rather than the three-point-source approach of steered systems. Moving sounds will pan across perfectly. The surround-sound outputs are completely clean as well.
Multisonic Imaging should not be confused with conventional matrix systems which perform poorly because they do not have enough channel separation to focus a sound source location. The Multisonic process provides at least twice as much separation across the front as conventional matrix systems. But more important than channel-separation specifications, it presents the sound from the correct direction.
In addition, it is better than any dynamic steering system at supplying tightly-focused front-center sound localization. It anchors dialogue to the video screen with clarity, with no dynamic side effects, and without losing focus at the left and right loudspeakers. Unlike steered systems, stereo background music or any left-right sound which may occur during center dialog can be simultaneously reproduced in full stereo during dialog. The experience of left-to-right stereo music combined with a voice focused in the center can be quite dramatic, especially if the listening/viewing position is off center.
Surround systems need to present the surround soundtrack as an environment-type sound, coming from all around the listener (hence the name). Multisonic Imaging accomplishes this in a highly effective manner by sending some surround information to the front left and right loudspeakers in addition to the surround loudspeakers. The listener is completely Asurrounded@ with the surround soundtrack.
The M86's SpreadSound function on the surround outputs provides a clean yet diffused and delocalized sound field for very realistic ambient sound.
In some movies, the surround soundtrack is used to localize sounds at the rear or sides, depending on loudspeaker placement. The Multisonic Imaging system also does this. Simply operating the surround loudspeakers at a higher volume setting causes the surround soundtrack to be localized toward the rear, away from the front. This approach allows for excellent environment surround or localized surround, depending on the desired effect.
Multisonic Imaging is a cost-effective and performance-oriented way to create a state-of-the-art home theater system. With this patented system, the listener receives unmatched performance from music videos and pure-music sources, as well as an outstanding sonic theater experience.
5 Benefits of Multisonic Imaging
• The listening area in which a well-balanced stereo image can be heard is much larger than with conventional systems. The stereo image remains stable and symmetrical even for way-off-center listeners. More people can simultaneously enjoy an excellent sonic perspective from a sound system.
• The stereo soundstage is more focused, especially in the central area. This provides an optimum presentation of the most important and prominent components of the stereo mix.
• The center channel enhances vocal intelligibility.
• It is a totally linear and high-fidelity process; it does not add harmonic, frequency, phase, or dynamic distortion. From a signal-quality standpoint, Multisonic Imaging is transparent, and it can easily be adjusted for very precise operation.
• It is extremely cost-effective and practical.
• It works perfectly with any type of stereo sound system and any stereo program material.
• The system sounds much cleaner; it has less distortion and more headroom by using three loudspeakers rather than two to create an effective, clean wavefront.
• A more continuous Awall of sound@ is created which provides even and well-balanced sound distribution. This is a great improvement over the two Ahot spots@ created by a two-loudspeaker system.
• The side and ambient components in the mix are directionally separated from the center, providing a more dramatic sonic presentation without changing levels in the mix.
• Listeners who are very near one of the loudspeaker systems will still hear all components of the mix from that loudspeaker, with the opposite-panned signal components somewhat attenuated but still audible in the mix. For large venues this is a very real improvement over discrete left-center-right systems.
• The direct-sound frequency response and sense of presence is improved through the use of three loudspeakers, instead of two, for the primary sound sources.
• When playing recorded music, the relative levels of prominent and ambient sounds in the mix can be adjusted to preference. The sound distribution among the loudspeakers can easily be adjusted to perfection for any stereo program material.
• Multisonic Imaging places sounds at any and all pan locations simultaneously. Unlike steered systems that can only place a sound in one specific direction at a time, this allows accurate reproduction of complex stereo mixes with many specific sound-source locations across the sound stage.
6 Multisonic Technical Description
The Multisonic Dispersion Matrix
The Multisonic Imaging process is both electronic and acoustic. The electronic portion of the system consists of the Multisonic Dispersion Matrix, a special circuit which combines the left and right input signals in a certain mathematically-precise way to create left, center, and right output signals. These three output signals will have the optimum 6 dB of minimum channel separation between any two of them. The transfer functions are as follows:
Left output = L - MxR
Center output = (1 - M) x (L + R)
Right output = R - MxL
where L and R are the input signals and M is an adjustable matrix factor (changed with the Multisonic Balance control) which can go from -1 to 1 but in general is set in the range of 0.4 to 0.7 with conventional program material. The "ideal" setting is 0.5, which works well with most stereo signals. The adjustment allows compensation for unusual recordings which are either too narrow (resulting in a stronger center), or too wide (resulting in a weaker center).
Most recordings have the bass energy centered so the amount of bass depends on the matrix factor. To avoid this effect, allowing the Multisonic Balance control to independently adjust typical vocal or instrumental center staging, a special internal crossover is implemented. This results in consistent bass response as the Multisonic Balance control is adjusted.
Increased Channel Separation
Conventional left-center-right matrix systems only achieve 3 dB of separation between left and center, or right and center. To increase the separation further, a steering scheme is typically used which has many problems such as unnatural imaging dynamics and the inability to localize all directions simultaneously.
The Multisonic system achieves twice that amount of separationC6 dBCbetween any of the output channels. This means the intended direction gets four times the power of the adjacent directions, which is enough for excellent localization. It achieves this for all directional components simultaneously without any artificial gain control or other dynamic distortion.
The Multisonic Dispersion Matrix alone does a fine job of imaging with it=s 6 dB of output channel separation. This would occur if the loudspeaker setup were far from optimum, the loudspeakers didn't match, or the room were very live or had difficult acoustics.
However with a good loudspeaker setup and reasonable acoustics, Multisonic Imaging increases the perceived channel separation to a much higher level due to the acoustic interaction of the three loudspeakers. In other words, the electrical channel separation is only one component of the imaging process. As a human perceives the system, the remaining Acrosstalk@ is effectively canceled out in the localization process. This is largely due to acoustic vector imaging.
Acoustic Vector Imaging
The acoustic operation of the Multisonic system also works according to the recorded direction of each signal component in the recording. Electronically, center-panned signals appear at full level in the center, and 6 dB down in the left and right loudspeakers. But through sound pressure (or intensity) vector addition of all three loudspeaker outputs, only the center loudspeaker is heard. This results in excellent center focus; in practice the left and right loudspeakers are inaudible with center signal components even for listeners who are quite off-center.
Signal components panned between left and center will cancel in the right loudspeaker, keeping it off or inaudible. They appear at the left and center loudspeakers, creating a phantom-center image between, at the correct location. This phantom image will be more clearly focused than the phantom center in two-channel systems because the loudspeakers are only half the distance apart. The right-center soundstage position works in the same manner.
Any signal components panned fully left or right will appear in the corresponding loudspeaker at full level and in the other two loudspeakers at 6 dB down. However, in this case the opposite side will be inverted (out of phase) so that the sound from the center and opposite-side loudspeakers will Adelocalize@ each other, leaving the intended side loudspeaker as the dominant audible sound-source location.
For signals panned almost fully left or right, the opposite loudspeaker combines with the center loudspeaker, by acoustic vector imaging, to further reinforce the intended direction. Acoustic vector imaging is the same principle used in TV sets and small portable stereos to expand the sound beyond the loudspeakers so that it sounds like they are farther apart than they really are.
For example, consider a signal panned mostly to the left. This results in signals appearing at the center and right loudspeakers, which by themselves create an image to the left of the center loudspeaker, because the right loudspeaker's signal is inverted. Meanwhile,
the left and center loudspeakers create a phantom image between them, mostly toward
the left. These locations coincide, so that all three loudspeakers contribute to the correct sound-source location.
Reduced Image Doubling and Comb Filtering
One of the major flaws of two-speaker left-right stereo systems is that they rely on a phantom image to create the center sound-source location. The center is the most important part of most music and movie program sources, yet as a phantom image between widely spaced loudspeakers, it has relatively poor localization.
One of the reasons for this is the different arrival times of the sound from each loudspeaker. If there were a real sound source in the center, each ear would hear transient sounds just once, as they are produced. But with a phantom image, since the ears are not exactly equidistant from the two loudspeakers, each ear hears a double image of every transient soundCthe sound from each loudspeaker does not arrive at the ear at the same time.
The distance between the ears prevents them both from being exactly centered, even at the best listening position. The resulting double image of phantom-center sounds gets much worse as the listener moves off center. It is unnatural and reduces the focus of the sound-source location.
Another problem with phantom-center imaging is that head shadowing and pinna reflections (the physical effects of the head and ears on the sound that enters the ear), also used to localize sound, are completely wrong. Simply put, the actual propagation directions of the left and right loudspeaker outputsCthe actual sound waves coming to the listener from the left and rightCare detected by the ear-brain system as being quite different from the single propagation direction resulting from an actual center sound source. As soon as the head movesCa localization techniqueCthe listener can hear that there are actually two sound sources at the left and right, not just one in the middle.
Two-loudspeaker configurations also cause cancellation of direct sound at certain frequencies due to arrival time differences. This is another result of each ear being closer to one loud-speaker than the other. At any frequency where this distance difference is one-half wavelength, or that plus any multiple of the wavelength, a significant degree of direct-sound cancellation will occur. This effect is known as comb filtering. The other ear will have a comb-filter effect also, and because of its different location, it will have a different set of canceled frequencies.
Having a different comb-filter effect at each ear makes it easier to distinguish a phantom center from a real center. The sound source will be blurred to some degree; in fact a similar approach is used electronically in some stereo synthesis schemes for the specific purpose of delocalizing the sound!
The Multisonic system uses a real center loudspeaker so that practically no time smearing or comb filtering occurs for the center of the soundstage. Also, the real center loudspeaker allows head shadowing and pinna reflections to properly localize the sound. For center images there is theoretically a small degree of interference from the side loudspeakers, but it will be minimal and inaudible because these loudspeakers create a phantom image coincident with the real center loudspeaker, and at a much lower level. At the left and right extremes of the soundstage, of course, there will be more comb filtering than a single loudspeaker would have, as in a two-loudspeaker system. However, with three loudspeakers operating one of them will dominate, so that the effect is minimal.
If sounds are placed at left-center or right-center, the result is basically a pair of loudspeakers creating a phantom image between them; the opposite loudspeaker does not have that sound in its signal at all because it is canceled out in the Multisonic Dispersion Matrix. In this case, the loudspeakers are half the distance apart compared to a two-speaker center-sound condition. The result is that the propagation directions are closer to correct, the delay differences are less, and the comb filtering starts at a higher frequency, making it less detrimental. All of this results in a phantom image which is more focused and convincing for left-center and right-center soundstage positions.
The left-minus-right difference signal contains predominantly ambient sound components (representing the room acoustics) on most audio recordings and surround information on movie soundtracks. The Multisonic Dispersion Matrix increases the relative level of these sounds in the left and right loudspeakers while maintaining complete cancellation of them in the center. When combined with rear loudspeakers producing the left-right difference signal, a very even, four-speaker surround effect is created. If rear localization of the difference signal is preferred to a true-surround presentation, this can be effectively achieved by raising the relative level of the rear loudspeakers.
This Miles Technology exclusive process is a high-fidelity method of intentionally and accurately localizing a sound source across a space, rather than at a point. SpreadSound is used at the surround outputs of the M86 to prevent the surround signal, which once separated out is a single monophonic signal, from localizing as a phantom center between adjacent surround loudspeakers. Phantom localization is desirable across the front soundstage with stereo sources but is usually considered a detriment at the sides and rear of a surround system.
SpreadSound works by shifting the phase of the audio signal a constant relative amount. A pair of SpreadSound outputs is designed to have a relative phase shift of 90 degrees at all frequencies above 100 Hz. By keeping the output signals in quadrature phase, a spreading effect is realized with no phantom localization. The effect is unnecessary below 100 Hz; at these lowest frequencies localization by humans does not occur. In this frequency range the signal is left in phase to maximize the low-frequency power output and the loudspeaker-array efficiency.
The SpreadSound circuit can be used with any monophonic program material to create a big, spread-out sound. With Multisonic Imaging, a mono source will focus tightly to the center. If a large soundstage is desired instead, such as with a mono music source, SpreadSound is a great way to achieve it without any of the detrimental side effects typical of stereo synthesis techniques. The M86 provides convenient selection of SpreadSound for the front soundstage.
An audio system with a Multisonic Imager and three loudspeakers has a power advantage over two-speaker systems. For a given choice of loudspeaker, the whole system can put out 50% more power and thus play significantly louder with the same distortion level. Loudspeaker distortion goes up as power is increased, especially at high levels. So if you keep the same listening level, there will be less distortion, since each loudspeakerCthe main distortion-generating componentsCwill be operating at only two-thirds power.
It is possible, and sometimes convenient and practical, to drive the three front loudspeakers with one stereo amplifier. This is possible because the center output is always an exact sum of the processed left and right signals in a Multisonic system. A bridged-center loudspeaker connection can achieve this.
The Imager=s right output signal, which goes to the power amplifier input, is inverted, and the right loudspeaker leads are also swapped to restore the correct right-channel polarity. The center loudspeaker then bridges the amplifier output (loudspeaker + to left amp +, loudspeaker - to right amp +), and it will have the correct signal and relative level.
There are two cautions in doing this: the left and right amplifiers must share a common ground (most do), and the amplifier must be capable of driving one-third the load impedance of one of the loudspeakers (many quality amplifiers can drive
2 ohms, allowing this approach with 8-ohm or 6-ohm loudspeakers). Alternatively, a separate amplifier can be used for the center.