App. #1
Application Notes

Application Note #1

TriSonic™ Imaging With The MTI-3


1. TriSonic Imaging

2. Where It Works

3. Making It Perfect

4. TriSonic Techniques

TriSonic™ Imaging

TriSonic™ Imaging is a patented 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" equidistant from the left and right loudspeakers. With TriSonic Imaging, a symmetrical stereo image is clearly heard from almost anywhere in the listening area. A comparison is shown below. Loudspeakers can even be placed across the long side of a rectangular room with excellent results.

Listening Area Comparison

Also, when compared to conventional two-channel systems, a TriSonic system provides much better focus and clarity across the front-line stereo soundstage.

Three loudspeakers are used across the front-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 program, and requires no special recording or encoding process to operate. The four-direction playback imaging typically used in movie soundtracks is achieved with outstanding results because no direction-steering is needed.

This system utilizes an adjustable electronic matrix circuit which achieves twice the separation of conventional systems in its output channels. The MTI-3 TriSonic Imager includes the matrix circuit and is combined with appropriate loudspeaker placement and loudspeaker-dispersion characteristics to provide TriSonic Imaging.

The low-bass level is not affected by the image process. Signals from 80 Hz to 20 kHz are imaged to the three loudspeakers according to the way they are recorded. Imaging (source-direction-sensing) is considered not to be perceivable by humans below 80 Hz, and this frequency range is slightly narrowed for consistent tonal balance.

Surround outputs, which correctly reproduce ambient or surround information from any recording, are also included on the MTI-3. This high-fidelity ambience extraction accurately reproduces the program source.

Monophonic sources are also improved by TriSonic Imaging and the MTI-3. Normally, mono sounds are clearly repro-duced at the center loudspeaker. However the MTI-3 also includes a SpreadSound™ function that spreads monophonic sources into a big, stereo-like soundstage. This unique spatial process maintains total clarity and uncolored sound.

Where It Works

There are many applications that can benefit from the use of TriSonic Imaging. All stereo audio systems which utilize loudspeakers will be greatly improved with TriSonic Imaging. Many systems which would traditionally be monophonic can now be radically improved, at very modest cost, using TriSonic Imaging. Here are some examples:

Auditoriums and Performing Arts Centers: TriSonic Imaging brings quality stereo sound to everyone at concerts and other performances. Live mixes can be panned out for wide stereo, without the coverage limitations of discrete left-center-right systems, and with results far superior to conventional two-channel stereo-comb-filtering problems and hot spots are gone. Practically all of a venue's seats will receive excellent, balanced stereo sound.

Houses of Worship: TriSonic imaging provides improved vocal intelligibility while 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 works well in difficult acoustic environments.

Nightclubs: Dance music with both dynamic and spatial impact is much more satisfying. True stereo sound is distributed over the entire dance floor and the entire audience area. The music is more impressive for a given SPL, and sound is distributed evenly without hot or dead spots. Even with a loud, gut-thumping beat, patrons and employees will find it more practical to interact.

Museum and Park Exhibits: Multimedia exhibits and entertainment shows should accommodate a group of people with true stereo audio. The way to achieve this is with TriSonic Imaging. Everyone, including those who are off to the sides, will be able to experience the benefits of quality stereo sound with correct soundstaging.

Foreground Music Systems: With TriSonic Imaging, music is clear but not overpowering. This is a perfect applica-tion-for motivating, for selling...even at an unobtrusive level, people will be touched by a sense of presence of the music.

Touring Systems: TriSonic Imaging benefits both large and small touring systems. The vast majority of the audience will hear excellent stereo localization. And those who are very close to a particular loudspeaker will still hear the entire mix, with sounds panned to the opposite side still clearly audible.

Portable Systems: TriSonic Imaging enhances the per-formance of small portable systems, increasing the coverage area and providing a bigger sound. The low cost and small size bring a very practical improvement to the overall performance of any portable or temporary audio setup.

Recording Studio Monitoring: Stereo recordings can be optimally engineered with the increased clarity provided by TriSonic Imaging. Also, the increased listening area in the studio control room will be greatly appreciated by all those who need to "listen in" during the mixdown process. The MTI-3 has additional applications in recording and production, described in Application Note #4.

Television and Radio Production: Control and production rooms at broadcast facilities often present a difficult en-vironment for accurate stereo monitoring of source material. TriSonic Imaging solves this by increasing the stereo listening area so 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.

Home Theater Systems: TriSonic Imaging is ideal for home theater systems. Any stereo TV broadcast or videotape will provide outstanding results, superior to steered surround decoders, since TriSonic Imaging can simultaneously image any and all directions without steering or dynamic modifica-tion. It is a truly phenomenal improvement in home video! Compatible with all program sources, the entire audio setup can also be used for superior music-only playback.

Home Stereo Systems: Using a TriSonic Imaging system is the very best way to listen to CD's, tapes, broadcasts, or other program sources. You will be surprised at the amount of previously-covered detail you will hear in your favorite music. It is not necessary to sit in the center of the room to hear good stereo. Furniture does not dictate poor audio imaging, nor does good imaging dictate a compromised furniture setup. This allows both excellent stereo imaging and creative, pleasing interior design!

Schools: Typical uses are in auditoriums, gymnasiums, and music rooms. Even if the room has poor acoustics such as a gym, or the audience is very spread out such as in a music rehearsal room, a TriSonic system will greatly improve the sound. With all sources, live or recorded, there will be more clarity and detail. This higher standard of sound quality can be achieved very cost-effectively with TriSonic Imaging.

Making It Perfect

Most rooms are designed for factors other than the sound system, resulting in some compromise of audio performance. The perfect room with the very best loudspeakers in the truly optimum arrangement is uncommon. In practice, systems are often set up in live rooms, with an unmatched center loudspeaker, and with loudspeaker placement dictated by architecture or convenience. Even so, there are variables which can be adjusted to optimize a TriSonic system for surprisingly good results, even in the worst conditions.

The primary factors which affect imaging performance are

1. Room Acoustics

2. Loudspeaker Selection

3. Loudspeaker Placement

4. Loudspeaker Aiming

5. Loudspeaker Level Balance

6. External Signal Processing

7. Program Channel Balance

8. Signal-source Quality

Room Acoustics

Generally, for the best audio imaging, the fewer the room reflections, the better. Live rooms are notorious for preventing clear soundstaging, while dead rooms yield a good, tight sound closely representing the original recording. Outdoor systems typically provide excellent results, unless there is a hard wall interfering or creating a slap echo.

There are some trade-offs in room design. An acoustically-live room requires less power for a given sound level and a live room tends to cover aberrations in loudspeaker performance, making them less noticeable. These attributes are at the expense of clear, sharp soundstaging.

A good compromise is to use a live-end/dead-end room treatment. In this case the area at the front of the room, behind and around the loudspeakers, is made as dead as possible. Using carpeting or acoustic tiles on the walls, floor, and ceiling at the front provides a good wave front with minimal interference from reflections. The back portion of the room, treated with relatively hard but irregular surfaces (for diffusion), provides acoustic energy reflection for a "present" sound quality and increased loudness.

Often the room is a "given"; its reverberation and reflection characteristics are pre-determined by the architecture or other factors. In this case, the other things listed below can be done to optimize the resulting sound.

Loudspeaker Selection

The loudspeakers, of course, should be high-quality systems with adequate power handling, bandwidth, and smoothness for good sound. Beyond that, there are mainly two factors which affect stereo imaging: matching and directivity.

If the loudspeakers do not match at all, they can still provide three-point imaging in a TriSonic system. But the smoothest, clearest sound results when there is good matching of frequency and phase response. This allows the adjacent loudspeakers (left and center, or center and right) to create effective phantom-image source locations between them.

Therefore it is best if the three loudspeakers are identical. In many cases, the center loudspeaker must be different. This can work very well as described later in this document.

Loudspeaker directivity is another important factor to consider. As with any sound system design, it is desirable for the loudspeakers to provide smooth, wideband sound to every listener they cover. Meanwhile for maximum intelligibility and system accuracy, it is best to minimize the energy radiated away from the listeners, toward walls or any other reflective surface. Careful control of radiation patterns is very helpful.

Loudspeaker Placement

For full soundstage width, it is best if every listener can hear all three front loudspeaker systems. In most cases this can be achieved to a satisfactory degree.

There are two important factors to consider with loudspeaker placement. One is the actual location of each loudspeaker as observed by the listener. This affects the shape of the perceived soundstage. The other factor is the distance from each loudspeaker to the listener. This affects the time alignment, which is important for quality sound reproduction.

TriSonic Imaging is actually very tolerant of imperfect arrangements. However, the best approach is to optimize the physical arrangement, from the point of view of the listeners, and to understand the ramifications of the resulting time alignment. The standard setup shown below is a good arrangement for maximum effective soundstage width covering the whole audience. The architectural setup shows another practical arrangement, often demanded by the architecture, which slightly narrows the soundstage but is very effective given the stage and seating-area design.

Loudspeaker Aiming

The loudspeakers should be aimed basically toward the center of the audience. In larger rooms, further adjustment toward the opposite end of the audience area--up and across the room--will be helpful to increase coverage. Aiming the loudspeaker axes this way provides higher acoustic radiation toward the listeners who are more distant from the loudspeaker, which is very helpful to increase the number of people hearing excellent stereo sound staging.

This same principle is typically employed when a loudspeaker cluster is designed to cover a specific audience area using different direct radiators and/or horns. This further improves coverage and TriSonic performance. Keep in mind that each cluster--left, center, and right--should individually cover the whole audience. That would be perfection; even if such coverage is impossible to achieve, the system will still work very effectively with TriSonic Imaging. The nature of TriSonic is that listeners not in the optimum coverage area still hear a great mix!

Loudspeaker Level Balance

The level balance of left, right, and center loudspeakers is important for best TriSonic Imaging. This is a simple yet important adjustment. One only needs to listen, and adjust the power amplifier gain controls, for the best resulting sound.

The Setup switch on the MTI-3 provides a simple way to achieve this. All one needs to do is press Setup, turn the balance to left, and walk the audience area with any music source playing. Listen to the resulting sound and decide whether it is exactly halfway between the left and center loudspeakers. If not, adjust the left amplifier gain to move the left-center phantom image to the correct midpoint. Move the Balance control to the right, and repeat for the right-center phantom location.

When the system correctly reproduces left-center and right-center phantom locations, it will reproduce all soundstage locations perfectly. Just be sure to listen from a range of locations near the center of the listening area. This listening approach results in outstanding stereo performance.

External Signal Processing

If a crossover is used for a mono subwoofer system, then the crossover can be patched ahead of the MTI-3. This allows the use of a stereo two-way crossover and is a good choice where the discrete center and surround inputs are not used.

For higher crossover frequencies, or where the discrete inputs are used, three crossovers are needed at the MTI-3 outputs. To the extent possible, the three crossover/speaker groups should be adjusted for matching sonic results.

The use of other types of line-level signal processing, such as compression, equalization, or noise reduction, should fol-low the same approach used for multi-stack systems.

Overall processing which relates to the program signal (such as SPL limiting, or program equalization) should be in the stereo signal ahead of the MTI-3.

Direction-specific room equalization, protective limiting, or any processing related to the individual loudspeakers or loudspeaker stacks should be applied to the TriSonic outputs after the MTI-3 to affect the corresponding individual loudspeaker.

Program Channel Balance

It is important for the program source to have correct channel balance. Modern program sources usually do, but when the signal goes through various equipment, the levels may be altered. As with all stereo systems, an imbalance of even 1 dB in level may alter the perceived stereo sound-source placement. Therefore, for best imaging from various sources, it is a good idea to carefully adjust the balance from recorded sources for correct centering of the stereo image. Once the balance is set, no further adjustment should be necessary.

Signal-source Quality

As one might expect, the technical signal quality of the program source affects its ability to provide good stereo imaging. Poor quality sources have random phase errors between the channels which degrades stereo localization. Live mixes, compact discs, VHS-HiFi, and 8mm video tape are excellent sources for stereo imaging. FM broadcast and stereo television are also excellent in most cases.

TriSonic Techniques

TriSonic Balance

The TriSonic Balance control effectively controls the "width" of the mix and the center-to-side ratio. The program material must be a stereo mix to be able to adjust this.

One option in setting this control, for a recorded or pre-mixed signal source, is to adjust it so that all three loudspeakers have the same loudness. To do this it usually helps to be somewhat off-center in the listening area. Simply adjust the TriSonic Balance control until the center and side loudspeakers seem to have the same output level. Under this condition, you will have the maximum possible separation and optimum TriSonic dispersion for that program source.

Another method is to listen carefully to the mix and adjust the control for a good level balance between the prominent, central parts of the mix (such as lead vocals, if present and centered), and the side or ambient parts of the mix. If the central portion of the program material is either too dominant or too weak, TriSonic Balance can be used to compensate and correct the situation. Recordings vary in the way they are mixed, and for the most critical and optimum results, adjustment can be made to optimize each recording.

Many recordings are made with narrow soundstaging. This studio mixing approach provides more consistency for monophonic reproduction. By setting the TriSonic Balance control toward "sides", typically at the 1 o'clock or 2 o'clock position, a full-width perspective can be restored without sacrificing image clarity.

Some recordings are recorded with a wide image, or they may seem to have a weak central area. Older or poorer-quality stereo recordings, especially if they are on cassette tape, may have a weak center due to poor phase accuracy between the left and right channels. Also, certain recording techniques (such as stereo synthesis) or microphone setups (such as a wide-spaced pair) can cause a weak center. In any of these situations, the center can be restored by setting the TriSonic Balance control more toward "center" to refocus the central area.

In many installations, it is not desirable or practical to make adjustments for particular recordings. This is fine! The control can be left in the straight-up position, or adjusted for good balance according to the room and sound system and left there. Then, every program source will be imaged exactly as it was recorded.

Mixing Live With TriSonic Imaging

Mixing live in stereo with a TriSonic Imaging System is easier than with a conventional stereo P.A. system. Sound can be panned to any location desired without concern for the audience being able to fully hear everything. As a sound is panned further to the left or right, the same signal begins to emerge in the opposite side, ensuring total coverage.

For example, if an instrument is panned fully to the left, it will also come from the right loudspeaker at a level 6 dB lower. It will also be inverted-this is part of the imaging process-but listeners very close to the right loudspeaker and far from the left loudspeaker will not notice the inversion. They will hear it at a lower level from the right loudspeaker, and that instrument will sound slightly farther away, as it should, since it is on the left.

Normally the TriSonic Balance control is left centered. However, it can be used to change the pan control range available at each input of the mixing console. When set toward "Center", it will limit the pan control range to a narrower soundstage; if set toward "Sides", it will extend the pan control range.

The Discrete Center and Discrete Surround inputs can be used with direct outputs or subgroup outputs on the mixing console. This will provide an extra degree of pinpoint focus and high intelligibility for center signals, for use where appropriate. The surround input is useful for special effects where a rear direction is desired with no signal from the front system, rather than full surround which would include the left and right loudspeakers.

Simply using your ears to determine the best pan locations and mix levels always works. If the mix location is not too far from the center of the listening area and within the coverage area of all three loudspeakers, the audience will hear much more closely the same mix that you hear.

Mixing for Two-Channel Transmission or Recordings

When mixing for a recording or for stereo broadcast, one needs to keep in mind that there are potential listeners who do not have a TriSonic system. For this reason, it is necessary for the TriSonic Balance control to be calibrated properly. If not, then the stereo mix will sound different on two-speaker playback systems.

Correct calibration is easily achieved. One approach is to simply set the MTI-3 TriSonic Balance control to the "Norm" position (straight up). This will be very close to ideal.

Another approach is to listen to familiar mixes and set the TriSonic Balance control for optimum sound. The user should initially verify the TriSonic mix on a conventional system until familiar with the TriSonic sound.

TriSonic Stage Monitors

TriSonic Imaging can be used for stage monitors. This is a good way to satisfy several musicians on stage while using only two monitor-send channels for a stereo monitor mix.

Simply set up three monitors, equally spaced across the front of the stage or in the desired area to cover. Using a separate TriSonic Imager, send the stereo monitor mix to its left and right inputs. Pan each musician's signal to their specific location on stage. Then everyone will hear the full mix, but with their own instrument or voice louder than the rest of the mix in the monitor loudspeaker nearest to them.

Using an Unmatched or Displaced Center Loudspeaker

In certain installations, it will be desirable or practical for the center loudspeaker to be different from the side loudspeakers. In typical auditorium situations, the center loudspeaker needs to have a wider dispersion angle in order to cover the entire audience area.

Architectural considerations often dictate a different type of center loudspeaker, and in many cases it must be located higher or forward from the side loudspeakers. While the theoretical principles of stereo imaging are based upon matched and aligned sound sources, good imaging still can be achieved with an unmatched or displaced center loudspeaker.

In this case, observation of the following will restore optimum imaging capability:

1. Use a separate amplifier or amplifier group for the center loudspeaker. Insert an equalizer between the MTI-3 center output and the crossover or amplifier. Adjust the equalizer so that the center loudspeaker sounds as much as possible like the side loudspeakers. Or, you can equalize the side loudspeakers to match the center, or equalize everything separately so that the end result has as much similarity as possible in the sound character from the three loudspeakers.

2. Then carefully adjust the center-amplifier gain according to the setup procedure in this manual. The center amplifier will very likely need to have a different gain setting from the left and right amplifiers. When using a non-matching center loudspeaker, the bridged-center configuration is not recom-mended.

For more information about TriSonic Imaging, the MTI-3, or other Miles Techonology products, call 800-280-8572.

Application Notes