App. #5
Application Notes

Application Note #5

Professional Surround Sound with
the MTI-3 TriSonic Imager

Many New Applications

An audio system with surround sound offers a real competitive advantage in many types of commer-cial establishments. Expectations for sound quality have increased, and surround sound is recognized as an important part of the sonic experience.

The Miles Technology MTI-3 TriSonic Imager provides high-quality surround outputs for various demanding applications such as A/V productions, touring and live sound, nightclubs, auditoriums, bowling centers, houses of worship, DJ systems, boardrooms, multimedia rooms, foreground music systems, exhibits and displays, and any other information or entertainment venue where stereo sound is reproduced.

Working with any ordinary or specially-mixed program source, the MTI-3 creates and routes surround signals and maintains excellent signal quality throughout the system.

The MTI-3 also provides surround sound for live production work. The discrete surround input and the derived surround signal are processed with our unique SpreadSoundÔ circuit which prevents point localization. It envelopes the audience with a big, spacious sound. To create surround-sound recordings with the MTI-3, see Miles Technology Application Note #4.

Derived Surround Signal Source

The derived surround signal is created from the left and right inputs; it is essentially a left-minus-right difference signal. This approach is fully compatible with all video material and audio-only sources. The difference signal provides complete separation of the surround output from the center output. It comprises mostly ambient and room sounds in acoustic recordings and reverberation sounds in remixed or ambient-processed recordings.

Discrete Surround Operation

The discrete surround input allows a separate program signal to be routed to the surround loudspeakers. The MTI-3 mixes this signal, at unity gain, with the derived surround signal and provides SpreadSoundÔ processing to the result, for outstanding surround-sound performance.

Signal Processing

The MTI-3 does not dynamically alter the program with any steering or gain riding. The signal's exact dynamics and spatial properties are left intact. This is one of the reasons it sounds so good.

A switch allows selection of wideband derived-surround, suitable for production applications or where the program material is of consistent high quality. The switch can be set for a limited bandwidth of 100 Hz to 7 kHz for use with potentially-noisy video sources, or for a more natural-sounding ambience in some applications.

Surround Presentation

The surround signal is split into two outputs using the SpreadSound process. This prevents direct localization of ambient surround sounds by effectively spreading the sound between the loudspeakers. This greatly reduces surround localization, while preventing "in-your-head" and "out-of-phase" effects. It also increases the depth and warmth of the surround sound field far more accurately and cleanly than digital decorrelation methods.

SpreadSound works by providing two outputs which are in quadrature phase (90 degrees phase difference) at all frequencies from 100 Hz to 10 kHz. The frequency response at each output is flat. Each output by itself sounds identical to the surround input. But when the two outputs are reproduced with two loudspeakers, the audible result is a very smooth, nonlocalized sound field.

Surround sound can often be implemented effectively with two loudspeakers located at the back or sides of the listening area. But in larger venues, more coverage and better distribution is needed. SpreadSound surround provides a way to drive any number of loudspeakers while maintaining a smooth surround effect, minimizing listener perception of the loudspeaker locations.


TriSonic surround works perfectly with ALL types of stereo program material, and is totally faithful to high-fidelity audio recordings. There are no artifacts or side effects. It also works perfectly with stereo video soundtracks, and is dramatically effective (and superior to other systems) with surround-sound video material. The optional SpreadSoundÔ function works with all audio

signal sources.

System Interconnection

Using the MTI-3, it is very simple to create and route the surround audio. Fig. 1 shows the typical system interconnection for surround operation. This approach applies to practically all types of sound systems.

Fig. 1

If TriSonic Imaging at the front is not needed, the MTI-3 can be used just for the surround outputs. The TriSonic outputs can also be used for regular stereo (set the TriSonic Balance control to the 9:00 position) or for mono (simply use the center output).

Loudspeaker Placement

In simpler or smaller systems, two surround loudspeakers may be used. These should be driven with the separate Surround 1 and Surround 2 outputs from the MTI-3.

In larger systems, more surround loudspeakers should be used. These provide even sound distribution over a larger audience area. With SpreadSound, the surround sound is improved further since a larger sound is heard rather than a "hot spot" at the nearest loudspeaker.

The basic concept is simple: adjacent or directly-opposite loudspeakers should receive different SpreadSound signals. See Fig. 2. This concept applies for any number of surround loudspeakers. If each audience member can hear two loudspeakers with different SpreadSound signals, rather than just one (which would be the result if a single surround audio signal were used) then the effect is an improved perception of ambience from the surround sound system.

Fig. 2

Level Adjustment

The surround loudspeakers need to be balanced correctly with each other and with the front loud-speakers. One way to do this is to individually balance each surround channel with the front system, while listening in the appropriate area near the surround loudspeaker. The MPR-450 multichannel power amplifier provides controls for this. Then with actual program material playing, balance all of the surround loudspeakers together with the master surround-level control for correct overall balance when listening in the center of the audience area.

More Information

For more information regarding the MTI-3 or other Miles Technology products, call us at 800-280-8572.

App. #5
Application Notes