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TACTEX CONTROLS MTC EXPRESS

May 1, 2001
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There have been a number of developments in controllers that work beyond the MIDI specification's confines. An important new entry in this field is Tactex Controls' MTC Express (see Fig. 1). The MTC Express features a pad made from Smart Fabric, a soft material that gives users a multipoint, three-dimensional control surface. The result is a programmable controller that is highly responsive and lets musicians realize the potential of electronic instruments.

INSIDE THE PAD

The MTC Express's Smart Fabric is based on a patented pressure-sensing technology developed by the Canadian Space Agency. Smart Fabric consists of thin cellular elastomers, typically made of urethane or silicon, with an outer skin that protects the fabric and serves as a wear surface. Intended for space-based robotics, the fabric provides touch-based feedback to remote telerobotic operators.

Within the Smart Fabric are overlapping pressure sensors, called taxels, connected to fiber-optic cables. An LED generates a measured amount of light, which is sent to the taxels through the fiber-optic cables. Pressure on the taxels restricts the flow of light. Each affected taxel returns a reduced amount of light to a light sensor, which tells the unit's CPU the amount of pressure exerted in a specific location.

The MTC Express's active area contains a grid of 72 taxels, with taxels spaced about one centimeter apart. Although future Smart Fabric products can be almost any size and configuration, the MTC Express has a 5¾-by-3¾-inch control surface, which is roughly the size of a mouse pad or a small Wacom art tablet.

GENEROUS CODING

The MTC Express includes the necessary hardware and cross-platform application program interface (API) for developing custom instruments and controllers. The API consists of a set of C-language libraries to which developers can link their custom code.

Cycling 74's Max and MSP are the best environments for developing your ideas for the MTC Express. Researcher Matt Wright and others at the Center for New Music and Audio Technology at the University of California at Berkeley have created external drivers for the Max graphical programming environment.

Most of the development work in Max has been to create external objects that include the MTC Express Pad as an input. Of course, a resourceful programmer can make application-specific input drivers for any Mac or PC program by using the API libraries.

After I unpacked the controller and installed the Tactex object, it only took me five minutes to get things working enough to receive coordinates and pressure readings when I touched or tapped the pad. I am an experienced Max programmer, but a novice user could use the Tactex pad to create custom controllers within a day.

SEE ME, FEEL ME

The MTC Express's internal electronics scan the sensing surface and convert the pressure and coordinate data of five points into digital information that is sent to the computer through a serial connection at 115 kbps. A computer program interprets this information and then creates a map of the contact points. That is different from how an art tablet operates: the computer gets the pressure information from the tip of a stylus and communicates information about that single point.

MTC Express Specifications
Pressure Resolution 8 bits
Transmission Rate 115 kbps
Minimum Activation Pressure 0.4 psi
Connector DB9 or Mac serial adapter
Dimensions 0.8" (H) × 7.5" (W) × 6.69" (D)
Weight 17 oz.

Fig. 2 shows how the same left-hand position on the pad can be interpreted differently, depending on the computer algorithm. Fig. 3 gives an example of how the pad can be used for tracking a moving point across the pad. Those visual examples show the variety of ways the MTC Express can be used. In the context of musical control, they suggest an array of exciting possibilities.

For example, the control surface can be set up as multiple dynamically interactive control areas. In one mode, the pad's surface can be used as a collection of virtual volume faders, like on a mixer. In another mode, note density and filter settings for a synthesizer can be controlled by finger position and pressure.

I wanted to use the pad to develop a map of virtual keys across the surface. The layout was based on the tonality diamond of microtonal-music pioneer Harry Partch. When I discerned the scanning pattern of my Tactex object, I was surprised to see a stream of numbers coming from locations I had not been touching. The MTC Express owner's manual revealed that I needed to normalize the pad before using it so that a baseline surface pressure could be determined.

Normalizing the pad eliminates the individual variability in taxel response. The procedure can be automated in software and made part of the initial startup of software using Smart Fabric. Once normalized, the taxels' range of pressure response is 0 through 1,023. This degree of accuracy is finer than the 0 to 127 data byte of a standard MIDI controller. After I normalized the pad, I received reliable results.

In my review unit, some taxels were slow to recover after being touched. When an area is depressed, each taxel should return immediately to its normalized state. However, that didn't always happen. The manufacturer says this problem has been fixed on subsequent pads.

Once properly calibrated, the pad sends out two numbers for each taxel that receives pressure. The first number represents the location, and the second is the degree of pressure. As pressure and location change, the new values transmit with the next scan.

After I finished creating my tonality diamond, I built a mixer control surface. I then realized that those different designs could be separate states within a single instrument design. The possibilities are staggering.

FUTURISTIC SCRATCH PAD

MTC Express's most intriguing aspect is that it provides continuous control of multiple user-definable parameters simultaneously. Imagine controlling pitch, filter cutoff, and synth amplitude at the same time without adapting traditional keyboard performance techniques. This could lead to specialized instruments based on a synth's capabilities, rather than outdated acoustic models.

An attractive attribute of the pressure-based approach of the MTC Express is its physicality. The performer deals with the surface in a discreet way, so the music is the direct result of fine muscle control and purpose, which have always been the prerequisites of making music but have been limited by most electronic controllers. The MTC Express is a great tool for realizing the promise of electronic musical-instrument design.


Scot Gresham-Lancaster is an electronic technician, computer-music lecturer, and recording engineer at California State University at Hayward's music department, as well as the fuzz in the electronic-music trio, Fuzzybunny.

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