The MTC Express (see Figure 1), from Canadian manufacturer Tactex Controls, is a touch pad controller with a unique multi-touch capability. Though it has other uses in a variety of applications including graphics, the device is definitely being marketed with computer music in mind.

Specifications
Using an internal web of fiber-optic strain gauges (produced under license from the Canadian Space Agency), the MTC Express senses multiple points of pressure applied to its surface simultaneously. Each contact point sensed by the pad provides its data, consisting of x, y, and pressure values, at a sampling rate of 200 Hz.

Software support is provided for both Macintosh and Windows computers in the form of libraries, example C code, and a demo application. For the Macintosh, an external for Cycling '74's Max is also provided with an associated ".help" patch.

The pad generates its multi-touch data starting from a 6 x 12 grid of deformation measurements gathered by the fiber optic sensors. This data is reduced within the pad to produce a list of contact points. The raw values on the grid are available through the device's Application Programming Interface (API) as well.

The unit is housed in red milled aluminum, into which the Tactex logo is etched. The active sensing area is 5.75 x 3.75 in (14.5 x 9.5 cm). This area has a black polycarbonate surface which is printed with a 3 x 3 grid in white. The entire unit is only a little over one centimeter thick. A single cable permanently attached to the rear of the pad forks off into both a nine pin serial interface and a "wall-wart" power adapter. A DB9-to-Mac serial cable is included for Macintosh use.

Impressions
The multi-touch capability of the MTC Express does indeed make it unique. There are other touch pad controllers on the market, some with built-in effects, and pen tablets have been around for years, providing multiple axes of control on a two-dimensional surface. But the ability to play using multiple simultaneous gestures on a single, portable, affordable control surface is entirely new and promises a wealth of performance uses.

I have been beta testing the Macintosh version of the MTC Express for several months. Along the way, I wrote several Max/MSP patches and made an alternate Max external to communicate with the pad. The process of compiling the external was easy thanks to the detailed documentation and example code that is provided. The documentation is provided in Adobe Acrobat format as a combined owner’s and developer’s guide. The information is comprehensive, and treats Windows and Macintosh users with equal courtesy. Happily, no installation of drivers is required to use the system—it uses the native serial drivers on each platform. So, although no information is given specifically for Linux developers, it seems likely that a savvy programmer could get it up and running on that operating system without too much trouble.

The setup process for Max/MSP consists of installing the Tactex Max external and normalizing the pad. Normalizing is a slightly tweaky process designed by Tactex to allow for variations in sensitivity between different pads. It can also be seen as a way to work out the trade-off between sensitivity and noise in the data. To begin, you run the Tactex demo application or invoke the normalization mode in the Max external. Then, you run a finger over the pad, traveling over the entire surface with an even touch. The demo program helps you out with a display showing the area covered. The pressure values generated are saved in a file to which the Tactex object refers on each startup. After a few tries you get the hang of it, and can then vary the pressure you apply in order to affect the pad's sensitivity.

I imagine that for consumer applications of their technology, Tactex will have to come up with a way of normalizing each pad during the manufacturing process. Probably they already have. But the MTC Express is not yet a consumer product: its audience is limited for the moment to Max users and C programmers, so it makes sense to let these picky early users experiment with the normalizing process for themselves.

The touch sensing works extremely well, with positional accuracy of about one millimeter. Multiple simultaneous touches spread widely apart on the pad are equally accurate, but when moved closer to one another they exhibit a proximity effect which pulls the results together. It's possible that this could be corrected by software in the future. When two touches get close enough together, they are merged by the pad, which limits the number of usable simultaneous touches to about four.

The dynamic range of the touch sensitivity is surprisingly large. The pad will recognize a light touch, yet still produce sensibly changing data when you bear down as hard as possible with a finger. When this much pressure is applied, the pad takes a few seconds to return to a zero reading after the pressure is removed while the flexible backing of the sensor returns to its rest state. But at moderate amounts of finger pressure, this delay effect does not appear at all.

The polycarbonate surface is pleasant to the touch, not too rough on the fingers, and it seems fairly durable. In the time that I've been working with the pad, a grid of small round indentations has become worn into the surface, seemingly corresponding with the structure of the internal sensors. These can easily be felt, but don't seem to affect the pad's sensitive response. The solid aluminum enclosure is pleasantly rugged in this time of cheaply available but cheaply built plastic peripherals. I would not hesitate to kick it around some if I had a mind to! Unfortunately, though, the cable which provides both power and data connections is permanently attached to the rear of the device. This makes packing a bit more difficult and warrants treating the device with care.

My only serious gripe about the MTC Express is with its connectivity. I imagine many performers currently using PowerBooks will be interested in this device. Unfortunately, recent generations of Apple computers lack native serial ports. To attach the MTC Express to a PowerBook, the only solution I have found is to use a serial-to-USB converter. And all such converters I've tried so far introduce signal latency on the order of 20-30 ms. So, unfortunately, I can't recommend the pad for the "finger drumming" I had hoped to do, or other such musical applications where a stable response of less than 10 ms is needed.

If conducting or mixing is more your style of performance, or you are not after real time musical control, the extended latency should not be a problem. And, if you have a computer with native serial hardware, you can experience the 5-ms latency which the pad offers. The problem is not strictly with the pad, but with the lack of any good connection option for current computers. The Magma PCI expander for Powerbooks combined with a PCI serial card might offer another possible solution, but I haven't been able to test this.

Overall, I have found the MTC Express to be an exciting and usable controller. I think the few compromises made in its design are reasonable ones. Its multi-touch capability, ruggedness, and portability all give it useful potential for any computer musician. If you are a laptop rocker looking to add more "live" to your live show, this functional unit could be an especially pleasant option, and quite affordable at US$ 495.

More information on the MTC Express and the Max/MSP demos are available on the company's Website (www.tactex.com).