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The basic operating principle consists of establishing an electric field over the control surface whose phase varies as a function of position. When the user touches the surface, the signal at that point is capacitively coupled by his hand to an adjacent pickup surface, and the phase information is extracted from the signal. Time multiplexing utilizes the same circuitry to give outputs for both axes.

Figure 2–Diagram of one-dimensional Trazor

Figure 2 is a diagram of a one-dimensional Trazor. A low-voltage alternating field is established along the length of the control surface whose phase, relative to one end, changes continuously as a function of distance from that end. At points in between, the signal from each end is attenuated in proportion to the distance from that end. The phase angle is determined by the vector sum of the two signals.

When the control surface is touched with a finger, a signal is coupled into the finger with a phase uniquely characteristic of the point touched. A pickup surface is located so the user's hand passes over it to touch the control surface. A portion of the signal coupled from the control surface to the finger is capacitively coupled into the pickup surface, which is connected to an amplifier. A phase detector recovers the phase information from the output of the pickup amplifier, producing an output proportional to the position of the point touched.

It is possible to provide two-axis operation by duplicating the circuitry for the second axis and operating the two sets of circuitry at different frequencies. It is simpler, however, to use the same circuitry for both axes by time multiplexing.

A level detector connected to the output of the amplifier produces a binary output signal to indicate the presence of a finger on the control surface. This circuit can be omitted if it is not needed.

Figure 3–Cross-sectional view of demonstration model

Figure 3 is a cross-sectional view of the demonstration model shown in Fig. 1. Although the control surface and pickup surface were separately fabricated in this model, they can both be produced from the same material if the impedance of the pickup surface is small relative to the input impedance of the amplifier.

The idea of the Trazor originated while its inventor was thinking about ways to make electronic musical instruments that would be easier to play than conventional instruments but more versatile than most electronic