This design shows the computer system used to monitor dancer movements for the Ender's Game ballet. It consists of various dancer-mounted sensors connected to a 68HC11 computer. A data lransmission channel is used to format a serial bit stream with suitable formatting to move the data via an FM radio transmitter to a receiving system. The entire sensor, computer and transmitter system is mounted on the dancer. The receiving system consists of an FM receiver, a 68HC11 computer and MIDI interface. Its purpose is to receive the serial data stream Erom the dancer and reformat it for input into the MAX system running on a MacIntosh computer where the various sensor commands are reworked and elaborated for the musical purposes of the dance. The overall system is shown in Figure 1.

Figure 1: The Overall System

Figure 2: The Dancer-Mounted System

The two accelerometers are mounted in a vertical orientation, one on each leg below the knee. They consist of a tube containing two magnets and an analog Hall Effect Sensor to measure the magnetic field between the two magnets. One magnet levitates above the other in its repulsive magnetic field. The sensor presents an analog voltage that is proportional to the distance of the floating magnet above the fixed magnet (and sensor). If the dancer "stomps" his foot then the upper magnet is accelerated toward the lower (fixed) magnet thus causing the magnetic field between the magnets to decrease. This is a time-varying value that will be "peak-detected" by the computer. It will send a new accelerometer message whenever a peak is detected and it will append the maximum value of the peak.

The four "Finger Switches" are mounted on the finger-tips of one hand. A magnet is sewn into the opposing thumb such that the dancer can place the thumb-magnet in contact with the surface of each sensor without touching the sensors on the adjacent finger-tips. Hall-Effect switches are used so that the only information that is transduced is whether the magnetic field is greater than a threshold value set by the switch specified. The dancer uses these switches for various "two-state" selections determined by the MAX program on the MacIntosh computer. The computer serializes a block of acquired data and Pulse-Width Modulates the serial bit stream for the FM transmitter. The PWM bit-stream will directly (DC) modulate the FM transmitter's carrier.

1.2 The Receiving System: The architecture of the Receiving System is shown in Figure 3: The FM receiver will present the serial data channel to the Demodulator where the data clock will be recovered. The 68HC11 synchronous serial port converts the incoming serial stream into byte serial form with the aid of the recovered clock. The byte-serial data is reserialized into asynchronous form with suitable MIDI instructions and transmitted at the MIDI data rate to the MIDI adaptor. The MIDI adaptor drives the MIDI cable.

Figure 3: The Receiving System

2	Transmitting System Subsections

2.1 The Accelerometer: The accelerometer consists of a Hall Effect Magnetic field sensor piaced between two cylindrical permanent magnets. The magnets are placed in a close-fitting cylindrical tube with their North poles facing. One magnet is attached to the housing such that it and the Hall-Effect sensor are at rest with respect to each other. The second magnet "floats" above the first in a mutually repelling magnetic field. When the body of the cynlindrical containing tube is accelerated along its longitudinal axis the position of the "floating" magnet changes thus varying the magnetic field intensity at the Hall-Effect sensor location between the two magnets. The output of the Hall-Effect sensor is a voltage that represents the value of the magnetic field intensity at the sensor's location. For our purposes, we can AC couple the sensor's output voltage to suitable amplification circuitry since we are interested in only the impulses that are applied to the sensor container, not the steady state value of the magnetic field.

The AC coupled signal is amplified and level shifted before being applied to the Analog-to-Digital converter in the computer as shown in Figure 4.

Figure 4: The Accelerometer

The potentiometer output is level-shifted and amplified to match the input range (0-5V) of the Analog-to-Digital input port of the MC68HC11 computer as shown in Figure 5.

Figure 5: The Limb-Angle Sensor

Figure 6: The Potentiometer Connector