The Radio Drum
Chopper Circuit 
Progress Report 1
Progress Report 2
Final Report
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The original Radio Drum processes the eight analog signals produced by the four antennas in the radio drum and sends a serial bit stream to be interpreted by the host computer. Most of the processing is performed in the Radio Drumís black box. The black box incorporates older, slower chips and ROM to process the signals which causes latency in the system and makes the virtual instrument sluggish, unreliable and limits the expression of the performer in real-time. Delicate gestural movements are difficult to detect and the black box is difficult to reprogram. 

One Channel Chopper Circuit

To enhance the Radio Drum, an 8 channel chopper circuit (AutoCAD Release 13) can be implemented to bypass the black boxís processor and send the eight analog signals directly to an A/D audio interface connected to the host computer. Modern-day processors can perform the data-acquisition and real-time processing to minimize latency, improve response and make the Radio Drum an effective virtual instrument. 

The chopper circuit must be implemented to overcome limitations introduced by audio interfaces. Todayís A/D converters are AC coupled via a series capacitor (highpass filter with cutoff frequency at 20 Hz). This restriction imposed by the series capacitor makes it impossible for the data acquisition software to detect gestures that produce slow signals. Signals created by slow delicate movements of the performer will be virtually DC and will not pass through the interface. The chopper circuit can be implemented to solve this problem. 

The chopper circuit will modulate the gestural analog signals onto a carrier signal at a rate of 44100 Hz/N, where N is a power of two to satisfy the Nyquist rate (sampling theorem). Most likely N will equal eight or sixteen. The resulting output from the chopper circuit is a square wave carrier wave, which is Amplitude Modulated with the gestural signal. If the gestural signal is DC the modulated signal will have constant amplitude. The data acquisition software will demodulate the eight modulated analog signals and will use filters to perform amplitude and peak detection to process the gestural data and produce MIDI. 

The chopper circuit is required to modulate signals ranging from close to DC to the maximum rate at which a drummer can produce a drum roll (approximately 250 Hz). The chopper circuit has eight analog inputs, eight analog outputs a clock divider circuit and a capacitive circuit to minimized Electromagnetic Interference (EMI) in the circuit. The following figures show the 8 channel chopper circuit.