Design of drum trigger MIDI converter

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Supervisor:
Dr. Benesóczky Zoltán
Department of Measurement and Information Systems

This thesis project deals with the design of a drum trigger midi converter, and the construc-

tion and measurement of a breadboard. A drum trigger’s function is to create a voltage

impulse from a beat on the drum’s body which can be processed afterwards. During the

work, I created a tool which consists of a snare drum and a properly installed piezo sensor.

I matched the signal, generated by the piezo, to the peak detector. The peak detector

includes a capacitor, the voltage of which jumps up when receiving an input by the pulse,

and then decays with a large time constant.

The resulting signal is connected to an Atmel ATMega128 A-D converter input. It is

not a good idea to analyze continuously the analog signals, because they are permanently

multiplexing between the 8 channels of the A/D converter, therefore the values are not exact

enough. In order to lessen the task of the microcontroller, the analog circuit implements

some additional features.

The circuit monitors the jumps in the signal with a differential grade, and generates

impulses induced by the jumps. The falling edge of these impulses is ideal for sampling

from the peak detected signal. This is important when measuring the beating strength.

I connected the generated impulse to the external interrupt pin of the controller with a

signal level adjust transistor.

The software has to be able to process multiple input channels. The fundamental idea

was to put a state machine in the main program, where every input channel has a state,

which processes the corresponding data. I used the A/D converter for timing of change

of the states. If the free running mode of the A/D converter is used, an inside interrupt

occurs in every 52 µs. These are suitable for timing the state machine. The software sends

a MIDI message after every trigger event.

MIDI (Musical Instrument Digital Interface) is a data communication protocol between

musical instruments. The sound module receives the MIDI message, and generates a drum

sound, with a volume level depending on the attack velocity.

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