Overview of a Three-Unit Percussive Drum Pad MIDI Controller - Prof. Todd D. Morton, Study Guides, Projects, Research of Engineering

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Western Washington UniversityETec 474 Paul J. Faughnan Instructor: Todd D. Morton 4 / 19 / 05

Drum Pad MIDI

Controller

Hardware Description

This percussive MIDI controller has three units: a 5-volt DC power supply, a modified practice drum pad, and a control unit. The power supply is a common wall plug type with a +5VDC regulated output. The maximum current output is rated at 3A, and it terminates in a 2.5mm coaxial plug. The drum pad is a Remo tunable practice pad modified with a six-inch piezoelectric film transducer manufactured by Measurement Specialties Inc. The transducer signal is transmitted over a shielded twisted pair terminating in a 3.5mm stereo audio plug. The cable’s shield is connected at the plug side only to avoid a ground loop. The remainder of the hardware is contained in the control / interface. The MIDI message is an asynchronous, serial bit stream composed of a status byte and one or two data bytes. The most significant bit is always 1 for a status byte, and 0 for a data byte. The scope of the project is limited to three types of messages. Program change allows the user to select a bank of related sounds. The sole data byte represents one of 128 (2 7 ) program numbers. The program number stays the same until a new program change message is received. Note on and note off messages contain data bytes for note number and velocity information. For keyboard instruments, the program number typically refers to the type of instrument, such as piano, and the note number refers to the pitch, such as middle C. In drum controllers, the program numbers frequently represent drum sets with associated note numbers for various drum and cymbal sounds. While indirectly related, velocity information is more immediately related to the force with which a note is triggered (or released). As with program numbers, the MIDI standard allows for 128 note numbers and velocity values. The piezoelectric transducer generates an AC signal proportional to the mechanical stress of the film. This makes it an ideal “velocity” sensor.

The shield is grounded on the control / interface end only to avoid a ground loop. The transducer signal is fed to the precision rectifier formed by U1B, U1C, R9, R10, D1 and D2. Ideally, this circuit’s output should be a full-wave rectified version of the transducer input with negative peaks traversing from +5VDC. The limitations of the LM324 require a reduced voltage level at this point (+3.9VDC approximately). This negative full-wave rectified signal is then summed at U2A with a threshold level determined by sensitivity resistor R7 and buffered by U1D. Since the summer is an inverting configuration, the negative peaks of the transducer signal will be converted to positive peaks, and, the lower the DC sensitivity level, the smaller the resultant signal peaks. The summed signal is also amplified in accordance with the setting of gain resistor R11. The signal is next fed to a two-pole low-pass filter at U2B before it is input to the HC12’s A-to-D converter at ANALOG IN. The DIGITAL OUT signal is generated by the serial communications interface (SCI) of the HC12. The MIDI electrical specification calls for the interface circuit shown. J3 is a five-pin DIN connector with pin 2 grounded. Again, the specification calls for the cable shield connected to pin 2 to be grounded only at the MIDI OUT side. R23 and R24, connected to pins 4 and 5, respectively, form a current loop. The current loop drives an opto-isolator at the MIDI IN side, in accordance with the electrical specification. Switching inverters Q1 and Q2 allow current sourcing without loading the output of the SCI.