Kawai R-100 MIDI-Controlled Mod

A mid-80’s drum machine, modified for MIDI-controlled sound chip selection, circuit bending and pitch. Any MIDI sequencer or keyboard can be used. No changes are made to the exterior of the machine.

The Kawai R-100 drum machine classic was modified using an Arduino Nano, the circuitbenders.co.uk “R-ROM Switcher” and a HC-SR08 board. The Nano reads incoming note data on MIDI channel 2 to control the HC-SR08 and ROM Switcher board. Incoming MIDI data can

  • Control the playback pitch of the entire machine
  • Select sound ROM chip
  • Control a couple of the circuitbend connection points

The pitch mod: The HC-SR08 is a development board for the Analog Devices AD9850 Direct Digital Synthesis chip. The output DDS frequency is controlled by The Nano, reading MIDI notes and pitch bend. The output square wave is used as a clock signal for the R-100’s Address Generation Unit. This means that when the Kawai R-100 tries to play any sound, the rate at which the sound sample data bytes are found in memory is set by the 9850 output.

Actually this can be done without the HC-SR08, only using a pwm output on the Nano. This is how I originally tested the idea. But while this would work fine for very low pitches, the closer you’d get to the R-100’s original pitch, the less pitch resolution you would get. Since I wanted to be able to play it in a resolution of semi-tones (or even cents, using pitch bend) I had to use a higher resolution clock generator :)

The circuit bend: There are pre-made circuit bend points in the ROM Switcher pcb. It’s sufficient to have the Nano connecting these points to ground, they don’t need to be interconnected. To get back to non-circuit bent sound, the Nano pins are left in a floating state. The sound quality of the bends are much like a comb filter or phaser.

The ROM chip selection: To enable one ROM chip, the original ROM-Switcher connects the Output Enable (or possibly Chip Enable) pin of that chip to ground. Others are in HIGH state. A very simple task for a microcontroller.

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