This page describes my latest DIY project.  I ported my AVR code to an ATTINY2313 to use as a MIDI processor.  This circuit is low enough power to operate from the MIDI-In connector.  The twelve DIP switches select the following functions:

    SD5  Note transpose on/off
    SD4  Channel transpose on/off
    SD3  Note reverse on/off
    SD2  Multi keyboard on/off

    SB7  Note transpose bit3  0/1             0-11 transpose up 0 to 11 semitones
    SB6  Note transpose bit2  0/1             12 transpose up 1 octave
    SB5  Note transpose bit1  0/1             13 transpose up 2 octaves
    SB4  Note transpose bit0  0/1             14 transpose down 1 octave
                                                                     15 transpose down 2 octaves

    SB3  Channel transpose bit3  0/1       0-15 selects MIDI channel
    SB2  Channel transpose bit2  0/1
    SB1  Channel transpose bit1  0/1
    SB0  Channel transpose bit0  0/1

Note transpose adjusts all MIDI note on and off commands by 0 to 11 semitones or +1, +2,  -1 or -2 octaves as selected by SB7 - SB4.

Channel transpose replaces the channel in all MIDI commands with the channel selected by SB3 - SB0.

Note reverse inverts the keyboard around key C4.  D4 is transposed to A3#, etc.  Note transpose operates after the reverse function.

Multi keyboard  splits and replicates the keyboard at key C4.  The notes below key C4 are scaled up two octaves and the notes at key C4 and above are scaled down 2 octaves resulting in two keyboards with key C2 and key C6 playing C4 (middle C).  Positioning your hands appropriately can 'reverse' the registers with your right hand playing the low notes and your left hand playing the high notes.  Add reverse for an even crazier mode as each multi keyboard reverses.  Note transpose operates after the multi and reverse functions

Below is a photo of the functional prototype.

Here is the pcb layout  I designed using the ExpressPCB mini service.  I later changed the two black schottky diodes to 1N4148 diodes and removed the gray zener diode.  The regulation varied slightly so I decided to use the crystal oscillator to ensure accurate serial data timing.  With the crystal oscillator I can let the voltage vary slightly.  It typically varies between 3.75 and 4 volts.  The square pins, three series resistors, and the reset circuit are required for in-system programming and could be omitted.  The unstuffed components are for optional external power and regulation.  The status led blinks briefly on each MIDI command received.

DJB-MIDI processor schematic

DJB-MIDI processor source code

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