mGB is a Gameboy cartridge program (You need a Flash Cart and Transfer hardware) That enables the Gameboy to act as a full MIDI supported sound module. It works with the old DMG Gameboy as well as GBC/GBA.

Current Source code available upon request. (Its a bit of a mess without comments.
mGB Change Log

    Feb 2 2009 1.3.0
        Rewrote 90% of code into assembly for much faster performance- especially noticeable on DMG.
        Changed note behavior. Removed Monophonic note memory to increase performance.
        Envelope does not retrigger if the notes overlap and have the same velocity- Good for arpeggios / broken chords.
        Note off has a slight delay so immediate retrigged notes don't cause "clicking" effect due to turning off the synth.
        Added screen off mode for great signal-to-noise ratio, longer battery life, and better performance on DMG. (To toggle the screen mode hold Select and press A.)
        Created back-end routine that prioritizes processes for better performance.
        Added 8 "noise" shapes to the Wav synth for more interesting effects.
        Made Wav pitch sweep stable and changed it so it glitches out at values above 8.
    Nov 5 2008 Version: 1.2.4
        Fixed small bug with the indicator arrow, it was offset vertically a bit.
        Fixed bug with unexpected behavior with large PB Ranges
        PB Range Max is now 48 notes. (hehe)
        Octave Shift max is now -2/+3
        Added some Octave shift logic. If the current note is greater than what the GB can play due to octave shifting, it will select the lower octave note, so no off key notes will play.
        Added Gameboy Color fast-cpu mode- better performance with newer Gameboys.
    Oct 28 2008 - Version: 1.2.3
        Added Note markers above synth number, so you can tell if the synth is on.
        Added PB wheel reset to MIDI panic (Start button)
        Code more efficient (I'm sure there is still more to do here)
        nitro2k01 @ http://gameboygenius.8bitcollective.com rewrote my gamejack serial function to make it as fast as possible. THANKS!!
    Oct 25 2008 - Version: 1.2.2
        Added Program Change messages to mGB
        Rewrote MIDI data input for mGB. (Rewrote the function in ASM to make it faster)
        Added Controller Priority. While changing parameters on the gameboy itself, MIDI messages will not overwrite your changes while your editing them. This is a good live mode feature
    Oct 23 2008 - Version: 1.2.1
        Found & Fixed various bugs in 1.2.0
        Changed help text. Made it more clear.
    Oct 23 2008 - Version: 1.2.0
        Change interface a bit
        Added presets
        Optimized code
    Oct 20 2008 - Version: 1.1.0
        Added Interface
        Changed Wav CCs Around to make more consistent with the Pu Synths.
    Oct 4 2008 - Version: 0.1.2
        Fixed bug with Wav Synth hanging after sequencer stop.
        Fixed bug with Wav Synth not resetting monophonic keyboard note triggers

mGB Button shortcuts

    Start: MIDI Panic
    Select + Dpad: Select multiple synths for editing.
    Select + A: Toggles the screen on or off, better battery life, less noise, and faster response.
    Select + B: Copys all parameters on screen while cursor is not on preset number.
    B: Pastes all parameters while cursor is not on preset number
    A + Dpad: Change parameter value
    To load/save presets, put the cursor on the "PRESET" number, and hit B for load, Select+B to save

mGB MIDI Implementation

Note: the name and number at the bottom left of the screen indicates the midi CC of the selected parameter.

    PU1 - MIDI CH1
        Program Change: 1 to 15
        PB: Pitch bend - up to +/- 12
        cc1: Pulse width - 0,32,64,127
        cc2: Envelope mode - 0 to 127, 16 possible steps
        cc3: Pitch sweep
        cc4: Pitchbend Range
        cc5: Load Preset
        cc10: Pan
        cc64: Sustain- Turns off note off. <64 = off, >63 = on

    PU2 - MIDI CH2
        Program Change: 1 to 15
        PB: Pitch bend - up to +/- 12
        cc1: Pulse width - 0,32,64,127
        cc2: Envelope mode - 0 to 127, 16 possible steps
        cc4: Pitchbend Range
        cc5: Load Preset
        cc10: Pan
        cc64: Sustain- Turns off note off. <64 = off, >63 = on

    WAV - MIDI CH3
        Program Change: 1 to 15
        PB: pitch bend - up to +/- 12
        cc1: shape select : 16 possible on a 0 to 127 range
        cc2: shape offset : 32 possible on a 0 to 127 range
        cc3: Pitch Sweep speed. 0=Off, 1-127 speed.
        cc4: Pitchbend Range
        cc5: Load Preset
        cc10: pan
        cc64: Sustain- turns off note off. <64 = off, >63 = on

    NOISE - MIDI CH4
        Program Change: 1 to 15
        PB: pitch bend +/-24
        cc2: envelope mode - 0 to 127, 16 possible steps
        cc5: Load Preset
        cc10: pan
        cc64: (sustain) turns off note off. <64 = off, >63 = on

    POLY MODE - MIDI CH5 - Plays Pu1/Pu2 and Wav in poly
        Program Change: 1 to 15
        PB: pitch bend +/-2
        cc1: See cc1
        cc5: Load Preset
        cc10: pan
        cc64: (sustain) turns off note off. <64 = off, >63 = on