MIDI Hero (part Ⅱ)

Turning a thrift store Guitar Hero controller into a MIDI controller (continued)

Well, it looks like we’re going to PyConAU and to celebrate the MIDI Hero guitar is going to get some sensor upgrades …

New Controls

There were three things I wanted to add to this project:

On-board Synthesis (Audio Out)

This would be cool because the instrument could have its own voice rather than being just a synth input device.
More fret controls, possibly a fretless slider

The five buttons are pretty limiting, and there’s plenty of room on the ‘fretboard’ for more controls.
Nicer strum control, possible a piezo or similar sensor

The clicky strum thingy is quite annoying, I’m interested to see how a piezo or force sensor would feel to play.

So let’s work out how those might work:

On-board Synthesis

On-board synthesis with Karplus-Strong shouldn’t be too hard.

While CircuitPython supports I2S it’d probably be easier just to run audio straight out the A0/A1 analog outputs. We’re not exactly after high fidelity here and I2S needs several pins and a separate I2S DAC board.

More Frets!

Some more fret buttons would be nice, and there’s also a possibility for analog input from something like a Soft Potentiometer which would allow for continuous movement much like a Lap Steel Guitar.

Better Strums!

The nice thing about the soft pot mentioned about is that it is thin and can just get surface mounted to the controller. This saves a lot of hassle removing internal structures which keep the controller rigid.

A similar device called a Force sensing resistor might be usable as a better strum sensor. Another possibility is to use a piezo sensor on the inside of the plastic case, or even just a tiny microphone to detect the sounds of fingers whacking on the plastic.

Software would track the peak force/pressure/signal and use this to assign the “velocity” of the MIDI event.

The existing strum control could either be left as is (two separate digital inputs) or wired into the analog strum circuit somehow.

Control Positions

Endless Possibilities

There’s many possibilities for adding extra controls. For a start, there’s a large rectangular space on the faux fretboard, between the lowest ‘fret button’ and the strum sensor. This is about 180mm long, and about 45mm at the wider end and 35mm at the narrower end. There are little fret ridges but there can be sanded flat if necessary.

The neck is also quite square, and so there’s a flat strip along the bottom of the neck which is about 240x18mm which is quite easy to get fingers on.

The MCU board takes up a lot of space in the top side of the lower bout, but there might be some room around the whammy bar for another control knob, or maybe the lower or upper horns could host some more controls.

Finally, that weird blobby headstock could maybe fit something or another in it? I’d consider sawing it off but that would make the controller hard to hang

I don’t really want to keep pulling this thing apart, and I’d like to have some spare stuff built in to fiddle with at PyConAU so I think I’ll take the approach of using up as many I/O pins as possible … not everything is going to turn out to be useful, but it’s a platform for experimentation so that’s okay.

Available I/O

Sticking with the Adafruit Metro M4 Express currently in it, we’ve got quite a few pins to play with:

Pin	Function	Use
A0	Analog Input or Output	Stereo Jack (T) *
A1	Analog Input or Output	Stereo Jack (R) *
A2	Analog Input or Digital I/O	Whammy Bar
A3	Analog Input or Digital I/O	Fret Slide (soft potentiometer)
A4	Analog Input or Digital I/O	Strum Sensor (force dependent resistor)
A5	Analog Input or Digital I/O	Knob 1
SDA	SDA or Analog Input A6	Knob 2
SCL	SCL or Analog Input A7	Knob 3
D0	Digital I/O or Serial1 RX	5 pin MIDI *
D1	Digital I/O or I2S SDI or Serial1 TX	5 pin MIDI *
D2	Digital I/O or I2S MC	Fret 1
D3	Digital I/O or I2S BCK	Fret 2
D4	Digital I/O	Fret 3
D5	Digital I/O	Fret 4
D6	Digital I/O	Fret 5
D7	Digital I/O	Fret 6 (additional)
D8	Digital I/O or I2S SDO	Fret 7 (additional)
D9	Digital I/O or I2S WS	Fret 8 (additional)
D10	Digital I/O	Strum Down
D11	Digital I/O	Strum Up
D12	Digital I/O	Select Button
D13	Digital I/O (red LED)	Start Button
SCK	Digital I/O or SPI SCK	**
MISO	Digital I/O or SPI MISO	**
MOSI	Digital I/O or SPI MOSI	**

*: Eventually? Leave them just in case.

**: These are on a different header so not as convenient.

So that’s an additional analog fret slider, a strum force sensor, three extra fret buttons and three multipurpose knobs to fit in. Plus a handful of passive components. And we have to work around the internal structures of the controller enough that it will stil screw back together nicely.

Then the onboard software will map each of these things to MIDI Messages such as notes, velocities, pressures, pitch bends and control changes.

See you at PyConAU!

packing my bags for PyConAU

I was in a rush so I just soldered things to other things at random and kept going until almost everything worked:

Pin	Function	Use
A0	Analog Input or Output
A1	Analog Input or Output	Whammy Bar
A2	Analog Input or Digital I/O	Slide sensor
A3	Analog Input or Digital I/O	Microphone
A4	Analog Input or Digital I/O	Pressure sensor
A5	Analog Input or Digital I/O	Knob
D0	Digital I/O or Serial1 RX	Start Button
D1	Digital I/O or I2S SDI or Serial1 TX	Select Button
D2	Digital I/O or I2S MC
D3	Digital I/O or I2S BCK
D4	Digital I/O	Strum Up
D5	Digital I/O	Strum Down
D6	Digital I/O	Fret 8 (additional)
D7	Digital I/O	Fret 7 (additional)
D8	Digital I/O or I2S SDO	Fret 6 (additional)
D9	Digital I/O or I2S WS	Fret 5
D10	Digital I/O	Fret 4
D11	Digital I/O	Fret 3
D12	Digital I/O	Fret 2
D13	Digital I/O (red LED)	Fret 1

Hopefully not too much of it will break in transit.

UPDATE 2023-08-21

Well it went to PyConAU, it survived, the overall conclusion was that:

The latency makes it practically unplayable, this needs fixing.
I need to do something about debouncing & also a curve to make the pressure pad more expressive.
I really like the idea of having the pressure pad actually control a parameter rather than the velocity or aftertouch.
The “soft pot” components worked okay, but they’re not necessarily a nice feel, and the big ones are very expensive. I might try making some custom boards for Capacitive Touch Sensing for frets instead.
It sure is fun to make annoying boop noises though

Thanks to everyone who popped past and said hi and had a go at it, and also cTrix who was busy filming but I got to chat to at the airport on the way home …

... and another thing ...