Something Unreal

Up to 15 notes at once on an Arduino using no timers! Well, the quality drops a lot as the number of playing notes increases, but still!

[Watch in HD]

This is a demo of a MIDI synth I’m developing for the Arduino. Its sound is currently very basic – it has no concept of different instruments, can only produce square waves and noise, and each MIDI channel can only be at one of 3 different volume levels. It has no fixed sample rate, and is always producing a new sample as quickly as possible, which is slower when more notes play at once (in practise, the sample rate ranges from about 20 KHz down to about 6 KHz).

It supports pitch-bends, modulation, monophonic/polyphonic MIDI channel mode, and some percussive notes. It also recognises some sysex messages, including GM/GS/XG “reset” messages and GS/XG messages to set a MIDI channel’s percussion mode.

To use the code yourself (hardware info):

If you want the Arduino to accept MIDI data from “real” MIDI hardware (through a MIDI socket), you’ll need to build a circuit with an optocoupler and connect that to the Arduino’s serial RX port, and change #define UseRealMIDIPort False to #define UseRealMIDIPort True (this affects the baud rate used). Due to laziness, while testing, I used a program called “Hairless MIDI<->Serial Bridge” and the virtual MIDI cable driver “MIDI Yoke” to send MIDI data straight over the Arduino’s USB serial connection, instead of building the proper circuit.
The code controls one “port” on the Arduino (a group of 8 pins determined by the specific Arduino board model), which connects to an 8-bit DAC (a simple R2R resistor ladder) to give an 8-bit audio output. I’m using port C on the Arduino Mega, because that neatly corresponds to digital pins 37 (LSB) to 30 (MSB), but it may work on other Arduino boards as long as there is a port where all 8 bits are mapped to digital pins, with minimal changes to the code. The output port (PORTAudio and DDRAudio) would need changing to one consisting of 8 usable pins, and the maximum number of playing notes at once (NumSoundChans) could either be reduced (will save CPU time and memory) or, in the case of the Arduino Due, increased.

You can download the code for the current version here (13.2 KB). You will also need the Fast Division library (info). Note that the code includes most of the above hardware info in the form of comments. =)

P.S. The MIDI in the video is being played on MIDITester. I did not make the MIDI, and I don’t know who did. Please, people, at least credit yourself in the metadata ;_;