Teensy SN76489 MIDI Synth

This project contains code and instructions to use a SN76489 sound chip as a MIDI controlled synth, driven by a Teensy 2.0 microcontroller.

You can see this in action in the YouTube demo.

Schematic

The schematic is also available in the Fritzing file schematic.fzz.

Getting it Running

1. Install the Arduino IDE
2. Install the Teensy software for the Arduino IDE
3. Open the teensy-sn76489-midi-synth.ino file in the Arduino IDE
4. Under the Tools menu option select Board -> Teensy 2.0
5. Under the Tools menu option select USB Type -> MIDI
6. Under the Sketch menu option select Upload
7. In your DAW of choice select the "Teensy MIDI" output device and use the MIDI channels as described in the "MIDI Configuration" section below

MIDI Configuration

Channel 1

• Controls square tone 1
• Pitch wheel is supported on this channel
• Note velocity is linked to the 16 level attenuation of square tone 1
• The brightness of LED 1 is linked to the attenuation level of square tone 1

Channel 2

• Controls square tone 2
• Pitch wheel is supported on this channel
• Note velocity is linked to the 16 level attenuation of square tone 2
• The brightness of LED 2 is linked to the attenuation level of square tone 2

Channel 3

• Controls square tone 3
• Pitch wheel is supported on this channel
• Note velocity is linked to the 16 level attenuation of square tone 3
• The brightness of LED 3 is linked to the attenuation level of square tone 3

Channel 4

• Controls the noise generator
• Note velocity is linked to the 16 level attenuation of the noise generator
• The brightness of LED 4 is linked to the attenuation level of the noise generator
• MIDI CC 1 (mod wheel) on this channel controls the frequency of square tone 3, which is used by notes F4 and C5 to control the noise shift rate

Unlike the other MIDI channels only 8 notes produce sound on this channel, these control the different settings of the noise generator. The notes are:

Note	Noise
C4	Periodic noise, shift rate = clock speed (Hz) / 512
D4	Periodic noise, shift rate = clock speed (Hz) / 1024
E4	Periodic noise, shift rate = clock speed (Hz) / 2048
F4	Perioic noise, shift rate = Square tone 3 frequency
G4	White noise, shift rate = clock speed (Hz) / 512
A4	White noise, shift rate = clock speed (Hz) / 1024
B4	White noise, shift rate = clock speed (Hz) / 2048
C5	White noise, shift rate = Square tone 3 frequency

NOTE: If you use square tone 3 to modify the shift rate of the noise channel you will interfere with any tones playing on channel 3.

To Do

Here are some features I might look to add in the future:

• Attack and decay support (via CC control)
• 3 voice polyphony on a single channel
• Legato support
• Arpeggiator support
• Mod wheel controlled vibrato on square waves

Credit

Initial inspiration for this project was taken from the (more advanced) CHIP_BASED_SYNTHESIZERS repository by Brian Peters.