NoiceSynth - A Compact RP2040 Synthesizer

A pocket-sized, battery-powered MIDI synthesizer built around the Raspberry Pi Pico. It's designed to be housed in a small enclosure (like an Altoids tin) and features an I2S DAC for quality audio, an OLED display for visual feedback, and a TRS MIDI input.

This guide provides the blueprint for building your own.

Features

• Compact & Portable: Designed to be powered by a LiPo battery and fit into a small tin.
• High-Quality Audio: Uses an I2S audio module for clean, low-noise sound output.
• MIDI Connectivity: Standard 3.5mm TRS-A MIDI input for control with external keyboards and sequencers.
• Expressive Controls: Features a rotary encoder for navigation and a potentiometer for volume control.
• Visual Feedback: A crisp 128x64 OLED display shows synth parameters and status.

Hardware Requirements

Component	Description
MCU	Raspberry Pi Pico or Pico W (RP2040)
Audio Output	I2S DAC Amplifier (e.g., MAX98357A or PCM5102A) with a 3.5mm headphone jack.
Display	0.96" SSD1306 128x64 I2C OLED Display.
Controls	1x Rotary Encoder with push-button, 1x 10kΩ Potentiometer.
MIDI Input	1x 3.5mm TRS (Stereo) Jack.
Power	3.7V LiPo Battery and a 5V booster/charger board (e.g., Adafruit PowerBoost).
MIDI Circuit Parts	6N138 or 6N137 Optocoupler, 1N4148 Diode, various resistors (see diagram).

Powering Your Synth

You can power the synth either from a portable LiPo battery or directly from USB.

Battery Operation (Portable)

To make the synth truly portable, use a LiPo battery and a 5V booster/charger board. This provides stable power to all components.

LIPO BATTERY WARNING: LiPo batteries are powerful but require careful handling.

• Never use a LiPo battery without a dedicated protection and charging circuit like the PowerBoost.
• Do not puncture, bend, or short-circuit a LiPo battery.

1. Connect the LiPo battery to the JST connector on the PowerBoost board.
2. Connect the PowerBoost's 5V output to the Pico's VBUS pin (Pin 40). This powers the Pico.
3. Connect the PowerBoost's 5V output to the VCC/VIN pin of your I2S Audio Module.
4. Connect the PowerBoost's GND to one of the Pico's GND pins. This creates a common ground.

USB Operation

If you don't need battery power, you can run the synth from a USB source (like a computer or wall adapter). The wiring is simpler.

There are two ways to power the components:

Option 1 (Simplest): Power everything from 3.3V The PCM5102A DAC chip runs natively on 3.3V. This is the easiest and safest wiring method for USB operation.

1. Power the Pico by connecting its micro-USB port to a USB power source.
2. Connect the Pico's 3V3(OUT) pin (Pin 36) to the VCC/VIN pins of all components: the I2S Module, OLED, Rotary Encoder, Potentiometer, and MIDI circuit.
3. Ensure all components share a common ground with one of the Pico's GND pins.

Option 2: Power I2S Module from 5V (with protection) If you prefer to give the audio module a separate 5V supply from USB. For good measure, adding a Schottky diode (e.g., 1N5817) is recommended to protect against any potential reverse voltage.

1. Power the Pico via its micro-USB port.
2. Connect the Pico's VBUS pin (Pin 40) to the anode (+) of a Schottky diode. Connect the cathode (-) of the diode to the VCC/VIN pin of your I2S Audio Module.
3. Connect the Pico's 3V3(OUT) pin (Pin 36) to power the OLED, Encoder, Potentiometer, and MIDI circuit.
4. Ensure all components share a common ground.

Wiring & Connections

Establish a common ground by connecting the ground from your PowerBoost board to the Pico, and then to all other components' GND pins.

Component	Pico Pin	Description
I2S Audio Module
VCC	3.3V or 5V Source (see Power section)	Power for the amplifier
GND	Common GND	Ground
DIN (Data)	GP11 (Pin 15)	I2S Data Out
BCLK (Bit Clock)	GP9 (Pin 12)	I2S Bit Clock
LRCK (Word Clock)	GP10 (Pin 14)	I2S Left/Right Clock
SCK (System Clock)	GND	PCM5102 Only: Connect to GND for internal PLL
SSD1306 OLED
VCC	3V3 (OUT) (Pin 36)	3.3V Power
GND	Common GND	Ground
SDA	GP4 (Pin 6)	I2C Data
SCL	GP5 (Pin 7)	I2C Clock
Rotary Encoder
+ (VCC)	3V3 (OUT) (Pin 36)	3.3V Power
GND	Common GND	Ground
CLK	GP12 (Pin 16)	Encoder Clock
DT	GP13 (Pin 17)	Encoder Data
SW	GP14 (Pin 19)	Encoder Switch
Volume Pot
VCC	3V3 (OUT) (Pin 36)	3.3V Power
GND	Common GND	Ground
Wiper (Output)	GP26 (Pin 31)	ADC0 for volume reading
MIDI IN
(Circuit Output)	GP1 (Pin 2)	UART0 RX for MIDI Data In

MIDI IN Circuit (TRS Jack)

MIDI requires an opto-isolated input to protect the Pico from electrical faults. We will use a Type A TRS MIDI wiring standard, which is the most common.

• TRS Tip: Connects to MIDI Pin 5 (Current Source)
• TRS Ring: Connects to MIDI Pin 4 (Current Sink)
• TRS Sleeve: Connects to MIDI Pin 2 (Ground/Shield)

Here is a schematic for the input circuit:

TRS JACK
  TIP ---- 220Ω ----+----|>|---- (Pin 6 of 6N138)
(MIDI 5)            |  (Diode)
                    |
 RING --- 220Ω -----+----------- (Pin 5 of 6N138)
(MIDI 4)

6N138 OPTOCOUPLER
  Pin 8 (VCC) ----> 3.3V (from Pico)
  Pin 7 (VB)  ----> GND
  Pin 6 (E)   --+-> 10kΩ Pull-up Resistor -> 3.3V
                |
                +-> To Pico GP1 (UART RX)
  Pin 5 (GND) ----> GND

This circuit correctly isolates the incoming MIDI signal and inverts it for the Pico's UART receiver.

Software Setup

1. Install RP2040 Board Core:

   • In the Arduino IDE, go to Preferences and add this URL to "Additional Boards Manager URLs": https://github.com/earlephilhower/arduino-pico/releases/download/global/package_rp2040_index.json
   • Go to Tools > Board > Boards Manager, search for "pico", and install Raspberry Pi Pico/RP2040.
   • Select Raspberry Pi Pico from the board menu.

2. Install Libraries:

   • Install the following from the Arduino Library Manager:
     • Adafruit GFX Library
     • Adafruit SSD1306
     • An I2S Audio Library for the Pico (e.g., I2S by Earle F. Philhower, III, which is often included with the core).

3. Upload Code:

   • Once everything is wired, you can upload your synthesizer code to the Pico.

Controls & Usage

The specific function of the controls will depend on your code, but here is a common layout:

• Rotary Encoder (Turn): Navigate menus, change synth parameters (e.g., waveform, filter cutoff), or select notes.
• Rotary Encoder (Press): Enter/exit edit mode for a parameter, or trigger an action.
• Volume Potentiometer: Controls the final output volume before it goes to the headphone jack.

Ideas for Unorthodox Sounds

The beauty of a programmable synth is the ability to go beyond simple subtractive synthesis. Here are some fun ideas to code:

1. Glitchy Wavetable Synthesis:

   • Concept: Store several single-cycle waveforms (sine, saw, square, triangle) in arrays. The encoder selects the primary waveform.
   • The Twist: Add a "glitch" parameter. When activated (e.g., by a long press of the encoder), the code starts intentionally misreading the wavetable. It could randomly jump to a different table, read sample points backward, or apply bitwise operations (XOR, AND) to the sample data before sending it to the DAC. This creates a source of controlled digital chaos and unexpected textures.

2. Karplus-Strong Physical Modeling:

   • Concept: This algorithm simulates a plucked string. A buffer (delay line) is filled with random noise (the "pluck"), then played back and fed back into itself through a simple low-pass filter.
   • The Twist: Use the controls in non-standard ways. Map the volume pot to the filter cutoff or the feedback amount instead of volume. High feedback can cause the "string" to resonate infinitely, like an e-bow. Map the rotary encoder to the length of the delay line to change pitch, but allow it to be modified while a note is playing, creating bizarre pitch-bending and warping effects.

3. Chaotic Oscillators:

   • Concept: Instead of a standard oscillator, generate sound using a mathematical logistic map, like x_n+1 = r * x_n * (1 - x_n). The output x is a value between 0.0 and 1.0.
   • The Twist: Map the output x directly to the audio sample value. The potentiometer controls the r parameter. At low r values, the output is stable or oscillates simply. As you increase r past ~3.57, it becomes chaotic, generating complex, noise-like, but still structured tones. This gives you a controller that smoothly transitions a sound from a pure tone into pure noise and back again.

Happy building, and enjoy your new tiny synth!