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digital-synth-pra32-u2

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PRA32-U2 is a 4-Voice Polyphonic Synthesizer for Raspberry Pi Pico 2/RP2350

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# Digital Synth PRA32-U2 v2.18.2

- 2026-08-11 ISGK Instruments
- <https://github.com/risgk/digital-synth-pra32-u2>

## Overview

- **PRA32-U2** is a 4-Voice Polyphonic Synthesizer for Raspberry Pi Pico 2/RP2350
    - Built-in Chorus and Delay FX
    - Controlled by MIDI -- PRA32-U2 is a MIDI sound module
    - Having the function of writing the parameters to the user programs and the flash
    - PRA32-U2 is an upgraded model of PRA32-U (for Raspberry Pi Pico/RP2040), but some specifications differ
- Modifiable with Arduino IDE and Arduino-Pico (by Earle F. Philhower, III)
- An **I2S DAC** hardware (e.g. Pimoroni Pico Audio Pack) is required
- Optional
    - **[PRA32-U2/M](#pra32-u2m-pra32-u2-multi-timbre-edition-optional)** (PRA32-U2 Multi-Timbre Edition) can also be configured
    - **[PRA32-U2/P](./README-PRA32-U2-P.md)** (PRA32-U2 with Panel) and **PRA32-U2/M/P** (PRA32-U2 Multi-Timbre Edition with Panel) can also be configured by adding certain parts
- Prebuilt UF2 files (in the "bin" folder)
    - PRA32-U2: "Digital-Synth-PRA32-U2-Pimoroni-Pico-Audio-Pack.uf2" is for Raspberry Pi Pico 2 and Pimoroni Pico Audio Pack
    - PRA32-U2/M: "Digital-Synth-PRA32-U2-M-Pimoroni-Pico-Audio-Pack.uf2" is for Raspberry Pi Pico 2 and Pimoroni Pico Audio Pack

![PRA32-U2 (Pico Audio Pack)](https://raw.githubusercontent.com/risgk/digital-synth-pra32-u2/main/pra32-u2-pico-audio-pack.jpg)

## [Change History](./PRA32-U2-Change-History.md)

## [Parameter Guide](./PRA32-U2-Parameter-Guide.md)

## [MIDI Implementation Chart](./PRA32-U2-MIDI-Implementation-Chart.md)

## Synthesizer Block Diagram

```mermaid
graph LR
    subgraph V1[Voice 1]
        V1O1[Osc 1 w/ Sub Osc] --> V1OM[Osc Mixer]
        V1O2[Osc 2] --> V1OM
        V1OM --> V1F[Filter]
        V1F --> V1A[Amp]
        E[EG] -.-> V1O1 & V1O2 & V1F
        V1AE[Amp EG] -.-> V1A
    end
    V1A --> VM[Voice Mixer]
    V2[Voice 2] & V3[Voice 3] & V4[Voice 4] --> VM
    VM --> P[Panner] --> C[Chorus FX] --> D[Delay FX] --> AO[Audio Out]
    P --> C --> D --> AO
    N[Noise Gen]  --> V1O2 & V1OM & V2 & V3 & V4
    N -.-> L[LFO w/ S/H]
    L -.-> V1O1 & V1O2 & V1F & V2 & V3 & V4
```

## Wave Table Graphs

![Wave Table Graphs](https://raw.githubusercontent.com/risgk/digital-synth-pra32-u2/main/pra32-u2-wave-table-graphs.png)

## Preparation for Modification

- Please install **Arduino IDE**
    - NOTE: Large noise is generated during the sketch upload if other than Update Method: "Default (UF2)" is used
    - Info: <https://www.arduino.cc/en/software>
- Please install Arduino-Pico = **Raspberry Pi Pico/RP2040/RP2350** (by Earle F. Philhower, III) core
    - Additional Board Manager URL: <https://github.com/earlephilhower/arduino-pico/releases/download/global/package_rp2040_index.json>
    - This sketch is tested with version **6.0.0**: <https://github.com/earlephilhower/arduino-pico/releases/tag/6.0.0>
    - Info: <https://github.com/earlephilhower/arduino-pico>
- Please install Arduino **MIDI Library** (by Francois Best, lathoub)
    - This sketch is tested with version **5.0.2**: <https://github.com/FortySevenEffects/arduino_midi_library/releases/tag/5.0.2>
    - Info: <https://github.com/FortySevenEffects/arduino_midi_library>

## Features

### MIDI

#### USB MIDI Device

- NOTE: Select USB Stack: "Adafruit TinyUSB" in the Arduino IDE "Tools" menu
- To disable, comment out `#define PRA32_U2_USE_USB_MIDI` in "Digital-Synth-PRA32-U2.ino"
- MIDI Device Name: "Digital Synth PRA32-U2"

#### UART MIDI

- UART MIDI helps avoid noise caused by USB communication
- To disable, comment out `#define PRA32_U2_USE_UART_MIDI` in "Digital-Synth-PRA32-U2.ino"
- Modify `PRA32_U2_UART_MIDI_SPEED`, `PRA32_U2_UART_MIDI_TX_PIN`, and `PRA32_U2_UART_MIDI_RX_PIN`
    - Speed: 31250 bps (default, for DIN/TRS MIDI) or 38400 bps (for PC)
    - GP4 and GP5 pins are used by UART1 TX and UART1 RX by default
    - You can also use `SoftwareSerial` by making the following changes:

```cpp
        #include <SoftwareSerial.h>
        #define PRA32_U2_UART_MIDI_TX_PIN              (4)
        #define PRA32_U2_UART_MIDI_RX_PIN              (5)
        SoftwareSerial mySerial(PRA32_U2_UART_MIDI_RX_PIN, PRA32_U2_UART_MIDI_TX_PIN);
        #define PRA32_U2_UART_MIDI_SERIAL              mySerial
        ```

```cpp
        //  PRA32_U2_UART_MIDI_SERIAL.setTX(PRA32_U2_UART_MIDI_TX_PIN);
        //  PRA32_U2_UART_MIDI_SERIAL.setRX(PRA32_U2_UART_MIDI_RX_PIN);
        ```

- DIN/TRS MIDI is available by using (and modifying) Adafruit MIDI FeatherWing Kit, for example
    - Adafruit [MIDI FeatherWing Kit](https://www.adafruit.com/product/4740) (Product ID: 4740)
    - M5Stack [Midi Unit with DIN Connector (SAM2695)](https://shop.m5stack.com/products/midi-unit-with-din-connector-sam2695) (SKU: U187) in Separate mode
    - Kinoshita Laboratory [MIDI-UART interface-san Kit](https://www.tindie.com/products/kinoshitalab/midi-uart-interface-san-kit/)
    - 木下研究所 [MIDI-UARTインターフェースさん キット](https://www.switch-science.com/products/8117) (Shipping to Japan only)
    - necobit電子 [MIDI Unit for GROVE](https://necobit.com/denshi/grove-midi-unit/) (Shipping to Japan only)
    - necobit電子 [MIDI Unit Mini for GROVE](https://necobit.com/denshi/midi-unit-mini-for-grove/) (Shipping to Japan only)
- We recommend using [Hairless MIDI<->Serial Bridge](https://projectgus.github.io/hairless-midiserial/) on PC
    - On Windows, We recommend using [loopMIDI](https://www.tobias-erichsen.de/software/loopmidi.html) (virtual loopback MIDI cable)
    - On Mac, a virtual MIDI bus (port) can be created by using the IAC bus

### Audio Output

#### I2S (Default)

- Use an I2S DAC (Texas Instruments PCM5100A, PCM5101A, or PCM5102A is recommended), Sampling Rate: 48 kHz, Bit Depth: 24 bit
- NOTE: The RP2350 system clock (sysclk) changes to overclocked 153.6 MHz by I2S Audio Library setSysClk()
- Modify `PRA32_U2_I2S_DAC_MUTE_OFF_PIN`, `PRA32_U2_I2S_DATA_PIN`, `PRA32_U2_I2S_MCLK_PIN`, `PRA32_U2_I2S_MCLK_MULT`,
  `PRA32_U2_I2S_BCLK_PIN`, `PRA32_U2_I2S_SWAP_BCLK_AND_LRCLK_PINS`, and `PRA32_U2_I2S_SWAP_LEFT_AND_RIGHT`
  in "Digital-Synth-PRA32-U2.ino" to match the hardware configuration
    - Define `PRA32_U2_I2S_DAC_MUTE_OFF_PIN` and connect this pin to the I2S DAC mute off pin to reduce click noise when writing the parameters to the flash
- The default setting is for Pimoroni [Pico Audio Pack](https://shop.pimoroni.com/products/pico-audio-pack) (PIM544)
    - [Adafruit PCM5102 I2S DAC](https://www.adafruit.com/product/6250) (Product ID: 6250), [Adafruit PCM5100 I2S DAC](https://www.adafruit.com/product/6251) (Product ID: 6251), and GY-PCM5102 (PCM5102A I2S DAC Module) can also be used

```
    #define PRA32_U2_I2S_DAC_MUTE_OFF_PIN          (22)
    #define PRA32_U2_I2S_DATA_PIN                  (9)
    //#define PRA32_U2_I2S_MCLK_PIN                  (0)
    //#define PRA32_U2_I2S_MCLK_MULT                 (0)
    #define PRA32_U2_I2S_BCLK_PIN                  (10)  // LRCLK Pin is PRA32_U2_I2S_BCLK_PIN + 1
    #define PRA32_U2_I2S_SWAP_BCLK_AND_LRCLK_PINS  (false)
    #define PRA32_U2_I2S_SWAP_LEFT_AND_RIGHT       (false)
    ```

- The following is setting is for [Pimoroni Pico VGA Demo Base](https://shop.pimoroni.com/products/pimoroni-pico-vga-demo-base) (PIM553)

```
    //#define PRA32_U2_I2S_DAC_MUTE_OFF_PIN          (0)
    #define PRA32_U2_I2S_DATA_PIN                  (26)
    //#define PRA32_U2_I2S_MCLK_PIN                  (0)
    //#define PRA32_U2_I2S_MCLK_MULT                 (0)
    #define PRA32_U2_I2S_BCLK_PIN                  (27)  // LRCLK Pin is is PRA32_U2_I2S_BCLK_PIN + 1
    #define PRA32_U2_I2S_SWAP_BCLK_AND_LRCLK_PINS  (false)
    #define PRA32_U2_I2S_SWAP_LEFT_AND_RIGHT       (false)
    ```

- The following is setting is for [Waveshare Pico-Audio](https://www.waveshare.com/wiki/Pico-Audio) Initial Version (WAVESHARE-20167)

- The following is setting is for [Waveshare Pico-Audio](https://www.waveshare.com/wiki/Pico-Audio) Rev2.1 Version (WAVESHARE-20167) (CURRENTLY NOT RECOMMENDED)
    - NOTE: No sound unless using Arduino-Pico 4.4.0

```
    //#define PRA32_U2_I2S_DAC_MUTE_OFF_PIN          (0)
    #define PRA32_U2_I2S_DATA_PIN                  (22)
    #define PRA32_U2_I2S_MCLK_PIN                  (26)
    #define PRA32_U2_I2S_MCLK_MULT                 (256)
    #define PRA32_U2_I2S_BCLK_PIN                  (27)  // LRCLK Pin is is PRA32_U2_I2S_BCLK_PIN + 1
    #define PRA32_U2_I2S_SWAP_BCLK_AND_LRCLK_PINS  (true)
    #define PRA32_U2_I2S_SWAP_LEFT_AND_RIGHT       (true)
    ```

#### PWM Audio (Optional) (CURRENTLY NOT RECOMMENDED)

- PWM Audio can also be used instead of I2S (PWM Audio does not require an I2S DAC hardware)
    - NOTE: Probably smaller output volume than I2S DAC boards
    - NOTE: To avoid noise, the parameters will not be written to the flash when using PWM audio
    - We recommend adding RC filter (post LPF) circuits to reduce PWM ripples
        - A 1st-order LPFs with a cutoff frequency 7.2 kHz (R = 220 ohm, C = 100 nF) works well
    - See "PWM audio" in [Hardware design with RP2040](https://datasheets.raspberrypi.com/rp2040/hardware-design-with-rp2040.pdf)
      for details on PWM audio
- NOTE: Select CPU Speed: "150 MHz" in the Arduino IDE "Tools" menu
- Uncomment out `//#define PRA32_U2_USE_PWM_AUDIO_INSTEAD_OF_I2S`
  in "Digital-Synth-PRA32-U2.ino" and modify `PRA32_U2_PWM_AUDIO_L_PIN` and `PRA32_U2_PWM_AUDIO_R_PIN`
- The following is setting is for Pimoroni Pico VGA Demo Base (PIM553)

```
    #define PRA32_U2_PWM_AUDIO_L_PIN               (28)
    #define PRA32_U2_PWM_AUDIO_R_PIN               (27)
    ```

- KNOWN ISSUE: When using PWM Audio, signal discontinuity (missing a sample) occurs about every 80 ms in each L and R channel
    - Click noise is particularly noticeable in the high frequency band and sine waves

## Files

- "Digital-Synth-PRA32-U2.ino" is a Arduino sketch for Raspberry Pi Pico/RP2040/RP2350 core
    - Modify `PRA32_U2_MIDI_CH` to change the MIDI Channel
- "pra32-u2-make-sample-wav-file.cc" is for debugging on PC
    - GCC (g++) for PC is required
    - "pra32-u2-make-sample-wav-file-cc.bat" makes a sample WAV file (working on Windows)
- "pra32-u2-generate-*.rb" generates source or header files
    - A Ruby execution environment is required

## PRA32-U2 Editor

- "pra32-u2-editor.html": Editor (MIDI Controller) Application for PRA32-U2, HTML App (Web App)
    - Modify `PRA32_U2_MIDI_CH` to change the MIDI Channel
- We recommend using Google Chrome, which implements Web MIDI API
- Select "Digital Synth PRA32-U2" in the list "MIDI Out"
- Functions
    - PRA32-U2 Editor converts Program Changes (#0-15 for user presets, #16-31 for factory presets) into Control Changes
    - When Program Change #127 is entered or Control Change #111 is changed from Off (63 or lower) to On (64 or higher), "Random Synth" is processed
    - PRA32-U2 Editor stores the current control values and the user presets (#0-15) in a Web browser (localStorage)
    - Current parameter values and user presets (#0-15) can be imported/exported from/to JSON files
- When not using PRA32-U2 Editor
    - PRA32-U2 can also be controlled by MIDI without using PRA32-U2 Editor
    - Refer to "PRA32-U2-MIDI-Implementation-Chart.txt" for the supported functions
    - The default program is #0
    - Programs #0-31 can be modified by editing "pra32-u2-program-table.h"
    - PRA32-U2 Editor functions related to parameter writing
        - Write: Write the current parameters to PRA32-U2 (Program #0-15 and the flash)
        - Program Change: Send Program Change to PRA32-U2 directry
          (NOTE: The current parameters of PRA32-U2 will not be updated)

## PRA32-U2/M (PRA32-U2 Multi-Timbre Edition) (Optional)

- Features
    - Synths
        - Basic Channel + 0 (Default 1): Main Synth, Poly or Mono; The default program is #0; The FX parameters apply to all channels
        - Basic Channel + 1 (Default 2): Sub Synth 1, Mono; The default program is #1; The FX parameters are disabled
        - Basic Channel + 2 (Default 3): Sub Synth 2, Mono; The default program is #2; The FX parameters are disabled
        - Basic Channel + 3 (Default 4): Sub Synth 3, Mono; The default program is #3; The FX parameters are disabled
        - *Basic Channels + 1 to + 3 (Sub Synths) are processed only if Basic Channel + 0 (Main Synth) is in Mono modes*
    - Layering
        - Basic Channel - 3 (Default 14): Control Basic Channel + 0 and + 1 simultaneously
            - Results of Program Change: Program # + 0 for Basic Channel + 0 and Program # + 1 for Basic Channel + 1
        - Basic Channel - 2 (Default 15): Control Basic Channel + 2 and + 3 simultaneously
            - Results of Program Change: Program # + 0 for Basic Channel + 2 and Program # + 1 for Basic Channel + 3
        - Basic Channel - 1 (Default 16): Control Basic Channel + 0, + 1, + 2, and + 3 simultaneously
            - Results of Program Change: Program # + 0 for Basic Channel + 0, Program # + 1 for Basic Channel + 1, Program # + 2 for Basic Channel + 2, and Program # + 3 for Basic Channel + 3
        - Not to use this feature, comment out `#define PRA32_U2_ENABLE_LAYERING` in "Digital-Synth-PRA32-U2-M.ino"
- How to modify
    - Copy all files in the "Digital-Synth-PRA32-U2" folder, except for "Digital-Synth-PRA32-U2.ino", to the "Digital-Synth-PRA32-U2-M" folder
    - "Digital-Synth-PRA32-U2-M.ino" is a Arduino sketch

## [PRA32-U2/P](./README-PRA32-U2-P.md) (PRA32-U2 with Panel) (Optional)

## Simple Circuit for PWM Audio (Optional) (CURRENTLY NOT RECOMMENDED)

### Circuit Diagram

![Circuit Diagram](https://raw.githubusercontent.com/risgk/digital-synth-pra32-u2/main/pra32-u2-pwm-audio-circuit-diagram.png)

- This image was created with Fritzing.
    - Actually, it is necessary to use Raspberry Pi Pico 2 (instead of Raspberry Pi Pico)
- Adding 10 uF electrolytic capacitors (AC coupling capacitors) will cut the DC components of the audio outputs.
- NOTE: Connect an amplifier or an active speaker to the audio jack.
  Connecting a headphone or a passive speaker may cause a large current to flow and damage the devices.

### Actual Wiring Diagram

![Actual Wiring Diagram](https://raw.githubusercontent.com/risgk/digital-synth-pra32-u2/main/pra32-u2-pwm-audio-bread-board.png)

- This image was created with Fritzing.
    - Actually, it is necessary to use Raspberry Pi Pico 2 (instead of Raspberry Pi Pico)

## Customization Examples

- Files in the "customization-examples" folder
- **Digital Synth PRA32-U2 (Lite)**
    - Osc 1 Shape and Morph are disabled in Saw, Sqr, and WT
    - Runs on a single core
    - "Digital-Synth-PRA32-U2.ino.Lite-Core-0-Only.txt"
    - "Digital-Synth-PRA32-U2.ino.Lite-Core-1-Only.txt"
- **Digital Synth PRA32-U2/M (Lite)**
    - Osc 1 Shape and Morph are disabled in Saw, Sqr, and WT
    - Runs on a single core
    - "Digital-Synth-PRA32-U2-M.ino.Lite-Core-0-Only.txt"
    - "Digital-Synth-PRA32-U2-M.ino.Lite-Core-1-Only.txt"

## License

![CC0](http://i.creativecommons.org/p/zero/1.0/88x31.png)

**Digital Synth PRA32-U2 v2.18.2 by ISGK Instruments (Ryo Ishigaki)**

To the extent possible under law, ISGK Instruments (Ryo Ishigaki)
has waived all copyright and related or neighboring rights
to Digital Synth PRA32-U2 v2.18.2.

You should have received a copy of the CC0 legalcode along with this
work.  If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.

### For Your Information

If PRA32-U2 is to be embedded in instruments or others, it would be nice
(but not required) to display the following:

digital-synth-pra32-u2

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