Raspberry Pico 1 and 2 Simple FM synth Arduino Library

This is a simple DX-9 style 6-channel FM synth library for the Raspberry Pico 1 and 2.

Features

• DX-9 style 6-channel FM synth.
• Play files that are inside the flash using LittleFS!

Requirements

• Arduino IDE 2.3.2 and above
• Raspberry Pi RP2040/RP2350 board library (4.1.1)
• I2S DAC (PCM5102, or MAX98357)
• Modified MajicDesigns' MD_MIDIFile library to play MIDI files from LittleFS (Flash size: 128KB)
• arduino-littlefs-upload extension
• USB connection for UART, FM synth module debug outputs at Serial1

This branch uses the RP2040's and RP2350's interpolator module to generate the sine wave using DDS. The interpolator for now is using only one lane. The interpolator module has to be shared by 4 other operators, and it is multiplexed by saving and restoring the accumulators on each sample.

Only 4 operators are used, and each of the operator has an envelope for each FM channel: Dexed Reference.

Using MajicDesigns' MIDI parser with LittleFS support instead of MidiTones. Some MIDI files might not play properly in the system and it is currently being investigated too.

Build a circuit example (Default: BCLK=31, WS=32, DATA=34)

Installation and usage instructions

• Unzip the MIDI parser into the Arduino's Documents\Arduino\Libraries folder.
• Install the arduino-littlefs-upload extension into the C:\Users\<username>\.arduinoIDE\plugins\.
• At the Tools->Flash Size select Sketch:3968KB,FS:128KB before compiling.
• A sample midi file is there inside for testing.

How to init library

• At the beginning of the sketch, add this:

#include <LittleFS.h>
#include <MD_MIDIFile.h>
#include <fmSynth_picoI2sAudioDriver.h>

• For default pins (BCLK=31, WS=32, DATA=34) just init the library as follows with the instrument of your choice (see Supported Instrument List):

fmSynthPicoI2s("GUITAR");

• If you need to define your own pins, here is an example:

fmSynthPicoI2s(29, 31, "GUITAR");

How to copy the files into the flash

• Put the midi file into the sketch folder's Data folder. Make sure you have the same midi file name in that .ino too (SMF.load)!
• Get the Raspberry Pico to be in Boot mode.
• Ctrl-Shift-P in the Arduino IDE -> Upload LittleFS to.... Your midi files are now saved into the flash! :D
• Reset the Raspberry Pico and let the music play.

Supported Instrument List

• BRASS1
• TBULARBELS
• MARIMBA
• E.PIANO
• BELLS
• TOYKBD
• GUITAR
• FLUTE
• PIPEORGAN2
• HARPSICH1
• JAZZGUIT2
• LUTE

Limitations

• RP2040: since it does not have a floating point unit, gbmhunter's Fixed Point library is used there. The sample rate is limited to 22050Hz because resolution of the fixed point is only at 15:16.

• RP2350 (ARM Cortex-M33): floating point is used and 44100Hz sample rate is used.

• RP2350's RISC-V cores are not supported for now. Porting and testing still in progress.

The sounds and patches being output are not exactly DX-9 - it's only a rough approximation of that instrument. The envelope generator is a rudimentary ADSR state machine and it is not based on the more complicated designs of those of the other DX series. With that limited sampling rate and resolution, some of the patches may sound off if compared to listening to the actual DX-7 or DX-9 ones.

Volume Control

Note: This is still under testing, and sample values may wrap around and cause bad distortions if the volume multiplier is too high. I'm only setting VOLUME_DEFAULT = 1, VOLUME_LOUD = 2, VOLUME_LOUDER = 4 and VOLUME_LOUDEST = MAX_FM_CHANNELS as a start.

Some instruments can also make distorted noise when the volume is too loud. It is recommended to scale down if that happens! :D

Setting volume example:

tPlayer->setVolume(VOLUME_LOUDER);

MAX98357's Shutdown Mode (/SD_MODE)

If this project involves battery and using MAX98357 module, remember to also set the /SD_MODE pin to 0 to shut it down. Here is an example if you need to include this in the project:

#define MAX98357_SD 16

void setup()
{
    // Code for your player's startup etc...
    pinMode(MAX98357_SD, INPUT);
    // More code for your player's startup...
}

void loop()
{
    // When player is done playing, set the pin to 0:
    pinMode(MAX98357_SD, OUTPUT);
    digitalWrite(MAX98357_SD, LOW);
}

Main Updates

Update 01-Sep-2025 - Changed sine test to be more easier to access, and added the previous Patch Maker app from the older branches. (Note: This Patch Maker feature is extra janky! Use with caution!)

Update 31-Aug-2025 - Added primitive volume control.

Update 16-Feb-2025 - Arduino Library support.

Update 13-Feb-2025 - Added support for playing MIDI files from Flash using LittleFS and the modified MajicDesigns' MIDI parser.

Update 08-Feb-2025 - This is being ported to RP2350 and for Arduino platform. Sine test module added for convenience.

Update 04-June-2023 - A very large part of the code has been restructured - modules are now more isolated and clearly defined. The interpolator module is still coupled to the Oscillator - more plans to separate this too in the future. However, it is decided that the part where you can create and modify patches has been removed and planned to be relocated to another separate app. If you need to still create patches, you can check the following instructions. :D

Sine test

Due to the difficulty of porting this to another architecture and/or platform, a short sine test is inserted inside. There should be only a 440Hz sine wave being output when you add SINE_440HZ_TEST during initializing the object (use default pins only!):

tPlayer = new fmSynthPicoI2s(SINE_440HZ_TEST);

Patch Maker

If you need to create and tune your own patches, Patch Maker can assist you. This Patch Maker is derived from the earlier branch and it is now easily accessible by initializing the object with PATCH_MAKER:

tPlayer = new fmSynthPicoI2s(PATCH_MAKER);

The Patch Maker is still in works and the documentation is available at the PATCH_MAKER_README.md.

Benchmarks

Pico	Sample Rate (Hz)	Average time for one FM channel (uS)
RP2040 @ 125MHz, no interpolator, sine generation using Bhaskara I method	22050	14~16
RP2040 @ 250MHz, no interpolator, sine generation using Bhaskara I method	22050	7~8
RP2040 @ 125MHz, interpolator	22050	4~5
RP2350 (ARM Cortex-M33) @ 150MHz, interpolator	44100	1~3

The FM channel processing time can be sampled by getting the time before fmc[chnNum].generateSample() and after.