µKlavier – Digital audio synthesizer made with Micropython

Project Description

This project is an attempt to build a fully-functional hardware music synthesizer (aka "sound module") with MicroPython on embedded hardware. The aim is to build a polyphonic synthesizer that can be played and controlled via MIDI, as is typical for table-top or rack-mount sound modules, but can also be programmed using intuitive hardware controls and a touch screen.

Synthesis is based on a mixture of additive synthesis, subtractive synthesis, frequency modulation and sampling. Many ideas are drawn from classical digital synthesizers like the Yamaha DX7, Synclavier and Fairlight CMI. Still the aim is not to clone any of these, but rather use their unique features as an inspiration for a modern-day synthesis engine running on modern-day embedded hardware.

Implementing a modern-day synthesis engine also means using modern-day tools and programing languages. While it is straight-forward to relay on C/C++ and well-known frameworks like Juice for DSP audio programing, this is also an attempt to push the boundaries and use a higher-level language like Python, instead. At least in theory this should allow for more compact, robust and easier to understand code at the cost of raw performance. Hopefully, using the two native code emitters from MicroPython, it should still be possible to achieve acceptable performance. Another aim is therefor to examine how far Python can be used for complex real-time DSP and which benefits there are to it.

Project Status

Unfinished prototype in very early development. The current goal is to develop a proof-of-concept to evaluate the performance of MicroPython on a typical microcontroller for realtime audio synthesis. Once this is shown to be feasible, the real synthesizer can be implemented.

Build Instructions

The ./make.py script automates everything to fetch external dependencies (like the MicroPython source code) and build everything. For that purpose the source tree is split into several sub-projects defined in make.conf:

• main: The main DSP code implementing the synthesizer itself.

• panel: Additional code running on a separate microcontroller to drive a touchscreen user interface and several hardware encoders.

• research: Several small experiments to try out different things and better understand the usefulness of Python for DSP programming.

• hardware: Not a project to be built with make.pyper se, but hardware schematics and PCB layouts.

Only the main project is needed to build a minimal synthesizer that can be used with an external MIDI controller to access all parameters.

All build actions are always "out of tree", this is, make.py places all its output in a top-level directory called build. ./make.py clean can be used to delete this directory and start over from scratch. Although most build commands delete old build files anyway before doing something. Note, that unlike Make the script neither builds single source files (a build is always a full build) nor does it enforce to run certain build steps before others. Therefor, for an initial full build several commands must be run in the right order:

./make.py micropython fetch
./make.py micropython build
./make.py micropython app build

Some additional notes:

• The script should run on all major operating systems, but currently only Linux is tested.

• ./make.py micropython deploy can be used to flash a freshly built MicroPython firmware image to a device.

• Use ./make.py app build <directory> to build only a single sub-project.

• The script has an extensive help system. Use ./make.py help and ./make.py help <command> to learn more.

Build Dependencies

• A C/C++ toolchain to compile MicroPython (usually GNU GCC), aka build-essentials in Debian or Development Tools in Fedora.

  See "Build Dependencies" in the MicroPython documentation for more details.

• A recent Python installation (CPython) for everything else.

• KiCAD for the hardware schematics and PCB layouts.

Hardware Setup

The ultimate goal is to design and manufacture a custom PCB and housing. Until then a stock PyBoard v1.1 with some external peripherals is used for development and testing:

Notes on the PyBoard prototype:

• In this early stage the circuit still has some rough edges and is mostly untested. It will likely change as more experience is gained.

• UART6 for communication with the front-panel subsystem is currently unused, as this part will be developed later.

• The PyBoard already contains 4k7 Ohm pull-up resistors for its two I²C buses.

• The Audio Codec for testing is a MikroElektronika MIKROE-506 Breakout Board using the Cirrus Logic WM8731.

• It is unclear, whether the Audio Codec can be connected as shown in the drawing. As of 09/2022 the Micropython I²S API only supports half-duplex operation for either sending or receiving digital audio.

• Therefor currently only the DAC (marked as I²S_…_TX on the PyBoard drawing) is being tested, as we first need to work with upstream to check if and how full-duplex operation with a shared clock can be supported.

Possible enhancements for the custom PCB:

• Switch to a faster Microcontroller for better DSP performance, if needed.

• Use the remaining two UARTs for additional MIDI connectors, making it possible to integrate MIDI routing features into the firmware.

• Use the built-in ADCs to hook up expression pedals and/or some analog hardware controls.

• Consider using an ESP32 module to drive the front-panel and touch-screen, making use of its built-in WiFi and Bluetooth connectivity. This could allow for some interesting additional features like:

  • Receiving Bluetooth audio from mobile devices to play-along.
  • Exchange sample data and sound presets with an external computer.
  • ...

  Mass-data could be exchanged with the DSP Microcontroller via a shared SPI-Flash IC.

• Design a separate Audio/MIDI PCB using I²S and MIDI over UART to communicate with the main PCB for more flexible housing.

• Provide symmetrical audio inputs and outputs out-of-the-box.

• Maybe add USB audio/MIDI, if technically feasible.

• Use separate PCB ground and power planes for analog and digital circuits.

Copyright

Copyright © 2022 Dennis Schulmeister-Zimolong dennis@zimolong.eu

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License along with this program. If not, see http://www.gnu.org/licenses/.