The Raspberry Pi Pico has emerged as an incredibly capable platform for DIY MIDI synthesizers, offering dual-core ARM Cortex-M0+ processing power, generous GPIO, and affordable pricing around $4. Its combination of processing capability, built-in PWM and I2S audio output options, and native USB support makes it ideal for music electronics projects ranging from simple monotimbral synthesizers to complex polyphonic instruments.

Why Pico for MIDI Synthesis

The RP2040 microcontroller at the heart of the Pico provides several advantages for audio synthesis. Its dual-core architecture allows one core to handle MIDI input and user interface while the other dedicates itself to audio generation, ensuring glitch-free performance. The Programmable I/O (PIO) state machines can handle timing-critical tasks like generating PWM audio or managing protocols without burdening the CPU cores.

Clock speed of 133MHz provides sufficient headroom for real-time synthesis algorithms including additive synthesis, frequency modulation, wavetable playback, and even basic sample playback. With 264KB of RAM and support for external flash, the Pico can store wavetables, samples, and preset data. The built-in USB controller simplifies MIDI connectivity, allowing the Pico to appear as a USB MIDI device without additional hardware.

Several notable projects demonstrate the Pico’s synthesizer capabilities. A Cornell University student project showcased a polyphonic MIDI synthesizer using PWM output filtered to produce surprisingly clean audio. The project implemented multiple waveforms, envelope generation, and MIDI note handling, all documented with code examples that serve as excellent learning resources.

The PicoADK (Audio Development Kit) provides a complete hardware platform specifically designed for audio synthesis. Version 2 upgraded to the RP2350 microcontroller with expanded memory and processing power. The board includes dedicated audio codec chips, MIDI I/O ports, and a layout optimized for synthesizer development, making it an excellent starting point for serious projects.

Community members have ported popular synthesizer engines to the Pico. Synth_Dexed, an implementation of the classic Yamaha DX7 FM synthesis engine, runs impressively on the Pico. This brings 6-operator FM synthesis with authentic DX7 sound to a microcontroller platform, opening possibilities for standalone FM synthesizer instruments.

Implementation Approaches

Audio output from the Pico typically follows one of three paths. The simplest approach uses PWM (Pulse Width Modulation) with an RC low-pass filter to convert digital audio into analog signals. While limited in fidelity, this method requires only resistors and capacitors, making it perfect for learning and prototyping.

For better audio quality, I2S DAC chips like the PCM5102 connect easily to the Pico and deliver 16-bit or 24-bit audio at professional sample rates. These inexpensive DAC modules cost just a few dollars and dramatically improve sound quality compared to PWM output. The Pico’s I2S support makes integration straightforward with existing libraries handling the protocol details.

Advanced projects might use the Pico’s DMA (Direct Memory Access) controller to generate audio, offloading memory-to-peripheral transfers from the CPU. This technique achieves the lowest possible latency and highest sample rates, though it requires more sophisticated programming.

MIDI Implementation

MIDI input can come from several sources. USB MIDI is often the simplest approach since the Pico supports USB natively through TinyUSB library. Your Pico synthesizer appears as a standard USB MIDI device when connected to a computer or USB MIDI controller.

Traditional 5-pin DIN MIDI uses a simple optocoupler circuit for electrical isolation and voltage conversion. MIDI runs at 31.25 kbaud, an unusual rate that requires careful timing, but the PIO state machines on the RP2040 handle this elegantly. Circuit diagrams and code examples for DIN MIDI on Pico are readily available in the community.

MIDI over serial (TRS MIDI) has gained popularity with the Type A and Type B standards used by various manufacturers. A simple voltage divider or level shifter adapts 3.3V UART signals from the Pico to MIDI voltage levels. Some Eurorack manufacturers have adopted this approach, making Pico-based synthesizers compatible with modular systems.

Sound Design Capabilities

The Pico handles various synthesis techniques effectively. Additive synthesis combines multiple sine waves at different frequencies and amplitudes, creating complex timbres from simple building blocks. The processing power supports a reasonable number of partials while maintaining polyphony.

Subtractive synthesis using virtual analog oscillators and filters works well on the Pico. Basic filter designs like low-pass, high-pass, and band-pass filters can be implemented efficiently. State variable filters provide smooth frequency sweeps ideal for classic synthesizer sounds.

Wavetable synthesis stores single-cycle waveforms in memory and plays them back at different rates for different pitches. This approach sounds excellent and runs efficiently on the Pico. Wavetable scanning adds timbral motion by interpolating between different stored waveforms.

Conclusion

Raspberry Pi Pico synthesizer projects offer an accessible entry point into digital audio synthesis with professional results achievable even on a tight budget. The combination of adequate processing power, flexible I/O, and extensive community support makes the Pico an excellent choice for both learning synthesizer design and building practical musical instruments. Whether you’re implementing classic synthesis techniques or experimenting with novel algorithms, the Pico provides a capable platform that punches well above its price point.

Explore projects on GitHub, Instructables, and Hackaday to see what others have built and find code examples to jump-start your own Pico synthesizer journey.