RP2040 Audio

RP2040_Audio

A Eurorack module for stereo audio effects based on the Raspberry Pi RP2040 MCU

Abstract

I've been doing audio synthesis and effects in the Eurorack modular market for a number of years. I started out in the late 2000's using the Microchip dsPIC and migrated over to ARM Cortex M devices, mostly STM32 in order to get access to higher speeds, more memory and floating point math. This was my main platform for most of the 2010's until the great silicon drought of the 2020's made high-end STM32 devices almost impossible to get. Into the vacuum stepped the good folks at the Raspberry Pi Foundation with the RP2040 MCU which has a fair chunk of on-chip SRAM, dual ARM Cortex M0 cores and a decent complement of peripherals.

This project is a feasibility study of using the RP2040 MCU as an audio effects processor, with particular emphasis on performance with challenging DSP.

Hardware

The complete design consists of two PCBs:

• The system PCB - a single board containg all electrical components. Find it in the Hardware directory.
• The Front Panel - a PCB-based front panel that conforms to the Eurorack physical mounting standard. Found in the Hardware/audio_panel directory.

The system PCB has the following features:

• Standard Eurorack 16-pin shrouded power connector
• Two +/-5V CV inputs and associated analog signal conditioning
• Two +/-7V Audio inputs and associated analog signal conditioning
• Two +/-7V Audio outputs and associated analog drivers
• WM8731 stereo audio codec
• RP2040 and 4MB SPI flash with USB, SWD and serial interfaces for development
• 160 x 80 IPS LCD
• UI button

Firmware

A demonstration firmware is included that exercises all of the capabilities of the system, including the LCD, UI button, CV inputs and stereo audio I/O. It is a basic multi-effects unit that supports a complement of audio DSP algorithms that are easily extended by adding standardized modules to a data structure. As provided here just three algorithms are available:

• Simple pass-thru with no processing
• Simple gain control
• Basic "clean delay" with crossfaded deglitching during delay changes.

Other algorithms have been tested including phasers, flangers, frequency shifters, resampling delays and reverbs, but these are not publicly released at this time.

Findings

Overall the RP2040 is a capable device that can do a reasonable amount of audio DSP. The lack of floating point is not a huge incovenience if one is handy with fixed point arithmetic and the large on-chip SRAM is sufficiently large for reasonable audio data buffers. The PIO is handy for building interfaces and is capable of full-duplex I2S with no difficulty. The on-chip ADC underperforms its 12-bit data width however and requires a fair bit of post-processing to remove noise and jitter in the conversion results. The dual ARM cores are useful in dividing up user interface and DSP tasks to keep a snappy response, but even with two cores the system struggles to compete with higher-spec ARM systems.

TL;DR - you can use it for DSP, but I'd prefer a Cortex M7.