Guitar Pedal DSP Processor
Project Overview
Being a musician and mechatronics engineer, I decided to start a project that combined both, my first attempt at digital signal processing (DSP). Some initial research turned up a microcontroller built specifically for this kind of application: the Daisy Seed, a development board with a similar form factor to the Teensy boards. It boasts a powerful 480MHz ARM Cortex-M7 alongside a 192kHz/32-bit Texas Instruments TAC5242 audio codec. My first challenge for this project was developing a PCB to house the Seed, while also containing any other I/O or peripherals I needed or wanted. My criteria for this PCB were:
- 2 input jacks and 2 output jacks (flexibility to run in stereo or mono, with an effects loop)
- All inputs and outputs buffered (avoids impedance mismatching for instrument-level signals)
- MIDI output (want to later experiment with FFT and controlling synths from a guitar signal)
- Small enough to fit in a mid-sized effects pedal enclosure
- Headers for a TFT display
- Header to interface with an encoder daughter board
From these requirements I set out to develop a PCB. I also decided to use 4 rotary encoders with built-in pushbuttons as the user control for the pedal. This was for two reasons: fewer components to deal with, and less wiring. To minimise wiring further, the daughter board is interfaced using an MCP23017 I²C IO expander, which brings the total cables from 16 (4 per encoder) down to 6 (MISO, MOSI, Interrupt, GND x2, VCC).
This project is still a work in progress. As more is completed, more information will be added here. My upcoming milestones are:
- Assemble PCBs
- Write a test program and verify the Daisy toolchain is working
- Design and 3D print a temporary enclosure and mounting system
- Write an audio passthrough program
- Write my first DSP effect program
Skills Used
- Embedded Systems
- C/C++
- Daisy Seed3
- DaisySP/libDaisy
- KiCad
- Surface mount technologies
Challenges & Learnings
This is the first time I have developed my own PCB and chosen my own components for a project. A large part of what I have learnt is the ability to define the specifications I need, and then find components that meet them. This also meant sourcing footprints and models for these components to add to my KiCad library. A few of them weren't available premade, so I had to build them myself.