A research and hardware project building a polyphonic guitar synth pedal using a harmonic fingerprint lookup table (LUT) for note and chord detection.
Instead of traditional waveform resynthesis or naive pitch tracking, this system performs:
- Monophonic note detection via harmonic template matching
- Polyphonic chord detection using the template matching to help optimize NNLS or FFT
- Strike detection to separate attacks from sustain for better tracking
The long-term goal is a standalone hardware pedal combining:
- Real-time digital note/chord detection
- Synth/sample playback engine
- Analog preamp + overdrive section
- Flexible routing between clean, driven, and synthesized signals
- Python LUT prototype active - note + chord detection working offline
- Strike detection system in development
- Daisy Seed hardware prototype in progress
- JUCE project scaffolded for desktop/VST testing
- Analog front-end planned and partially prototyped
.png)
Single-note recordings are analyzed and stored as harmonic “fingerprints.”
Each note in the LUT contains multiple takes for robustness.
During detection, the system:
- Assumes each possible note
- Extracts harmonic features under that assumption
- Scores against stored templates
- Selects the best match
This avoids fragile single-f0 estimation and improves stability across strings and playing styles.
Chord detection does not use chord templates.
Instead:
- Single-note templates are converted into spectral templates
- The live spectrum is modeled as a nonnegative mixture
- NNLS determines which notes are present
- Results are pruned and thresholded
This allows detection of multiple simultaneous strings without pre-defining chord shapes.
Accurate detection requires separating:
- Attack transients (strike)
- Sustained harmonic content
Strike detection helps:
- Trigger new synth events cleanly
- Avoid re-triggering during sustain
- Improve chord onset recognition
This subsystem is currently under active development for real-time hardware use.
The final pedal will include a dedicated analog input and overdrive section.
The microcontroller expects a stable line-level signal.
A clean preamp stage is required to:
- Properly buffer guitar pickups
- Provide adjustable gain
- Condition signal before ADC
A functional prototype based on a CMOY-style op-amp design already exists and will be adapted for guitar-level input:
https://tangentsoft.com/audio/cmoy/
This will likely be redesigned onto a custom PCB.
Two overdrive-style circuits are being explored:
-
A classic op-amp soft-clipping design (Wampler reference topology)
https://www.wamplerpedals.com/blog/latest-news/2020/05/how-to-design-a-basic-overdrive-pedal-circuit/ -
A preamp-style overdrive inspired by the Tascam 424
https://aionfx.com/app/files/docs/424_preamp_documentation.pdf
The final hardware pedal will include an analog drive stage for:
- Traditional guitar overdrive tones
- Synth-through-drive textures
- Flexible signal routing options
There is ongoing investigation into whether analog preprocessing can improve polyphonic detection accuracy. Possible areas:
- Adjustable input gain
- Mild high-pass filtering
- Dynamic range conditioning
- Controlled saturation before ADC
A clean, well-scaled input signal is critical for stable harmonic fingerprint matching.
-
LUT - In Development/
Python prototype for LUT building and note/chord detection -
JUCE - In Development/
Desktop/VST prototyping environment (skeleton) -
DAISY - In Development/
Hardware prototype (Daisy Seed + Audio Shield) -
Documentation/
Diagrams, status reports, and supporting materials
A standalone polyphonic guitar synth pedal combining:
- Harmonic fingerprint note/chord detection
- Real-time synthesis
- Analog drive and tone shaping
- Hardware-friendly DSP
- Expandable routing architecture