Source-linked AI summary
PolyMap: A 64-Channel Polyphonic Guitar Pickup System
David Wieland, Jonas Roth, Christoph Studer
TL;DR
Conventional guitar pickups provide limited, shared sensing at predetermined positions and produce a mono analog signal. PolyMap samples each string at eight locations using 64 pickups, digitizes the signals inside an eight-string guitar, and transmits them via MADI for software processing. The system enables pickup-position editing and extensive per-string sound shaping, with 5.4 ms measured end-to-end latency.
Problem
Conventional guitar pickups generally sense all strings together at a few fixed positions and output a mono analog signal, limiting pickup-position choices during performance.
Method
PolyMap uses 64 pickups arranged across eight strings and eight locations, digitizes their signals inside the guitar, and transmits them via MADI to computer software.
Results
5.4 ms total end-to-end latency was measured with a 32-sample buffer at 48 kHz, while the hardware itself contributed around five samples.
Takeaways & Limitations
PolyMap enables pickup-position editing and per-string processing, including stereo spreading, separate effects or amplifiers, delay-based timbral effects, and phase-based harmonic shaping.
Abstract
from arXiv · showhide
In electric guitars, the vibrations of the strings are typically sensed by coils of wire combined with a magnet, called pickups. The pickups and their position along the strings contribute strongly to the instrument's sound. Most guitars feature one to three pickups, each spanning across all strings with fixed positions and generating a single mono output. The work of this Master's Thesis at ETH Zürich introduces a new pickup system called PolyMap, which senses each string individually and at multiple locations. The system is demonstrated with a custom-made eight-string guitar that contains eight pickups per string for a total of 64 pickups. The signals from these 64 pickups are individually digitized inside the guitar and transmitted over a multichannel audio digital interface (MADI), a low-latency digital audio interface, to a computer for further processing. PolyMap enables high-resolution sensing of an electric guitar's strings and enables extensive post-processing capabilities for musicians, audio engineers, and researchers. To the best of our knowledge, this is the first polyphonic guitar pickup system with such a complete feature set.
1. INTRODUCTION
PolyMap extends guitar pickup sensing from shared, fixed-position mono capture to individually sampled strings at eight locations, with all signals digitized and controllable in software. This enables pickup-position editing and extensive per-string sound shaping.
- PolyMap senses every string individually at eight different locations, expanding beyond conventional pickups that sense all strings together.
- The 64 pickup signals are digitized inside the guitar and sent over a single digital interface for simultaneous monitoring and recording.
- PolyMap moves pickup-position decisions from performance to editing, allowing musicians to adjust active pickups and their blends afterward.
- Individual string sensing supports stereo spreading and routing different strings or combinations to separate effects or amplifiers.
2. POLYMAP PICKUP SYSTEM
PolyMap combines 64 individually digitized pickups with FPGA-based MADI transmission and custom software for controlling and processing the signals. Its hardware and software expose per-string, multi-position pickup selection within a computer-based workflow.
- Hardware: 64 pickups are arranged as an 8×8 grid across eight strings and digitized by eight simultaneous 8-channel ADCs at 48 kHz and 24-bit resolution.The analog outputs are buffered and bandwidth-limited before digitization.
- Digital transmission: The FPGA converts ADC data into a MADI stream transmitted over coaxial cable to an external USB MADI interface.The encoding chain includes buffering, MADI status-bit insertion, 4B5B encoding, clock-domain crossing, synchronization symbols, serialization, and NRZI encoding.
- Pickup array: The system uses small directional Nu Capsules, whose wider bandwidth records more information than most traditional guitar pickups.Each pickup must fit beside neighboring pickups while primarily sensing the string above it.
- Control software: MADI exposes all 64 audio channels to the computer, while the custom plug-in selects active pickups and applies processing across them.The current plug-in implementation has been confirmed to work with Reaper because most DAWs do not support 64-channel tracks.
- Control software: PolyMap Studio offers manual per-pickup adjustment and virtual per-string pickup positioning with proportional blending between neighboring physical pickups.Manual controls include enabling, level adjustment, inversion, panning, and optional delay; virtual positions can be linked globally or controlled independently.
3. RESULTS
PolyMap achieves low end-to-end latency while revealing position-dependent spectral and amplitude changes, with measured noise dominated by the pickups.
- 3.1. Latency: 5.4 ms end-to-end latency was measured from pickup to computer headphone output, compared with 6.1 ms for a Presonus Audiobox USB 96.The hardware contributes around five samples and MADI adds 27 samples; DAW and operating-system buffering dominate.
- 3.2. Noise: −73 dBFS to −67 dBFS RMS noise was measured across channels.The measurement includes the pickups, buffering circuit, and analog-to-digital conversion.
- 3.3. Analysis of Different Pickup Positions: Figure 8 compares spectra from eight pickup positions along the open A string, from bridge-nearest Pickup A to neck-nearest Pickup H.The figure demonstrates how pickup position changes the observed harmonic spectrum.
- 3.2. Noise: −90 dBFS noise floor was measured when the pickups were unpowered, indicating that a significant noise source is power-supply noise coupling into the pickups.The unpowered measurement isolates the noise floor associated with the rest of the signal chain from noise entering directly through the pickups.
- 3.3. Analysis of Different Pickup Positions: Pickups farther from the bridge produce higher uncompensated output amplitude, while the strongest fundamental and first harmonic occur at the neck pickup.Higher harmonics peak progressively closer to the bridge, and amplitude dips correspond to likely standing-wave nodes modeled by a comb filter.
4. APPLICATIONS
PolyMap’s per-string, multi-position sensing supports spatialization, delay-like timbral effects, split amplification, and fine tonal control through fractional blending.
- 4.1. Spatial Audio: Panning each string differently across the stereo field creates an immersive spatial effect, especially for musical motion such as arpeggios.This effect is described as unavailable with a traditional guitar.
- 4.2. Delay Effects: Selective delay on multiple pickups of one string produces reverb- or fast-delay-like effects while also changing the timbre of delayed parts.The effect uses pickup signals rather than requiring an additional effects unit.
- 4.3. Split Processing: Different strings can be routed to separate amplifiers, such as lower strings to a bass amplifier and upper strings to a guitar amplifier.String-selective delay, reverb, clean, or distorted processing is also possible.
- 4.4. Fractional Blending: Reduced-volume blending of a second pickup provides finer control over the resulting sound than enabling both pickups at full volume.Inverting one pickup by 180 degrees can cancel much of the fundamental and emphasize higher harmonics and attack.
5. DEMO EXPERIENCE
The demonstration lets visitors experience PolyMap’s complete guitar-to-DAW signal chain, test plug-in configurations, and explore applications using recorded audio.
- 5. DEMO EXPERIENCE: Visitors can play the guitar while hearing how different PolyMap Studio configurations affect its sound.The demonstration covers the signal chain from guitar input through post-processing in the DAW.
- 5. DEMO EXPERIENCE: Visitors can also use the plug-in with pre-recorded audio in the DAW to explore PolyMap’s musical applications.The plug-in and a sample audio file are publicly available for download.