DIY Network-Ready Polyphonic Music Controller

Hans Peter Portner’s Chimaera project is a touch-less, expressive, network-ready, polyphonic music controller released as open source hardware. It is a mixed analog/digital offspring of the Theremin. An array of analog, linear Hall effect sensors make up a continuous 2-D interaction space. The sensors are excited with Neodymium magnets worn on fingers.

Portner’s Chimaera project

The device continuously tracks and interpolates position and vicinity of multiple present magnets along the sensor array to produce corresponding low-latency event signals. Those are encoded as Open Sound Control bundles and transmitted via UDP/TCP to a software synthesizer. The DSP unit is a mixed-signal board and handles sensor read out, event detection and host communication. It is based on an ARM Cortex M4 microcontroller in combination with WIZnet W5500 chip, which takes care of all low-level networking protocols via UDP/TCP.

The poly-magneto-phonic Theremin

In his project write-up, Portner explains:

With its touch-less control (no friction), high update rates (2-4 kHz), its quasi-continuous spatial resolution and its low-latency (<1 ms), the Chimaera can react to most subtle motions instantaneously and allows for a highly dynamic and expressive play. Its open source design additionally gives the user all possibilities to further tune hardware and firmware to his or her needs. The Chimaera is network-oriented and configured with and communicated by Open Sound Control, which makes it straight-forward to integrate into any setup.

The hardware of the Chimaera consists of two types of printed circuit boards and an enclosure. Multiple sensor units are daisy-chained to form the sensor array and connected to a single digital signal processing (DSP) unit.

Sensor unit

A single sensor unit consists of 16 linear hall-effect sensors spaced 5mm apart and routed to a single output through a 16:1 multiplexer which is switched by the DSP unit. Downstream the multiplexer, the analog signal runs through an amplification circuitry.

A modular hardware design consisting of identical sensor units and a single DSP unit embedded in a wooden case allows building devices with array sizes of 16-160 sensors.

The DSP unit is a mixed-signal board and handles sensor read out, event detection and host communication. It is based on an STM32F303Cx ARM Cortex M4 microcontroller in combination with WIZnet W5500, a hardwired 100Mbit IPv4/PHY chip taking care of all low-level networking protocols via UDP/TCP. The board’s analog part features 10 analog inputs providing connection points for the sensor units, leading to a maximally possible array of 160 sensors. Those analog inputs connect directly to three in parallel running 12bit analog-to-digital converters.

Schematic of the DSP unit (STM32F303Cx part)

Networking technology in a zero configuration setup has advantages in respect to long-distance transmission, operating system independence and inherent ability for network performances. We thus use the Open Sound Control (OSC) specification via UDP/TCP as low-level communication layer.

Schematic of the DSP unit (WIZnet W5500 part)