Tech Page: K-Bow Bull's-Eye

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Electronic music is primarily a keyboardist's game. Most electronic instruments are keyboard oriented, and even the language of electronic music — MIDI — is based on keyboard parameters such as Key Velocity and Sustain Pedal. As a wind player, I've spent a lot of time configuring synthesizers to respond well to MIDI woodwind controllers, but it isn't always easy.

Bowed-string players have it even harder. Many bow gestures are completely lost in translation, never affecting any part of the electronic sound. However, that could soon change thanks to the K-Bow, which was designed by industry veteran Keith McMillen.

The K-Bow provides an electronic controller for all bowed-string instruments. Available for violin, viola, cello, and bass, the K-Bow is the same size, weight, and balance as a conventional bow, but it's packed with technology that senses and communicates position, Velocity, and other parameters, allowing all of them to control various aspects of a synth's sound.

The bow itself is made of Kevlar and carbon fiber embedded with two full-length loop antennas at right angles to each other. In conjunction with a small RF (radio frequency) emitter that clips under the end of the fingerboard on any traditional instrument, these antennas enable the system to measure the distance between the bow and the end of the fingerboard. The emitter also includes four IR (infrared) LEDs that generate beams detected by a sensor in the bow frog (the piece that secures the bow hair at the end held by the player), which determines the distance between the frog and the fingerboard as well as the angle of the bow with respect to the top of the instrument.

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FIG. 1: In this screen from the K-Bow application, a player can monitor the output of the various sensors and control the DSPs, looping sequencer, vocoder, and volume levels of each module.

In addition to the IR sensor, the frog is crammed with electronics, including a grip-pressure sensor, a bow-hair-tension sensor, and a 3-axis accelerometer that measures the bow movement in three dimensions. Twenty op-amps precondition the sensor signals, which are then sent through a 12-bit A/D converter and processed by an 8-bit RISC CPU. All the information is transmitted wirelessly via Bluetooth from a transceiver controlled by a 32-bit ARM CPU. A 6 gm lithium-polymer battery provides a full day of use, and it can be recharged through a standard USB connector that also facilitates firmware updates.

The K-Bow comes with a software suite for Windows or Mac written in Cycling '74 Max/MSP (see Fig. 1), which lets users modify and extend it as they wish. Its basic capabilities include calibrating the system and adjusting the sensitivity for different playing styles. It also includes a 4-track looping sequencer and 13 DSP functions such as reverb, wah, and so on, which can be applied to any audio signal from a pickup or microphone. Each bow signal can be mapped to control any of several hundred parameters, including a 6-channel phase vocoder applied to existing audio files.

Neural-net technology lets you train the software to recognize various gestures made with the bow. For example, you could “write” the number 2 in the air to select track 2 in the sequencer, or move the bow up, down, right, left, back, and forth at different speeds to trigger different drum sounds. You could also move sounds around with the surround processor, which can handle up to eight audio channels.

The violin K-Bow with fingerboard emitter and software retails for nearly $4,000, which isn't cheap by any means, but it's a real bargain compared with the price of a good conventional bow. Even better, profits from the sale of the K-Bow go to the Beam Foundation, a nonprofit organization McMillen founded to promote new music. Making and supporting new music at the same time — now that's a double bull's-eye!