The Brain Opera Technical Systems

by Joseph A. Paradiso

[Highly-Trained Musicians]

Previous hyperinstrument collaborations with highly-trained musicians

There is such an enormous amount of innovative technology that has been developed for the Brain Opera that it is impossible to mention it all here. Brain Opera technology is a natural extension of the Hyperinstruments project, started at the MIT Media Lab in 1986 by Tod Machover and Joe Chung, and joined by Neil Gershenfeld in 1991 and myself in 1993. At first designed to enhance the virtuosity of some of the world's greatest performers, from Yo-Yo Ma to Prince, hyperinstruments started evolving in 1991 towards the development of expressive tools for non-professional musicians. The Brain Opera is the culmination to date of this work, and points the way to the further development of expressive objects (furniture, remote controls, clothing, etc.) and responsive environments (including living rooms, concert halls, and department stores).

Among the more significant new technical developments for the Brain Opera are the Harmonic Driving system, the Melody Easel, the Rhythm Tree, the Gesture Wall, the Digital Baton, the Singing and Speaking Trees, and the Sensor Carpet. Among the project's numerous software innovations are the Singing Trees (analysis of every nuance and "feeling" of vocal quality); Harmonic Driving (parametric algorithms that allow a piece of music to be shaped and "personalized" while it is playing); the Rhythm Tree (which analyzes multiple-person behavior to create a complex systemic reaction); the Performance Hyperinstruments (which forge an array of continuous gesture and discrete positional information into intuitive, natural controls); and the entire Brain Opera system, which is itself a complex networked environment capable of integrating new elements into an existing structure automatically or in human-assisted fashion.

Below is a graphical and technical discussion of the technological developments of each of the individual Brain Opera experiences:

1) Forest Stations

A floormat switch detects the user's presence, and starts the experience. The user then navigates through the interactive database using a hand-held piezoresistive mouse that detects the center of pressure of the thumb, and moves the pointer accordingly on an embedded color LCD VGA display.

[Forest Stations]

2) Harmonic Driving

The presence of a seated participant is detected when a light beam pointed across the chair is interrupted. The user controls the experience with a novel joystick made from a large, bendable spring. Two-axis bending angles are measured using capacitive sensing to detect the relative displacement between the spring's coils at its midpoint. Twist is also measured with a potentiometer that rotates through the relative angle between the top and bottom of the spring.

[Harmonic Driving]

3) Melody Easel

We use a pressure-sensitive IntelliTouch Screen from ELO, based on ultrasound propagation through the screen surface. This device produces 8 bits of pressure information, along with precise x and y screen coordinates. The figure shows an earlier version of the Melody Easel, where we postulated the addition of a shunt-mode Fish sensor array placed around the monitor to sense noncontact hand gesture above the screen. The Fish was dropped from the final implementation, since the touch screen interface itself produced ample parameters for the music generation software and made a sufficiently satisfying interface.

[Melody Easel]

4) Rhythm Tree

A simple microprocessor on each pad analyzes the signal coming from a piezoelectric (PVDF) strip, which picks up the strike. A set of simple parameters are extracted from a 5 ms sample of the PVDF signal after a significant peak has been detected, indicating a valid hit. These parameters include the polarity of the initial PVDF signal peak (indicating a top vs. side impact), the number of zero crossings detected (indicating a sharp elastic hit vs. a dull slap with the hand remaining on the pad), and the net integrated signal amplitude (producing 14 bits of velocity information). After a fast, bit-slice poll from the bus host, the struck pads send their data across a shared RS-485 serial network to a host processor, which formats the data into MIDI and passes it to the main computer running ROGUS music generation software. In order to simplify cabling, up to 32 pads can be daisy-chained (like a string of Christmas lights) onto a single host and bus line. We will have 10 such strings running in the Brain Opera Lobby. Each pad also houses a bright LED, which can be illuminated with a dynamically variable intensity. The pads are completely programmable via MIDI system-exclusive commands. We have written a Visual Basic application to adjust the parameters (i.e. trigger sensitivity, light flash options, trigger rejection flags, integration time,...) for individual pads or groups of pads, in order to rapidly configure the rhythm tree into a working configuration. In addition to these capabilities, the rhythm pad string used with the performance instruments is augmented with Gesture Wall sensors, enabling the hand motion above the pads to be tracked before the pads are struck.

[Rhythm Tree]

5) Gesture Wall

A performer steps onto a plate that has a harmless low-frequency (50 Khz), low-voltage (10 Volts) RF signal applied to it. This signal is then couples through the performer's shoes and is broadcast through their body to a set of four pickup antennas located on goosenecks around the perimeter of the screen. These signals change with the distance of the performer from the respective sensors (an LED mounted in each sensor glows with increasing intensity as the performer's body approaches). The sensor data is transferred to a PC running ROGUS, where it is analyzed for gestural characteristics. Before starting the experience, the user must calibrate out the coupling strength of their shoes and body mass, which vary considerably from person to person. This is accomplished by touching a reference pickup electrode, which adjusts the transmitted signal such that every participant radiates equally. A Fish sensor with linearizing log amplifiers is used for the sensing hardware, just as with the Sensor Chair. Hand-on-calibrator state is detected by breaking an IR beam directed across the calibrator surface.

[Gesture Wall]

6) Digital Baton

A small microprocessor in the baton samples signals from 5 pressure-sensitive resistors potted into the baton skin (to measure finger and hand pressure) and 3 orthogonal accelerometers in the baton base (to measure sweeping gestures and beats). These signals are sent through a wire to the host computer running ROGUS. A camera housing a position-sensitive photodiode looks at an infrared LED mounted at the baton tip. This camera is only sensitive to the 20 kHz signal emitted from the LED; all other light sources are ignored. The photodiode in the camera directly produces a signal that determines the horizontal and vertical coordinates of the baton tip; no video processing is required. See below for an older conceptual implementation of the Digital Baton.

[Digital Baton]

[Sensor Floor Pad]

7) Audience Sensing in the Performance Space

A Sensor Floor, composed of a 6 x 10 foot mat surface atop a matrix of 64 pressure-sensitive piezoelectric (PVDF) wires, measures the position and intensity of footsteps, turning them into MIDI note events. The upper body motion is sensed above this region with a pair of quadrature-demodulated 2.6 GHz Doppler radars with beams formed by flat, 4-element micropatch arrays. These devices produce MIDI controller values corresponding to amount of motion, velocity, and direction of motion, projected normal to the radiators. In addition, an 8-channel MIDI-controlled ranging sonar system (using simple pulse-echo detection with 40 kHz piezoceramic heads and a TVG) has been developed to monitor remote distance to people and objects from 1 to 25 feet away.

[Sensor Chair]

8) The Sensor Chair

This is a quick description written in November, 1994, outlining the Chair system, as first developed for a performance with Penn and Teller

As labeled on the chair layout diagram, above, the copper plate (A) affixed to the top of the

chair cushion is a transmitting antenna being driven at roughly 70 kHz. When a person is

seated in the chair, they effectively become an extension of this antenna; their body acts as a

conductor which is capacitively coupled into the transmitter plate.

Four receiving antennas (B) are mounted at the verticies of a square, on poles placed in front of the chair. These pickups receive the transmitted signal with a strength that is determined by the capacitance between the performer's body and the sensor antenna. As the seated performer moves his hand forward, the intensities of these signals are thus a function of the distances between the hand and corresponding pickups. The pickup signal strengths are digitized and sent to a Macintosh computer, which estimates the hand position. A pair of pickup antennas are also mounted on the floor of the chair platform, and are used to similarly measure the proximity of left and right feet, providing a set of pedal controllers. In order for a performer to use these sensors, he must be seated in the chair, and thus coupled to the transmitting antenna. Other performers may also inject signal into the pickup antennas if they are touching the skin of the seated individual, thus becoming part of the extended antenna system. The sensor antennas are synchronously demodulated by the transmitted signal; this produces a receiver tuned precisely to the waveform broadcast through the performer's body and rejects background from other sources.

A pair of footswitches (D) are incorporated in this system to provide sensor-independent triggers. These are used for changing parameters when the foot pedals are dedicated to generating musical sounds, or for instigating t riggers when the performer is not seated, hence is unable to use the sensors.

The hand sensor antennas (B) are composed of a copper mesh encased inside a translucent plastic bottle. A halogen bulb is mounted inside this mesh which is illuminated with a voltage proportional to the detected sensor sign al (thus is a function of the proximity of the performer's hand to the sensor), or driven directly by the Macintosh computer as a MIDI light-instrument. Four lights are mounted below the platform (F); these are correspondingly driven by the foot-sensor signals or directly through MIDI. A digital display (E) is also mounted on one of the sensor posts; this is similarly defined as a MIDI device, and is driven by the Macintosh to provide performance cues (i.e. amount of time or triggers remaining in a particular musical mode, etc.). The sensors are used to trigger and shape sonic events in several different ways, depending on the portion of the composition that is being performed. The simplest modes use the proximity of the performer's hand (or head in the case of Teller's closing bit) to the plane of the hand sensors (z) to trigger a sound and adjust its volume, while using the position of the hand in the sensor plane (x,y) to change the timbral characteristics. Other modes divide the x,y plane into many zones, which contain sounds triggered when the hand moves into their boundary (i.e. the percussion mode). Several modes produce audio events that are also sensitive to the velocity of the hands and feet.

[Sensors Response to Hand Gestures]

Data showing response of sensors to hand gesture

[Block Diagram of Chair Electronics]

Block Diagram of Chair Performance Electronics

[Penn and Teller]

Penn and Teller performing at the sensor chair debut, MIT Kresge Auditorium, Oct. 1994.

9) Summary of Embedded Electronics Cards Designed for this Project

Fishbrain Board: HC11 with MIDI, RS-232, bootload, 4-chnls of analog conditioning, proto area, user port, etc. Essentially a functional block for embedded MIDI or serial controllers.

Updated Fish: Fish, with bootload and a few minor tweaks added.

Gesture Wall Utility: Card with 8-channel light driver, Fish autocalibrator, MIDI input interface, and connector changeouts. Plugs into Fish.

Calibrator Corrector: Adjustable nonlinear correction for Gesture Wall autocalibrator.

Autocalibrator Hand Sensor: Card with sensor electrode matched to hand size, with optical hand-down detection, buffer amplifier, and LED drivers.

Brain Opera Drivers: Small cards based around AD712, which buffer fish signals and send them down RJ-11 cable. Flat frequency compensation and low front-end gain will produce very few drift problems. LED also present, which the Gesture Wall utility lights to make glowing sensors.

New Log Amps: New, more accurate, simpler 4-channel log amps based around Burr Brown Log100JP. Used in the Gesture Wall.

Harmonic Driving Utility: Card with 8-channel light driver, MIDI input interface, LED drivers, and buffers for twist pot and occupant-detector photocell. Plugs into Fish.

Quad Buffer Amplifiers: 4 channels of Fish buffer amplifiers based on AD713, also with frequency flattening and low front-end gain for low drift.

Digital Drumpad: PVDF sampling and LED drive with PIC on RS-485 bus.

Drumpad Concentrator: Card that sits with a Fishbrain at the base of the drumpad bus; essentially an intelligent UART for the HC11 with RS-485 drive.

Sensor Floor Interface: Card with buffer amplifiers, peak detectors, and multiplexers to interface to a Fishbrain and scan 64 channels of analog input from PVDF wires impregnated into a carpet to detect footsteps and measure their energy/location.

Doppler Radar Head: Card with micropatch phased array and electronics (local oscillator, quadrature diode demodulator, opamp driver) to measure motion.

Doppler Radar Analog Processor: Card with direction determinator and envelope followers and filters to convert the doppler signals into a triad of voltages that conform to direction of motion, amount of motion, and rapidity of motion.

Pulsed Sonar Head: Card to manage a simple TOF sonar, and produce an output gate (going low when ping goes out, high on return), signal envelope, and an analog voltage proportional to range. These signals, plus power and trigger inputs are applied via an RJ-45 cable.

8-Channel Sonar Dispatcher: Card to allow a Fishbrain to manage up to 8 independent pulsed sonar heads.

Warning; not all of these cards are currently ready for flawless production; some require hand-patches.

10) Students and Collaborators

Forest Stations: Patrick Pelletier, Will Oliver

Harmonic Driving: Matt Gorbet

Melody Easel: Kai-Yuh Hsiao

Rhythm Tree: Ara Knaian, Josh Smith, Matt Reynolds

Digital Baton: Theresa Marrin, Chris Verplatse

Sensor Floor: Craig Alber

Doppler Radar: Matt Reynolds

Sensor Chair: Ed Hammond, Pete Rice, Eran Ergozy

Object Design: Maggie Orth, Ray Kinoshita, Sharon Daniel

Electronics Fabrication: Rick Ciliberto, Joel Rosenberg