The T-Box (formerly known as the Tralf) is a third generation instrument, based on initial versions first by Geof Holbrook and later by Geof Holbrook and Eileen TenCate. It makes use of ultrasound sensing to detect the performer's gestures in the air above the instrument.

The T-Box hardware has gone through a number of stages over the course of its development. The basic design uses ultrasonic transmitter and receivers and senses the distance between using the amplitude of the signal at the receiver, with the incoming signal passed through an envelope follower to extract the amplitude.

In the first two versions of the instrument, both the generation of the 40 kHz square wave for the transmitters and the envelope following of the received signal was performed using analog circuitry. This resulted in some issues regarding the stability of the square wave and also the response of the envelope follower. The envelope followers attack and decay coefficients were fixed, while the oscillator exhibited some drift and required regular retuning.

The current version of the T-Box makes use of 2 Atmel microcontrollers in order to deal with these issues. Square wave generation is performed on board an ATTiny26 running at 4 MHz. Two digital I/O pins are used to drive each ultrasound transmitter. This pins are kept out of phase to allow for a 10 volt swing in the square wave amplitude even though the microcontroller itself is powered at 5 volts.

Analog to digital conversion of the incoming signal, envelope following and communication with the computer are performed on an Arduino Mini board, together with an Arduino MiniUSB adapter. The Arduino board contains an Atmel AtMega168 running at 16 MHz, which is capable of performing the necessary calculation and communication tasks. It also performs analog to digital conversion for a potentiometer on the back of the T-Box, which is used to control the coefficients of the envelope follower code.

The T-Box's physical form is that of a rectangular wooden box, mounted on top of a microphone stand. Four ultrasound receivers are mounted on top of this box. The control for the envelope follower is one the back, facing the performer.

Two hand-pieces are connected to the sides of the box by flexible cabling. Each hand-piece is worn by the performer and contains an ultrasound transmitter and 4 switches, one for each finger.

Communication with the computer uses a serial link over a USB cable. This link runs at 115200 bps, allowing an update rate of ~250Hz for the sensor data.

The software for the T-Box is implemented in Max/MSP. It processes the incoming data from the T-Box, including some filtering, smoothing and downsampling of the data from the switches.

Synthesis is currently being performed using a Max/MSP patch which implements granular synthesis.

• There are some more images of the T-Box in the gallery
• A Youtube video of Joe Malloch demonstrating the T-Box at Wired magazine's NextFest in L.A.