Teabox: A Sensor Data Interface System
Jesse T. Allison and Timothy A. Place
Conservatory of Music, University of Missouri - Kansas City
Ltap@electrotap com
@'~p 'electrota,com
Abstract
Artists have long sought after alternative controllers,
sensors, and other means for controlling computer-based
musical performance in real-time. Traditional techniques
for transmitting the data generated by such devices typically
employ the use of MIDI as the transport protocol. Recently,
several devices have been developed using alternatives to
MIDI, including Ethernet-based and USB-based sensor
interfaces.
We have designed and produced a system that uses S/PDIF
as the transport mechanism for a sensor interface. This
provides robust performance, together with extremely low
latency and high resolution. In our system, data from all
sensors is multiplexed onto the digital audio line and
demultiplexed in software on the computer using standard
techniques. We have written demultiplexer objects and
plugins for Max/MSP and Jade, as well as a MIDI
Conversion program for interapplicaton uses, while others
are in the works for PD, SuperCollider, and AudioUnits.
1 History / Development
Practitioners of interactive music frequently require sensing
devices to obtain gestural control of various signalprocessing parameters. While much attention is given to
new and novel sensing devices, and to the interactive system
itself, the interface that connects the two is often
overlooked. This "middle man" which negotiates between
the analog sensors and the digital computer is frequently a
source of bottlenecks, latency variations, and system
instability, not to mention considerable expense. These
attributes combine to make such systems inadequate for a
wide range of expression and impractical for many artists
[1].
The Teabox is an interface developed in an attempt to
resolve these difficulties. Our initial experiences with the
Teabox have shown it to perform favorably when compared
to most commercially available systems.
The current implementation of the Teabox is the fifth
generation in a series of solutions utilizing audio hardware
to transmit sensor data. The initial solution, an interface
housed in a real tea box, converted continuous voltages
from two sensors into square waves, using 555 timers, and
transmitted them to the computer over two audio lines. The
data was represented as the frequency of the square waves.
The computer decoded the sensor information by counting
the zero-crossings of each signal. This solution worked well.
However it inefficiently used bandwidth and quickly filled
all available analog audio inputs on a computer.
A second revision improved on this design by replacing
the square wave oscillators with sine wave oscillators. The
new box collected data from three sensor sources and mixed
the sine wave signals they generated onto a single audio line
to send to the computer. The computer then separated the
signals using band-pass filters and analyzed each signal
individually. The data was represented by the amplitude
envelope of the sine waves, which was converted into a
control signal in the software.
The third approach we took manifested itself in a project
we called the SensorBox. The SensorBox accepted 6 sensor
inputs and two audio inputs. The data from each sensor was
carried as the amplitude of a sine wave which was located in
the 18Khz-20Khz frequency range, and piggy-backed on the
two audio inputs. This allowed for the sensor data to be
transmitted to the computer on the same line as the audio
input.
The fourth experiment was to attempt the multiplexing
of a number of sensor voltages and sending the data into an
analog audio input. However, the drawbacks of this system
were ringing and overshoot in the anti-aliasing filters on
most audio interfaces. This ultimately led us to a fifth
revision that has turned out to be incredibly successful.
2 The Teabox
The Teabox uses the S/PDIF digital audio format to
transmit 8 channels of sensor data via an audio interface to a
computer. The Teabox conditions the sensor inputs, samples
the voltages, and transmits the data via optical TOSLINK
or coaxial RCA cables to an awaiting audio interface. The
transmission speed is hardware selectable to accommodate
standard audio sampling rates. At the computer, the data is
normalized and can be accessed and used in Max/MSP or
PD through the teabox- object, in Jade through the
analysis.teabox module, and in various other audio
programs through the Teabox MIDI Conversion program.
Proceedings ICMC 2004
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