ABSTRACT
The Accordiatron is a new MIDI controller for real-time performance
based on the paradigm of a conventional squeeze box or concertina. It translates
the gestures of a performer to the standard communication protocol of MIDI,
allowing for flexible mappings of performance data to sonic parameters.
When used in conjunction with a real-time signal processing environment,
the Accordiatron becomes an expressive, versatile musical instrument. A
combination of sensory outputs providing both discrete and continuous data
gives the subtle expressiveness and control necessary for interactive music.
Keywords
MIDI controllers, computer music, interactive music, electronic musical
instruments, musical instrument design, human computer interface
INTRODUCTION
The Accordiatron was initially conceived as a class project for a course
in human-computer interfaces at the Center for Computer Research in Music
and Acoustics (CCRMA) at Stanford University. Its designers brought their
respective expertise in product design and interactive performance together
to create a new device that aims to combine aesthetic appeal, comfortable
operation and useful output from gestural sensing.
THE NEED FOR NEW CONTROLLERS
There exists a highly debated problem in interactive music that is
characterized by a lack of correlation between performance gestures and
their musical consequences. This can create a difficulty in perception
and indeed reception of interactive music; the music can often be reduced
to a "challenge" of seeking some relationship between the actions of the
performer and the resulting sound. The listener can feel a gap between
him or herself and any meaningful purpose to the music. The remedy to this
problem is certainly partly the responsibility of the composer, who, if
concerned, should try to make more accessible music, but there are some
limitations inherent in the instruments used in these compositions that
prevent the bridging of this gap.
The practice of employing traditional instruments in interactive performance
situations, either by retrofitting the instrument with various sensors
or simply tracking its audio output, can prove to be problematic for the
audience. Musical listeners have an ingrained urge to identify the source
of a sound. They have a similar expectation of the nature of the sound
that will be produced by an instrument. Presently, people have little difficulty
accepting the fact that a computer or a recording can be a sound source
and have come to expect all sorts of sounds from these media. However,
in the case where a traditional instrument is used as a controller for
computer music, these expectations are abandoned. The paradox in seeing
a familiar instrument and hearing sounds other than those associated with
the instrument can be difficult to overcome. In such a case, there is no
apparent source for the sound; only this contradiction. The listener is
left with the task of trying to associate specific performance gestures
with sonic events. In itself, this can be a novel game, but it is certainly
not the goal of every composer of interactive music. Traditional instruments
are limited in this context in that they are designed for the sounds they
produce, not the gestures associated with playing them. There is therefore
a need, in the authors' opinion, for new gestural controllers designed
specifically for interactive music situations.
DESIGN INSPIRATION
Upon setting out to create a new controller that could function as
an electronic instrument, the authors devised a set or requirements for
the new device. Experience with other gestural controllers for interactive
music, such as the Theremin, generated several criteria for the nature
of a new controller, while traditional instruments determined others.
Multi-Dimensionality
Traditional instruments are capable of producing variations in numerous
dimensions such as pitch, loudness, timbre, articulation, etc., all of
which can be altered at varying rates and applied to determine auditory
parameters in real time. In order to be versatile and as useful as an acoustic
instrument for interactive applications, an electronic controller similarly
needs to provide multi-dimensional data (several unique streams) capable
of simultaneously controlling multiple sonic parameters. This has been
one limiting factor of the Theremin as a controller, rather than as an
instrument: the performer has control in only two dimensions, pitch and
loudness.
Discrete and Continuous Data
A further requirement was that the sensory output provide a combination
of discrete and continuous data. This is necessary to suit diverse interactive
musical applications, where both singular, isolated events and smooth changes
can occur. This is again a limitation of other controllers, like the Theremin,
where the only outputs are continuously changing.
Visual Intrigue
In order to attempt to satisfy a live audience's desire for visual
and auditory correspondences as described above, a new controller needed
to have strong visual appeal. A performance instrument should be interesting
to watch as well as to hear, otherwise part of the purpose of live performance
is lost.
Constrained Motion
The final requirement was that the gestures of the performer be constrained
in some way. While free motion in space may be visually expressive, it
can be difficult to translate to musically useful information. Furthermore,
there are several such systems in existence. The goal in this case was
to design an instrument that could be physically manipulated. Motion with
limited constraint allows for the control and precision required for musical
expression.
THE SQUEEZE BOX PARADIGM
The paradigm of the squeeze box satisfies many of these design requirements.
A number of buttons on the instrument allow for discrete events, while
the squeezing motion of the hands and arms is continuous. Sensing the unique
button presses as well as the constrained hand motions in several directions
allows for multi-dimensional output. The squeeze box was also a compelling
starting point because of the expressive physical engagement of the performer
and the subsequent value for live interaction. Capturing gesture as an
input for electronic music seems to be a very effective means of overcoming
the distance between the audience's expectations for a musical performance
and the static nature of computer hardware.
In designing the Accordiatron, three types of motion borrowed from
the squeeze box were chosen for continuous positional sensing. These are
the distance between two ends panels attached to the hands (amount of "squeeze"
or "pull"), and the rotation of each hand in relation to the mechanism
connecting the two end panels. These are natural motions associated with
playing a squeeze box, and while the angle of the hands is not essential
in a performing role, it occurs nonetheless. The discrete sensors in the
Accordiatron are in the form of a series of buttons, borrowed directly
from the design of a concertina.
THE ACCORDIATRON
The way in which the Accordiatron breaks from the mold of its acoustic
counterpart is in the flexibility to apply the sensory data from these
motions and buttons to any number of sonic parameters. In a traditional
concertina, button presses determine which note the instrument sounds,
while the velocity of the squeezing action (and hence the pressure applied
by the bellows to the reed) determines the loudness (and incidentally the
timbre) of the resulting sound. The output of the Accordiatron could easily
produce the same results, simply by mapping button presses to note events
in a synthesizer and mapping the rate-of-change of the distance between
the two ends to the overall loudness. However, the Accordiatron is capable
of much more. In conjunction with a real-time audio processing environment,
it can control any aspect of sound.
The Communication Medium
This is achieved by the use of the MIDI protocol[1]. The Accordiatron
outputs MIDI data from a standard 5-pin MIDI connector and can be connected
to any device equipped to receive MIDI. It specifically uses control change
messages on channels 1-3 for the three continuous controllers, using controller
numbers 16-18, which are designated as general purpose controllers. Each
is assigned a value from 0 to 127 (the standard 7-bit encoding of MIDI)
based on the sensed position on the controller. Only changes in position
are transmitted so that filtering of repetitive data by a receiving device
is unnecessary. The MIDI standard therefore prevents truly continuous data
transmission, but the MIDI transmission rate allows for roughly 1000 total
events per second.
Button presses are encoded as note-on messages, each of the 10 buttons
with a unique note number from 60-69, on a unique channel from 4-13. A
note-on with velocity 127 is transmitted when a button is pressed, and
another with velocity 0 upon release. No data after the initial note-on
is sent while a button is held down. Button presses are therefore transmitted
as truly discrete events.
The Accordiatron presently contains one extra non-performance button
that transmits a note-on number 70 on channel 14. This can be used for
mode changes or as an additional trigger for setup. Future Accordiatron
versions could contain two more such buttons either for transmitting messages
on channels 15 and 16, or for changing aspects of the Accordiatron's internal
program.
The Guts
At the heart of the Accordiatron is the Parallax Basic Stamp II programmable
microprocessor[2]. The Accordiatron's program, based on code by Craig Sapp[3],
is therefore flexible and can easily be modified in future versions. The
continuous sensory input comes from 3 potentiometers that are mechanically
rotated by the motions of the performer. The distance between the two ends
is sensed by a logarithmic pot attached to one of the linkages with its
angle-of-rotation changing as the mechanism opens and closes. The other
two potentiometers linearly measure the rotation of each the end panels.
These yield output voltages proportional to the motions, whose ranges are
adjusted in three operational amplifier circuits, in order to maximize
the 0-5V range of the analog-to-digital converters (ADC's). The sensory
data is digitized by two 12-bit 8-channel MAXIM ADC converters, the first
5 bits of which are ignored to suit the MIDI standard. The button presses
are similarly digitized based on high-low voltage changes. The entire system
is run from a regulated 5V from a standard 9V battery.
DESIGN
The starting point for the physical design of the Accordiatron was
a traditional squeeze box. It evolved from that model into its final form
by incrementally eliminating the non-essential details and resolving the
issues that came with the integration of the circuitry and mechanical elements.
The approximate dimensions, the kinds of motions the designers wanted to
capture, and the combination of continuous and discrete inputs (in the
form of buttons) all come from its acoustic counterpart. The reasons for
the differences between the two range from prototyping considerations to
issues of ergonomics and interface design.
From the squeeze box the designers took the basic layout, two end panels
with buttons assigned to each hand and a mechanism between them with some
form of continuous control. By isolating the kinds of gestural motion of
the performer, a mechanism that could translate that motion to three twist-potentiometers
was devised. 1/4 inch acrylic sheet was chosen as the material for the
linkages (and end panels) because of its physical and visual characteristics
as well as the ease of fabricating the parts with a Computer-Aided Design
(CAD) software package and a laser cutter. Custom hardware was machined
from aluminum for the hinges at the end panels and the attachment points
of the potentiometers. The remaining hardware consists of off-the-shelf
stainless steel and aluminum fasteners used to assemble the various parts.
The end panels each contain nylon hand straps, five buttons with one
for each finger, and an offset pad for the palm of the hand. This allows
for improved leverage for the fingers and a more natural hand position.
The buttons are off-the-shelf parts, but have been modified with levers
hinged over them in order to provide better action and allow for a range
of hand sizes.
All of the circuitry is contained on the inside of the right-hand panel.
The wiring from the buttons and potentiometer on the left side are carried
along the linkages through eye-hooks to the right, where they are soldered
along with the rest of the wiring to the circuit board. Stops preventing
interference from the mechanism and an additional cover-plate mounted to
the end panel protect the circuit board.
The careful consideration of these details in the design serves two
purposes. From the standpoint of the user, the Accordiatron feels very
much as a musical instrument should. The mechanical movement is smooth,
the action of the buttons is very similar to valves or keys, and the feel
of the Accordiatron in the hands of the musician is quite natural. For
the audience the exposed electronics and mechanism and their movements
become a point of interest during a performance and give visual clues linking
the viewer's expectations with the sounds being created.
APPLICATIONS
The Accordiatron is well suited for interactive performance applications.
The following URL contains both video and audio examples of a performance
of a piece for the Accordiatron composed and performed by Michael Gurevich
using the Max/MSP programming environment on the Macintosh: http://www-ccrma.stanford.edu/~gurevich/accordiatron/
This piece demonstrates some of the capabilities of the Accordiatron.
The buttons in this case are mapped to wavetable synthesis using accordion
samples, on one hand to single notes, on the other to clusters of notes.
These are convolved in real time with a sound file. A cross fade from the
raw sound of the two sources through the convolution of the two is controlled
by the distance between the two ends. The angles-of-rotation of the end
panels control various other parameters, including the playback rate (pitch)
of the accordion samples, reverb room size and several kinds of modulation.
This certainly does not represent the full extent of the musical possibilities
of the Accordiatron when used with Max/MSP, but is an indication of its
usefulness as a performance instrument in this interactive context.
ACKNOWLEDGMENTS
We would like to thank Jonathan Berger, Bill Verplank, Max Mathews,
Tamara Smyth and Aaron Hipple for their invaluable guidance and assistance
in this realization of this project.
REFERENCES
1. Harmony Central: MIDI Tools and Resources. Available at http://www.harmony-central.com/MIDI/.
2. Edwards, Scott. Programming and Customizing the Basic Stamp Computer.
McGraw-Hill, 1998.
3. Sapp, Craig. "Electronic Music Devices for Basic Stamp IIsx". Available
at http://devices.sapp.org/micro/stamp2sx/.