Proceedings of the COST G-6 Conference on Digital Audio Effects (DAFX-01), Limerick, Ireland, December 6-8, 2001
DISCOVERING EXPRESSIVE REALTIME PARAMETERS:
THE PERFORMING LAPTOP ARTIST AS EFFECT DESIGNER
Dan Trueman
R. Luke DuBois
Curtis Bahn
Music Department
Colgate University
Computer Music Center
Columbia University
iEAR Studios
Rensselaer Polytechnic Institute
dtrueman@mail.colgate.edu
luke@music.columbia.edu
crb@rpi.edu
1.
INTRODUCTION
The laptop artist is a modern-day jack-of-all-trades; in a
way unlike any time in history, composers, performers,
instrument designers and engineers are often one and the
same person. In this case, the feedback loop between
musical
experimentation
and
software/hardware
development is very short, and can produce unusual,
idiosyncratic results.
Over the last several years, we (all "laptop artists" of this
sort) have been active as performers, composers,
software/DSP developers and hardware instrument builders.
In particular, we have been seeking more compelling
physical connections between body and digital sound,
creating personalized hardware/sensor interfaces [1] [2],
connecting them to custom signal-processing algorithms,
and generating electronic sound through various kinds of
speaker
arrangements
(both
outside-in
surround
configurations and inside-out spherical arrangements) [3]
[4].
In this paper, we focus on some of the software, signal
processing and synthesis algorithms that we have worked
with and discuss how our particular creative contexts have
influenced algorithm design. We emphasize our search for
"expressive parameters," algorithm parameters which we can
imagine performing. We discuss how physical interfaces
inspire this search, and in turn how the sonic results of a
particular algorithm inspire new physical interfaces. We
also discuss some of the music made with these algorithms
(much of which has been released on several CDs over the
last year [5] [6] [7]. Finally, we describe the publication of
an open-source software toolkit—PeRColate—our primary
software workbench that facilitates both low-level and highlevel development for laptop artists [8].
2.
SHORT/MEDIUM-TIME DELAY EFFECTS
We describe two algorithms that manipulate short/medium
time (10-1000ms) delay-lines. One, the "scrubber," is a
delay-line scrubber. Constructing a clean algorithm (one
that will not click or generate unwanted artifacts) which
allows one to scrub (vary the playback rate) of a finite
delay-line is tricky. Our solution involves using three
rotating buffers: one for recording, one for playback, and a
spare, which allows for clean crossfading during buffer
changes (we describe this algorithm more completely in the
final paper). While playback rate is clearly one of the
algorithm's most expressive parameters, three other
parameters that result from the algorithm's architecture are
equally powerful: buffer-size, overlap time (the amount of
overlap when switching and cross-fading buffers) and record
ramp (the envelope on the record buffer) are all interesting
to manipulate in performance. In our final paper, we
describe some of these applications, and provide musical
examples from our recent work. In particular, we illustrate
several compelling mappings from physical interface to
algorithm parameter.
The "munger," a granular-sampling algorithm, uses a
similar three-buffer scheme to generate clean granular
textures from a delay-line. Like the scrubber, this algorithm
has several obvious expressive parameters—grain-size,
playback rate—as well as a few less obvious parameters.
For instance, dynamically controlling the maximum delaylength, in the range of 10-1000ms (or more), allows the
performer to expressively create a sense of "tightness" (or
"looseness") around the input signal. Again, we describe
particular musical applications of this and other mappings.
3.
PHYSICAL MODELS
As is well known, many physical models offer intuitive
parameters for expressive control. We have worked
extensively tailoring physical input parameters to the body,
creating intuitive and kinesthetic connections to sound in
complex multi-dimensional performance environments.
Most of our efforts with physical models have favored
pushing these expressive parameters outside of their
"realistic" ranges, creating instruments or virtual players
which are either caricatures of the original model or simply
unrecognizable. We have also experimented with creating
hybrid models which embody features of physically
incompatible instruments. One particularly compelling
example is the "blotar," a combination of the flute, electric
guitar, and mandolin physical models, which takes
advantage of the similar design of the flute and electric
guitar models (in the final paper, we describe this model in
DAFX-1
Proceedings of the COST G-6 Conference on Digital Audio Effects (DAFX-01), Limerick, Ireland, December 6-8, 2001
more detail, which is an elaboration of the model first
introduced by Cook [9]. (reference DAFx98, Towards the
Perfect Audio Morph? Singing Voice Synthesis and
Processing)). In addition to the familiar expressive
parameters of the original models, we discovered that a
simple crossfade between the low-pass filter of the guitar
model and the one-pole filter of the flute model offered
unusual expressive possibilities (we refer to this kind of
hack as an SMH: "Stupid Musician Hack"). As usual, in
the paper and presentation, we will present several musical
applications of this and other models.
We have also made extensive use of Perry Cook's
Physically
Inspired
Sonic
Modeling
(PhISM)
algorithms[10]. In one application, models of several
different kinds of "shakers" (maraca, bamboo wind-chimes,
and guiro, for example) are "hung" from a violin bow that
is fitted with sensors to control the models; using the bow,
we can "shake" the models and dynamically alter their
resonances. We further describe several applications and
modifications of these algorithms for use with interactive
works for dance [11] and experimental digital instruments
for children [12].
4.
OLD SCHOOL and IDIOSYNCRATIC
Applied in realtime, with various kinds of physical control,
old fashioned signal processing algorithms, unmodified, or
reworked in simple ways, can yield musically interesting
results. Examples include ring modulation, "terrain", a 2dimensional wavetable scanner (which is particularly
effective when manipulated with a graphics tablet), and
"chase," a three-way signal comparator. For the laptop
artist, who always has an eye on the CPU meter, these are
cheap and effective and, while lacking some of the nonlinearity and richness of their analogue models, offer
flexibility that was impossible with hardware. Also of
interest (and examples of SMHs) are algorithms which
arbitrarily take advantage of software system architecture.
One particularly amusing example, "klutz," simply reverses
the samples in the computation signal vector of the
MAX/MSP environment; in this case, the signal vector size
becomes an expressive parameter. In the final paper, we
further illustrate these and other examples, and describe
various musical applications and physical mappings used to
perform these algorithms.
5.
NEW TERRITORY: THE FREQUENCY DOMAIN
The laptop has finally entered the frequency domain, and
with it, the laptop artist. We are just now beginning
explore the possibilities of manipulating frequency domain
data in realtime with various kinds of physical controllers.
Phase vocoding, naturally, is particularly enticing, and,
while much can be learned from the deep legacy of nonrealtime work in the frequency domain, the context is
different, as are the problems and possibilities—input-
output latency is of particular concern, and causes problems
when large FFT sizes are desired. But, more importantly,
given that we imagine these frequency domain
manipulations as realtime musical instruments that need to
provide aural feedback given specific physical controls
(imagine "bowing" samples in the frequency domain), the
goals and possibilities are different for the laptop artist. In
the final paper, we describe some of our beginning efforts
in the frequency domain, and anticipate, come conference
time, that we will have numerous musical examples to
present as well.
6.
PeRColate: AN OPEN SOURCE TOOLKIT
PeRColate is an open-source distribution of a variety of
synthesis and signal processing algorithms for Max/MSP
[reference], and now NATO [reference]. It began around a
(partial) port of the Synthesis Toolkit (STK) by Perry Cook
(Princeton) and Gary Scavone (Stanford CCRMA). Like the
STK, it provides a fairly easy to use library of synthesis
and signal processing functions (in C) that can be wired
together to create conventional and unusual instruments.
Also like the STK, it includes a variety of precompiled
synthesis objects, including physical modeling, modal, and
PhISM class instruments; the code for these instruments
can serve as foundations for creating new instruments (the
blotar is one example) and can be used to teach elementary
and advanced synthesis techniques. Since its first release (in
February 2000), PeRColate has come to include many more
objects not from the STK; some are from RTcmix and
others are of our own design (like scrub, the munger, and
the SID—Synthesis Isn't Dead—algorithms). In addition, a
library of PeRColate NATO video processing objects has
been created.
7.
REFERENCES
[1] Trueman, D. and P. R. Cook. “BoSSA: The Deconstructed
Violin Reconstructed.” Proc. of the International
Computer M, ausic Conference, Beijing, October, 1999.
[2] Bahn,
C.
R.
"SBASS:
Sensor
Bass."
http://silvertone.princeton.edu/~crb/Activities/sbass.htm
[3] Cook, P. R. and D. Trueman. "Spherical Radiation from
Stringed
Instruments:
Measured, Modeled,
and
Reproduced." Journal of the Catgut Acoustical Society,
November 1999.
[4] Trueman, D., C.R. Bahn and P. R. Cook, “Alternative
Voices for Electronic Sound:Spherical Speakers and
Sensor-Speaker Arrays (SenSAs),” Proc. of the
International Computer Music Conference, Berlin,
Germany, 2000.
[5] interface. "./swank." CD released by C74 Records.
[6] Bahn, C. R. "r!g." CD released by the Electronic Music
Foundation.
[7] the Freight Elevator Quartet. "Fix it in Post." CD released
by C74 Records.
[8] Trueman, D. and R. Luke DuBois. "PeRColate."
http://music.columbia.edu/PeRColate/
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Proceedings of the COST G-6 Conference on Digital Audio Effects (DAFX-01), Limerick, Ireland, December 6-8, 2001
[9] Cook, P. R., “Towards the Perfect Audio Morph? Singing
Voice Synthesis and Processing”, Proc. Workshop on
Digital Audio Effects (DAFx-98), Barcelona, Spain, 1998.
[10] Cook, P. R. “Physically Inspired Sonic Modeling
(PhISM): Synthesis of Percussive Sounds,” Computer
Music Journal, Volume 21, Number 3, September 1997.
[11] Bahn,
C.
R.
and
T.
Hahn.
"Streams."
http://silvertone.princeton.edu/~crb/Streams/streams.htm
[12] The JPMorganChase KIDS Digital Movement and Sound
Project. http://music.columbia.edu/kids/
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