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Updates of the WONDER software interface for using Wave Field Synthesis
Marije A.J. BAALMAN
Communication Sciences, Technische Universität Berlin
Sekr. EN8, Einsteinufer 17
Berlin, Germany
baalman@kgw.tu-berlin.de
Abstract
WONDER is a software interface for using Wave
Field Synthesis for audio spatialisation. Its user
group is aimed to be composers of electronic
music or sound artists. The program provides a
graphical interface as well as the possibility to
control it externally using the OpenSoundControl
protocol. The paper describes improvements and
updates to the program, since last year.
Keywords
Wave Field Synthesis, spatialisation
1 Introduction
Wave Field Synthesis (WFS) is a technique for
sound spatialisation, that overcomes the main
shortcoming of other spatialisation techniques, as
there is a large listening area and no “sweet spot”.
In recent years, WFS has become usable with
commercially available hardware.
This paper describes the further development of
the WONDER program, that was designed to
make the WFS-technique available and usable for
composers of electronic music and sound artists.
2 Short overview of WFS and WONDER
WFS is based on the principle of Huygens,
which states that a wave front can be considered as
an infinite number of point sources, that each emit
waves; their wavefronts will add up to the next
wavefronts. With Wave Field Synthesis, by using a
discrete, linear array of loudspeakers, one can
synthesize correct wavefronts in the horizontal
plane (Berkhout e.a. 1993). See figure 1 for an
illustration of the technique.
WONDER is an open source software program
to control a WFS system. The program provides a
graphical user interface and allows the user to
think in terms of positions and movements, while
the program takes care of the necessary
calculations for the speaker driver functions. The
program is built up in three parts: a grid definition
tool, a composition tool and a play interface.
Additional tools allow the user to manipulate grids
or scores, or view filter data.
For the actual realtime convolution, WONDER
relies on the program BruteFIR (Torger). This
program is capable of doing the amount of filter
convolutions that are necessary to use WFS in
realtime. On the other hand, BruteFIR has the
drawback, that all the filters need to be calculated
beforehand and during runtime need to be stored in
RAM. This limits the flexibility for realtime use. It
is for this reason, that a grid of points needs to be
defined in advance, so that the filters for these
points can be calculated beforehand and stored in
memory.
For a more complete description of the
WONDER software and the WFS-technique in
general, I refer back to previous papers (Baalman,
2003/2004).
Figure 1. The Huygens' principle (left) and the Wave
Field Synthesis principle (right).
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3 Updates to WONDER
Since its initial release in July 2004, WONDER
has gone through some changes and updates. New
tools have been implemented and the functionality
of some of the old tools have been improved. Also,
some parts were reimplemented to allow for easier
extension in the future and resulting in a cleaner
and more modular design.
The graphical overview, which displays the
spatial layout of the speakers and source positions
have been made consistent with each other and
now all provide the same functionality, such as the
view point (a stage view or an audience view),
whether or not to show the room, setting the limits
and displaying a background image.
The room definition has been improved: it is
now possible to define an absorption factor for
each wall, instead of one for all walls.
3.1 Grid tools
The Grid definition tool allows a user to define a
grid consisting of various segments. Each segment
can have a different spacing of points within its
specified area and different characteristics, such as
inclusion of high frequency damping and room
parameters for reflections.
The menu “Tools” now provides two extra tools
to manipulate Grids. It is possible to merge several
grids to one grid and to transform a grid.
The Merge-tool puts the points of several grids
into one grid, allowing the user to use more than
one grid in an environment.
The Transform-tool applies several spatial
transformations to the segments of a grid and then
calculates the points resulting from these
transformed segments. This transformation can be
useful if a piece will be performed on another
WFS system, which has another geometry of the
speaker setup and the coordinates of the grid need
to be transformed.
The filter view (fig. 2) is a way to verify a grid
graphically. It shows you the coefficients of all the
filters of a source position in a plot. The plot
shows on the horizontal axis the loudspeakers, in
the vertical direction the time (on the top is zero).
The intensity indicates the value of the absolute
value of the volume. Above the graph, the input
parameters of the gridpoint are given, as well as
some parameters of the grid as a whole. This filter
overview can be useful for verification of
calculations or for eductional purposes.
Another way to verify a grid is using the grid
test mode during playback with which you can
step through the points of a grid and listen to each
point separately.
3.2 Composition tools
With the composition tool the user can define a
spatial composition of the sound source
movements. For each source the movement can be
divided in sections in time and the spatial
parameters can be given segmentwise per section.
In the composition definition dialog it is also
possible to transform the current composition. The
user can define a set of spatial transformations that
have to be applied to the sources and sections
specified. After the transformations have been
applied, the user can continue working on the
composition. This tool is especially handy when
one source has to make a similar movement as
Figure 2. Filter view tool of WONDER. At the top,
information about the grid and the current grid point
are given. In the plot itself is in the horizontal direction
the speakers, in the vertical direction the time. The
intensity (contrast can be changed with the slider at the
bottom right) is an indication of the strength of the
pulse. The view clearly shows the reflection pattern of
the impulse response.
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another: just make a copy of the one source and
apply a transformation to the copy.
After a score has been created (either with the
composition tool or by recording a score), there are
four tools available in the “Tools”-menu to
manipulate the scores.
“Clean score” is handy for a recorded score.
This cleans up any double time information and
rounds the times to the minimum time step
(default: 25 ms).
“Merge scores” enables you to merge different
scores into one. It allows a remapping of sources
per score included.
“Transform score” allows you to make
transformations to different sources in a score.
The last tool is the “timeline view”, which
shows the x- and y-component in a timeline (fig.
4); it shows the selected time section as a path in
an x-y view. It is also possible to manipulate time
points in this view. While playing it shows a
playhead to indicate the current time. The concept
of this timeline view is inspired by the program
“Meloncillo” (Rutz). The timeline view allows for
a different way of working on a composition: the
user can directly manipulate the score.
3.3 Play interface
WONDER provides a graphical interface to
move sound sources or to view the movement of
the sound sources. The movement of sound
sources can be controlled externally with the OSC-
protocol (Wright, 2003).
Score control is possible by using the Transport
controls (fig. 3), which have been re-implemented.
The sound in- and output can be chosen to be
OSS, ALSA, JACK or a sound file. In the first
three cases the input has to be equal to the output.
The program BruteFIR (Torger) is used as audio
engine. The communication between BruteFIR and
WONDER can be verified by using a logview,
which displays the output of BruteFIR. Due to
some changes in the command line interface of
BruteFIR, the communication between WONDER
and BruteFIR could be improved and there is no
more a problem with writing and reading
permissions for the local socket.
The logview, which records (in red) the
messages that are shown in the statusbar of
WONDER, shows the feedback from the play
engine BruteFIR in black. It is possible to save the
log to a file.
Figure 4. The timeline view of WONDER. The selected time section is shown as a path in the positional
overview. The user can edit the score by moving the breakpoints or adding new ones.
Figure 3. Transport control of WONDER. The slider
enables the user to jump to a new time. The time in
green (left) indicates the running time, the time in
yellow (right) the total duration of the score. The
caption of the window indicates the name of the score
file.
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3.4 Help functions
The manual of the program is accessible via the
“Contents” item in the “Help”-menu. This displays
a simple HTML-browser with which you can
browse through the documentation. Alternately,
you can use your own favourite browser to view
the manual.
Additionally, in most views of WONDER,
hovering above buttons, gives you a short
explanation about the buttons' functionality.
4 External programs to control WONDER
There are two examples available which show
how to control WONDER from another program: a
SuperCollider Class available and an example
MAX/MSP patch.
Another program that can be used for external
control is the JAVA-program SpaceJockey
(Kneppers), which was designed to enable the use
of (customizable) movement patterns and to
provide MIDI-control over the movements.
5 Conclusion
WONDER has improved in the last year and has
become more stable and more usable. Several
changes have been made to facilitate further
development of the program.
Current work is to create an interface to EASE
for more complex room simulation and to integrate
the use of SuperCollider as an optional engine for
the spatialisation. It is expected that SuperCollider
can provide more flexibility, such as removing the
necessity to load all filter files in RAM and the
possibility to calculate filter coefficients during
runtime.
Current research is focused on how to
implement a more complex sound source
definition.
Download
A download is available at:
http://gigant.kgw.tu-berlin.de/~baalman/
References
Baalman, M.A.J., 2003, Application of Wave Field
Synthesis in the composition of electronic
music, International Computer Music
Conference 2003, Singapore
Baalman, M.A.J., 2004, Application of Wave Field
Synthesis in electronic music and sound
installations, Linux Audio Conference 2004,
ZKM, Karlsruhe, Germany
Baalman, M.A.J. & Plewe, D., 2004, WONDER -
a software interface for the application of
Wave Field Synthesis in electronic music and
interactive sound installations, International
Computer Music Conference 2004, Miami, Fl.,
USA
Berkhout, A.J., Vries, D. de & Vogel, P. 1993,
Acoustic Control by Wave Field Synthesis,
Journal of the Acoustical Society of America,
93(5):2764-2778
Kneppers, M., & Graaff, B. van der, SpaceJockey,
http://avdl1064.oli.tudelft.nl/WFS/
Rutz, H.H., Meloncillo,
http://www.sciss.de/meloncillo/index.html
Torger, A., BruteFIR,
http://www.ludd.luth.se/~torger/brutefir.html
Wright, M., Freed, A. & Momeni, A. 2003,
“OpenSoundControl: State of the Art 2003”,
2003 International Conference on New
Interfaces for Musical Expression, McGill
University, Montreal, Canada 22-24 May 2003,
Proceedings, pp. 153-160
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