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Csound Implementation of Physical Models of Woodwind InstrumentsJosep Mª Comajuncosas Nebot Introduction
IntroductionThe motivations behind this work came
from an article I read a year ago in the Journal of New
Music Research, vol. 24 (1995): "Physical Models
of Wind Instruments : A Generalized Excitation Coupled
with a Modular Tube Simulation Platform" by Gijs
de Bruin and Maarten van Walstijn. Most of the Csound implementation is
biased towards the clarinet by now, and most of the given
examples will be from my last clarinet model, which can
be found on my Csound page at My aim is to build a nice collection of virtual woodwind instruments of great quality to use them in my Csound compositions. In this article I summarize the current progress of the implementation and two already functional models are presented : a clarinet and the Grallot (a sort of doublereed clarinet). Basic implementationthe reedOne of the main differences between
this woodwind model and the ones implemented in the
Csound sources - from P. Cook´s Synthesis Toolkit
- is the simulation of the reed nonlinearity. which after some simplification and cosmetics becomes
I refer you to the bibliography and the
Csound code for a complete description of all the
parameters involved. The Bernoully pressure (pb) takes an important role
in double reeds but is neglected in clarinet-type single
reeds. The flow entering the reed as function
of the reed aperture and the pressure difference
between the mouth and the mouthpiece is assumed to obey
Backus equation for single reed instruments and a modified Backus equation for double reeds, taking into account the different reed geometry
(0.4e-6 is the reed aperture in
equilibrium for a clarinet, the instrument in which
Backus measurements were done). I´ll possibly experiment
with another set of equations described in the revised
Nederveen´s thesis. the clipping routineNotice that at very small reed
apertures (when playing hard) the Backus equation is no
longer valid and can lead to large numerical errors. Also,
the reed aperture cannot go to zero as in a real
instrument. To overcome that problem several strategies
have been proposed, like desining different computing
zones according to the reed aperture. allowing overblowing (MP3) and being parametrizable. This is vital
because the reed clipping has a strong effect on the
sound and makes possible a large variety of reeds and
playing styles. This clipping routine can be considered
as a phenomenological model of the reed curling up the
mouthpiece. coupling the reed with the boreTo couple the reed with the bore one
must be able to compute the outgoing pressure (p right) which will
be the value injected into the waveguide, from the flow Ub and the ingoing pressure
(p left).
According to Nederveen this term
behaves like a virtual volume added to the mouthpiece
cavity and one has to compensate for it when computing
the total delay time of the waveguide.
the boreThe bore is implemented as a recursive comb filter. Specifically, the current implementation is simple enough (i.e, a single cylindrical bore without a tonehole lattice) to allow a special implementation which makes use of a single waveguide of double length (half the wavelength of the desired note in the case of an open/closed cylinder) to simulate the upward and inward travelling waves. This method is similarly implemented in R. Cook´s woodwind and stringed instruments. the tonehole reflection filterAssuming that most of the radiation
coming out of the clarinet is due to the first open
tonehole, we can neglect the rest of the bore and derive
the reflectance and transmittance filters from a generic
clarinet tonehole and apply it to all pitches, as if the
clarinet was a sizeable cilinder with the tonehole at the
end of the bore. The filter design routine has been
implemented in Matlab by Maarten van Walstijn and matches
the frequency and phase response of the theoretical
tonehole response according to D.Keefe´s tonehole models.
The frequency response of the reflection filter has a
lowpass shape with a cutoff frequency of about 1,5 kHz,
which agrees well with Keefe´s measurements. The phase response of the refletance filter is taken into account when computing the total delay length, compensating for it in the main delay line. The final bore structure as shown below includes the main waveguide together with the tonehole reflection filter and a fractional delay filter to fine tune the delay length. Notice that the transmittance filter is outside the feedback mechanism and thus could be parameterizable (for tone and brightness control) without undesired effects on the tuning (MP3) Csound minutiaethe player and the instrumentAs the aim is to accurately model an
instrument it is conceptually advisable to split the code
into 2 Csound instruments. The first one works as the
player, that is, it is activated at each new event and
sends the appropiate performance values (such as flow and
reed parameters, pitch, volume and so on) to the second
instrument. integration schemeThe system of equations describing the
reed motion and the air flow through it are integrated as
follows: first integrate the reed motion (2nd order ODE)
with the Euler method. Thus we will get the reed aperture.
filters and delay linesThe filters employed to implement the tonehole (reflectance and transmission filters) are designed with Matlab as 1st or 2nd order sections and thus can be implemented in Csound with the biquad opcode. Though rather simple they approximate very well the actual tonehole response and allow an analytic computation of their phase delay, which is used to fine tune the instrument. The fractional delay filter is implemented as a simple interpolator (a first order Lagrange interpolator) but I´ll soon implement it as a 3rd order FIR filter with the filter2 opcode, which should minimize the high frequency losses. The delay line is implemented as a delayr/delaw pair, from where I extract the precise sample with the deltapn opcode. Notice a cubic interpolation could be possible without the explicit use of filter opcodes, making use of the new deltap3 opcode. I avoided it because I´m not sure if it is reliable enough. legato notes : the dual bore structureBecause the woodwind is left active
during the performance, it is possible to deal with
legato notes quite easily. Changing the pitch in a legato fragment could be done by shortening or enlarging the bore like in a slide flute, but this glissando sounds unnatural and causes the volume to increase during the note transition. The solution was to implement a second bore running in parallel with the first. At each new event the bore is switched with an appropiate crossfade time. The crossfade is not linear but sigmoidal, though the difference is subtle. The result is not unlike the transition time needed to change the fingering in a real instrument. I found that the optimum transition time depends on the note frequency and the speed of the fragment to be played. If it is too low a glissando is heard and the note takes longer to oscillate at the expected amplitude, if too short, the abrupt transition will cause noticeable clicks. recoding all the stuff with MacrosMore recently I decided to recode the instruments making use of the new Macro functionality. Thus the woodwind is built in a modular way: reed + bore + tonehole. The reed itself, for example, also includes macro calls showing its internal structure : reed motion + clipping routine + flow equation. Hopefully the code is more readable this way. Also it makes it easy to change part of the code, to apply it to different designs and eventually to port all the stuff to C++ or viceversa (that´s what I´m actually doing with Maarten´s C++ implementation of the clarinet). MIDI and Score implementationsBoth a MIDI and a Score activated
orchestra will have the "instrument" identical,
but the "player" code will differ depending on
how the event is activated. Some working examplesThe clarinet : single reed cylindrical woodwindUsually the clarinet is a good
candidate to begin this kind of modeling, because of its
relative simplicity : a nearly cylindrical bore with a
small bell which can be neglected for most pitches, and a
single reed is much simpler to model than the saxophone´s.
The clarinet mouthpiece can also be neglected at a first
approximation without so much discrepancy as in the sax.
Grallot : a double reed clarinetThis instrument is my first nearly
successful implementation of a double reed, which is far
more complicated because of the higher Bernoulli pressure
caused by the small reed apertures and the more
pronounced beating regime. the tuning problemDespite taking into account all the
parameters infuencing the pitch -those related to the (frequency
dependent) phase delay caused by the tonehole reflectance
filter, the (constant) delay induced by the extra flow in
the reed motion equation, and the fractional delay
filters- the instrument can be completely out of
tune sometimes. This was notorius in my Grallot,
probably because I neglected some crucial parameters (or
there could even be a serious mistake somewhere !).
Future workconical bores and mouthpiecesImplementing piecewise conical
waveguides is the next step in the model, to be able to
design bores of arbitrary shape. It is well known that
implementing conical sections can lead to numerical
instabilities and Csound could not work wery well until
double precision is implemented. Modelling a whole tonehole/register hole lattice might well be an exhausting experiment and I leave it for the future, as well as implementing the bell with the new Truncated IIR filters. defining playing stylesOnce you have a very
accurate woodwind instrument, sensitive to all the
playing parameters such as lips and mouth pressure, flow
pressure, fingerings, etc. how do you play such a beast
with a MIDI keyboard or even worse, with a numerical
Score written by hand? BibliographyA. Barjau, J. Agulló: Calculation of the Starting Transients of a Double-reed Conical Woodwind. Acustica, vol.69 (1989) A. H. Benade: Fundamentals of Musical Acoustics. Second edition. Dover publications, NY 1990. Gijs de Bruin, Maarten van Walstijn: Physical Models of Wind Instruments : A Generalized Excitation Coupled with a Modular Tube Simulation Platform. Journal of New Music Research, vol. 24 (1995) M. Pavageau: Synthèse d´un modèle physique d´anque double. Rapport du stage D.E.A. d´Acoustique Appliqué de l´Université du Maine. Septembre 1993 C. J. Nederveen: Acoustical Aspects of Woodwind Instruments.Revised edition. Northern Illinois University Press. DeKalb 1998 |