Presented at the 1996 International Computer Music Conference, Hong Kong. 1 Modeling and Control of Performance Expression in Digital Waveguide Models of Woodwind Instruments Gary P. Scavone [email protected] Center for Computer Research in Music and Acoustics (CCRMA) Department of Music, Stanford University Stanford, California 94305 USA Abstract Most digital waveguide models of woodwind instruments to date have provided only basic con- trol parameters such as frequency, breath pressure, and vibrato. It is clear that if these models are to gain popularity as real-time performance synthesizers or as composition tools, more flex- ibility is necessary. This paper discusses the implementation of expressive controls for flutter tonguing, growling, tone-bending, multiphonics, and variation of attack style. These effects are implemented on existing clarinet and flute waveguide instruments using the application SynthBuilder on a NeXTStep computer platform. Finally, control of these expressive effects using MIDI controllers is discussed. 1 Introduction struments is a particularly important aspect of in- strument performance and offers enormous expres- Research in physical modeling of woodwind in- sive flexibility to both the composer and performer. struments has largely focused on methods to ac- Further, this attack information is a critical ele- curately represent the instrument bore (V¨alim¨aki ment in distinguishing different instruments from and Karjalainen, 1994), toneholes (V¨alim¨aki et al., one another. Breath pressure modifications, such 1993), and nonlinear excitation mechanism (Scav- as flutter tonguing, growling, and singing into the one, 1995). The understanding of these issues is far instrument, are possible in all wind instruments, from complete and work should continue to improve though they are a more common element of wood- the existing models. However, there is a growing wind instrument performance. The production of demand by composers and musicians to use phys- multiphonics, achieved by non-traditional finger- ical models in new compositions and performance ings, is particular to woodwind bores with tonehole settings. In these cases, the models need more flex- lattices. ibility so as to produce a wide variety of sounds, both similar to real instruments and sounds which would be physically impossible in the real world. 2.1 Attack Variation This paper first presents methods for achieving such flexibility within the context of digital wave- A variety of attack styles are possible in wood- guide modeling. The control of these extensions us- wind instruments, ranging from breath attacks to ing MIDI controllers is discussed in the second part extremely percussive, “slap tongue” effects. Most of this paper. The implementation of performance models typically implement only breath-like styles expression within the context of stringed instru- of attack. Hard tonguing effects are achieved by ments was previously discussed by Jaffe and Smith using the tongue to briefly push the reed against (1995), but issues of real-time MIDI control were the mouthpiece facing, stopping the reed vibra- not considered. tions and air flow into the mouthpiece. The rapid increase in pressure and air flow into the mouth- piece upon removal of the tongue from the reed, 2 Modeling Performance Ex- together with noise produced by this highly tur- pression bulent initial air flow, produces the resulting at- tack sound. Lighter tonguing effects are created The expressive controls discussed here fall into by briefly interrupting the reed vibrations with the three principal categories – attack variation, breath tongue and lesser degrees of flow interruption. The pressure modification, and bore manipulation. The upper half of Figure 1 represents a common method attack or onset of sound generation in musical in- for implementing a breath attack. The breath noise scaler controls the level of noise present in the Growling and singing are audio rate modulations of steady-state sound. A tongued attack is imple- breath pressure. One interesting non-physical ex- mented with an additional burst of DC pressure tension possible in the digital domain is modulation and noise, as shown in the lower half of Figure 1. with speech signals, particularly fricative sounds. The tonguing envelope controls the magnitude and duration of the attack and should have a shape of Typical Breath Attack Implementation x Breath the form xe− . Scaling of the tonguing envelope Pressure Envelope corresponds to “hardness” of attack and provides Breath an important performance expression control pa- Noise Scaler rameter. The relative degree of air flow stoppage is Noise Generator X + controlled with the tonguing noise scaler. Breath Modulation Pressure Irregularity Output Typical Breath Attack Implementation Scaler Constant = 1 + X + Breath Oscillator Pressure Frequency Envelope Control Breath Modulator Noise Frequency X Scaler Breath Oscillator Noise X + Breath Figure 2: Breath Pressure Modulation System Pressure Output X + The implementation of these effects can be achieved using the system depicted in Figure 2. A Tonguing Noise sinusoidal signal of some desired modulation fre- Scaler Tonguing quency is added to the original breath pressure Envelope and the modulation frequency is randomly var- Switch Closed for Keyboard Control ied around its mean to attain a more realistic modulation signal. Modulation of breath pressure Figure 1: Tonguing System with speech can be achieved using recorded signals, though memory considerations in real-time DSP Slap tonguing is an effect whereby the reed is implementations often make this prohibitively ex- pulled away from the mouthpiece lay using a suc- pensive. A more desirable implementation provides tion force between the reed and tongue. When the real-time digital input via a microphone, which can elastic restoring force of the reed becomes greater be scaled and added directly to the breath pressure than the suction force between the tongue and reed, signal. As discussed later in conjunction with wind the reed separates from the tongue and “slaps” controllers, a breath pressure sensor sampled in the against the mouthpiece lay. Varying amounts of range of 2 kHz would be ideal for breath pressure breath pressure are then added to produce a range control and eliminate the need for most of the sys- of effects from dry to fully sounding. Implemen- tem in Figure 2. tation of this effect can be accomplished in sev- eral ways. One method involves the recording of 2.3 Multiphonics and Pitch Bending a dry slap with the mouthpiece removed from the instrument. This signal is then added to the nor- Multiphonics are a common contemporary perfor- mal breath pressure signal and input to the instru- mance technique produced on musical instruments ment’s nonlinear excitation. This effect can also be which have a tonehole lattice. Acoustically speak- achieved by approximating the slap with a prede- ing, non-traditional fingerings produce air column termined filter impulse response, which is added to resonances which are not harmonically related, but the breath pressure signal. which are strong enough to entrain simultaneous inharmonic reed oscillations. The resulting tone 2.2 Breath Pressure Modulation is heard as comprised of two or more synchronous and distinct pitches, or as a tone with a rough and Several extended performance techniques involve beating quality. the superposition of higher frequency components The most accurate method of modeling this with the DC breath pressure applied to the instru- phenomenon is to implement a full series of tone- ment. This type of modification is referred to here holes which exactly reconstruct the real instrument as modulation, though not in the strict sense of am- behavior. The present understanding of tonehole plitude or frequency modulation. Flutter tonguing, behavior and interaction does not allow realization accomplished using either the tongue or ventricular of this goal and designs using current tonehole mod- folds (false vocal folds), simply amounts to the ad- els would prove extremely difficult to properly tune. dition of a 15 – 30 Hz signal to the breath pressure. Further, the complexity of such a model would 2 make real-time performance difficult to achieve in ing ways to control these behaviors in real time most situations. An efficient, though non-physical, is far from simple given current MIDI standards technique for generating multiphonics is to add and controller technology. It is obvious that MIDI more bores to the model, each of different length. was not designed to handle extended techniques. In terms of digital waveguide modeling, this corre- Is it possible to adequately control these parame- sponds to the addition of more delay lines and a ters without inventing a new protocol? The clear summing operation for feedback to the excitation choice for controlling woodwind synthesis models mechanism, as shown in Figure 3. Each delay line is a MIDI wind controller. However, the few wind represents a particular resonance and set of over- controllers commercially available offer only basic tones, while the input scalers roughly control their features and prove inadequate for the control of relative strengths. most of the extended techniques discussed in this paper. The MIDI keyboard is also far from ideal in Delay Line Input Scalers this context, but it must be supported for histori- Delay Line cal and pragmatic reasons. The limitations of the Breath Nonlinear keyboard are sometimes circumvented by providing Pressure Delay Line Excitation Input