NORDIC ACOUSTICAL MEETING 12-14 JUNE 1996 HELSINKI Dept. of Computer Science, University of Copenhagen Universitetsparken 1, DK-2100 Copenhagen Ø, Denmark krist@diku.dk 1 INTRODUCTION Acoustical instruments can be divided into two classes, envelope-based instruments, and continuous-control instruments. Some instruments, such as the bowed string instruments, permit the execution of both techniques, envelope-based by plucking the string, or releasing the bow, and continuous-control by stroking the bow on the strings. Most instruments only permit one class of control, which can further be divided into several subclasses. These classes, or different control mechanisms will be outlined here, for some typical acoustic instruments, along with the perceptive outcome of this control. Based on this analysis, a model of continuous control of electronic music instruments is outlined, which permits the control of a large number of parameters on an instrument with less sensors. The continuous controls of electronic instruments are a possibility today when the instruments are becoming realistic enough, and especially considering that some of the new synthesis algorithms, i.e. physical modelling, are excellent for controlling many perceptually important dimensions of the sound. In some related topics [3] deals with the representation of continuous music signals, [4] adds rules for duration and soundlevel in computer performance. [5] discusses the shortcomings, and [6] proposes a replacement of midi. 2 THE PARAMETERS OF THE SOUND 2.1 The Amplitude One important control is the amplitude control. The amplitude control can be executed in two manners, limited control on a envelope, or continuous control, as in the bowed string instruments. The piano sound, for instance, has an envelope with the typical attack, decay, release behaviour. The instrumentalist has two parameters that controls the amplitude; the velocity, which controls the overall envelope, and the time each key is pressed, which decides the length of the decay period. 2.2 The pitch
The pitch has several different control techniques, the discrete pitch control, as in the flute, the contained pitch control, i.e. the pitch can be varied in a limited range, vibrato, as in the guitar, and finally there's the continuous pitch control, as in the violin. 2.3 The timbre The timbre is the amount of energy in the different harmonics, or frequency components, and the temporal evolution of this energy. Some instruments, as the violin, has control over many timbre dimensions, others have a fixed timbre. 2.4 The noise Some instruments permits the addition of a noise component on the sound. This can be breathing noise, bowing noise, tapping noise, etc. Also, playing at the limit of a resonant mode can increase the noise component. 2.5 The inharmonicity The bowed string instruments has control over the inharmonicity. The inharmonicity increases for instance in the violin, when bowing in an angle to the string, which increases the longitudinal vibration. See [1, p. 133] for a mathematical study of this phenomenon. 2.6 The space Most acoustic instruments permits to be moved and pointed at different directions. Moving the instrument changes the amplitude and the timbre of the sound, which is interpreted by the listener, who thus understands the location of the instrument. 3 THE PARAMETER CONTROL In the different musical instruments, there are several ways of controlling the different parameters of the sound. Some of the parameters have no control, as the pitch in a bell, whereas the violin has full control of the pitch. Some control patterns can be singled out, such as, no control, NC, discrete control, DC, contained control (vibrato), CC, fixed envelope, FE, adaptive envelope AE ( the length of the sustain period is controllable), limited continuous control, LC, and full continuous control FC. Table 1 lists the control of some typical instruments on the different parameters of the instrument. Table 1. The control structure of acoustic instruments Instrument Pitch Amplitude Timbre Inharmonicity Space Bell NC FE NC NC NC Guitar CC AE LC NC LC Violin FC FC/AE LC LC LC Trumpet CC FC LC NC FC Flute DC FC LC NC FC Voice FC FC FC NC FC
In the actual instrument, things are rarely so simple as depicted in the above table. In order to understand what is really happening in the physical instrument, the control of a few instruments is outlined below. 4 SAMPLE INSTRUMENTS. 4.1 The violin. The violin [1], [7], [8] is a bowed string instrument, which share the control characteristics with most of the other instruments in this family. The bowed instruments include the violin, the viola, the cello, and the contrabass. See [2] for a more thorough analysis of the violin control. 4.2 The trumpet. The trumpet [1], [10] is a lip-driven brass instrument, which produces sounds by buzzing the lips in the mouthpiece. This buzzing is produced when the blowing pressure forces the lips apart. The lips can vibrate only at certain frequencies, defined by the resonance of the horn, which in turn depends on the form, and the length of the horn. The trumpet can thus only be played at the modes of the instrument. The upper modes of the trumpet can be placed so as to correspond to musical intervals, but the lower modes lie too far apart to be useful in the diatonic scale. The gap is filled by changing the length of the cylindrical part of the horn. The trumpet has three valves, which lowers the tone one, two and three semitones. The valves are not tuned exact to the semitone, so pressing one and two semitones valves at the same time gives a slightly higher frequency than pressing the three semitone valve. This permits to adjust the tones. The three semitone valve also have a variable extension which permits to lower the tone an additional halftone. The range is about 2 1/2 octaves, but skilled players can reach even higher tones, by better control of the lips. Some tones can be reached either by changing the valves, or the lip form, but this is rarely used, the rule being to have as few valves open as possible. The tone ceases when the air flow stops, either by controlling the lungs, or by stopping the airflow with the tongue. The tongue is also used to get a faster attack, and to repeat the tones. The most common tongue forms are the k and the t forms, the t form being slightly sharper. The blowing force decides the amplitude and the timbre, more high frequency energy gets added with the blowing force. The form of the lips also influences the timbre, but it is very hard to change this without changing either the blowing pressure, or the frequency. Vibrato is introduced by hardening and softening the lips. A noise component, typical for the instrument, is added when playing at the border of the resonance. It is common to change the timbre on the trumpet by introducing an object in front of the mouth of the trumpet. This can be either the hand, or different kind of dampers. 4.3 The saxophone.
The saxophone [1],[9] was developed by Adolph Sax 150 years ago. It is a woodwind reed instrument, and includes soprano, alto, tenor and baritone models. The sound is created when the tongue is pressing the reed against the mouthpiece, thereby allowing it to vibrate when blowing air through it. The pitch is changed by opening and closing holes, with the help of a elaborate system. The instrument covers 2 1/2 octave, but higher frequencies can be reached by tuning the mouth to a higher mode. Some pitches can be reached by several different combinations, to facilitate the quick transition from one tone to another tone. The sound radiates from the open mouth of the instrument, and from the open holes. The combination of radiation gives a stable sound at about 1 meter from the end of the saxophone. To get an even tone that covers the whole range of the instrument, it is necessary to change the blowing technique for each tone. The blowing strength, the blow direction (up/down), the stiffness of the lips, the tongue, and the size of the cavity in the mouth can influence the stability of the sound. When the mouth is tuned to the pitch, the attack is facilitated, and the tone appears strong and stable. It is, of course, necessary to tune the mouth prior to starting the tone. The strength of the air blow decides the amplitude and the timbre of the sound. The upper harmonics gets more energy when the blowing force increases. The direction of the blowing jet is changed by moving the jaw back and forth. This changes the timbre of the sound. Moving the jaw back gives a softer tone, moving the jaw forth yields a more percussive sound. The size of the cavity in the mouth decides the stability of the tone. Deeper tones demands a larger cavity. Vibrato is produced by lowering the lower jaw and lip. This lowers the pitch of the tone slightly. The sound also softens slightly when lowering the jaw. Other parameters includes sitting/standing position, and, of course, the direction of the sound. When all the above parameters are tuned up, the saxophone changes pitch. Higher tones can thus be reached. The octave is the most common choice, but the fifth over the octave can also be reached. It is also common to introduce a noise in the sound by playing at the limit of the resonance. This noise can also be produced by "singing" into the mouthpiece. 3.4 Conclusion The musical instruments can thus be controlled by the fingers, hands, mouth and body in several different perceptual dimensions, including the amplitude, pitch, timbre, noise component, inharmonicity and space. Many of these controls are continuous. 4 DISCUSSIONS It is important to notice that the different parameters of an acoustic instrument cannot be set outside the limits of the instrument. This means, for instance, that whatever you do to change the timbre, the instrument is still recognisable. Many of these limits are not the product of any physical law, but the result of many years of practice to avoid the displeasing sounds.
Thus, the musician can place less emphasis on one or more of the parameters, and still be sure that they obey the laws of the instrument, while instead concentrating on another parameter. For instance, when prolonging one note, emphasis might be put on the stable amplitude, leaving the pitch and timbre fixed, but when playing a succession of fast notes, emphasis will be put on the correct pitch. In an electronic instrument, it is important to define the range of each parameter, avoiding values which displaces the identity of the instrument, but allowing a wide enough range so the musician is able to express himself through the instrument. Furthermore, it is necessary to define some typical functions for each parameter, for instance, envelope, fixed value, low frequency oscillating, or a sampled envelope. When the physical interface, as a musical keyboard, doesn t have enough manipulators, the musician can then remove one parameter from the sensor, leaving it on its trail, and assigning another parameter to the sensor, controlling another aspect of the sound. The musician can thus switch the control from parameter to parameter, by ensuring the correct behaviour of the other, non-controlled, parameters, which follows a defined vector. 5 CONCLUSION The control mechanisms of some acoustic instruments have been studied, and a list of typical controls have been shown. This list contains many more control possibilities than any electronic instrument. An approach to this problem is made by proposing a control mode which will permit the musician to control more parameters with less sensors. Musicians and performance experiments should be involved in further research with the proposed model. 6 REFERENCES 1. Fletcher, N. H. & Rossing T.D. The Physics of Musical Instruments. Springer-Verlag, 1988. 2. Jensen, K. 1996. The Control Mechanism of the Violin. Nam-96 Proceedings. 3. Desain, P. & Honing, H. On continuous Musical Control of Discrete Musical Objects. ICMC Proceedings 1993. 4. Friberg, A. Generative Rules for Music Performance: A Formal Description of a Rule System. CMJ 15(2), 1991. 5. Moore, F.R. The Dysfunction of MIDI. CMJ 12(1), 1988. 6. The ZIPI Music Interface Language, CMJ 18(4), 1994. 7. Martins da Fonseca, A. Private Conversation 1995-1996. 8. Hansen, K. Private conversation. 1996. 9. Thorsbro B. Private conversation. 1996. 10. Green T. Private conversation. 1996.