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An instrument's tone color refers mainly to the harmonic over tone structure of each musical note. Tone color or timbre is a complex acoustical phenomenon, and we are limited by present day technology in our ability to sense it or represent it as a control voltage. The closest thing to a "timbre" extractor that we have in present day musical products is a vocoder, which samples the signal's overtone structure in a number of different frequency bands, and converts the output of each frequency band into a control voltage. This would present an information overload in present instrument-controlled synthesizer architectures. A simpler method of approximating a "tone color" parameter makes use of the fact that, in most musical instruments, the overtone structure is "brighter" (contains more harmonics) as the instrument 's loudness increases. The relationship between brightness and loudness is not simple, but we can link loudness and timbre together and obtain pleasing results. So the bright- ness parameter is a control voltage derived from the loudness parameters.

In figure 1, the system components for parameter extraction in an instrument-controlled synthesizer are shown. In Figure 2, a musical sequence and its associated control parameters are displayed. By studying these diagrams, you will have a good idea of the process of parameter extraction.


ARCHITECTURE OF AN INSTRUMENT-CONTROLLED SYNTHESIZER

In the last section and in Figure 1, we constructed a system to derive control parameters from a musical signal input. This system is essentially the front end of an instrument-controlled synthesizer. It functions as the interface between the input signal and the synthesizer guts the same way as the mouthpiece and buttons on a saxophone function as the performer interface. You play the saxophone with your lungs, embouchure and fingers through the mouthpiece and buttons; and in the same way you play the instrument-controlled synthesizer with the output of your saxophone through the instrument-interface system.

The rest of the instrument-controlled synthesizer is essentially a voltage-controlled synthesizer, similar in many ways to all the synthesizers produced since Moog put out his first one in the 1960's. The available products differ markedly in their approach to synthesis systems, but they all employ adaptations of basic synthesizer elements. (In Part 2 of this article, we will go into specifics.)

The synthesis section connects to the instrument-control interface by means of the control parameters derived from the musical instrument input. Figure 3 shows this basic system, while figure 4 provides a detailed representation of the subsystem components.

The synthesis section consists of one or more voltage controlled oscillators, a voltage-controlled filter, voltage-controlled amplifier, and one or two envelope generators. The interconnection between the synthesis components and the control interface components is dependent on the complexity of the synthesizer. The system of figure 4 represents a complex system which makes full use of all the information we have gathered about the musical sound, as described earlier.

The voltage-controlled oscillators, which form the basic tone-generation system for the synthesizer, are controlled by the output of the pitch follower. Thus, the synthesizer will provide tones which track the pitch of the input signal, either in unison or at some musical interval selected by the user.

The VCO outputs are filtered by the voltage-controlled filter to provide the desired tone color. In a standard synthesizer, the voltage-controlled filter is swept by an envelope generator to create a spectral envelope. In the instrument-controlled synthesizer depicted here, we have the option to make the filter much more responsive to the musical information we derived from the input signal. By controlling the amplitude of the filter envelope with the brightness control voltage, we can make the synthesizer's tone color more life-like, responsive to the musical expression contained in the brightness of the original signal. Alternatively, we can control the voltage-controlled filter by the output the envelope follower, so that the filter's "contour" exactly duplicates the instrument's loudness envelope. This would simulate the familiar "envelope-controlled filter" musical effect, except that in this case a synthesized signal is filtered instead of the controlling instrument's own signal. Both methods of dynamic filtering are found on currently available instrument-controlled synthesizers.

The voltage-controlled amplifier, which provides the amplitude contour of the filtered signal, is driven by a control voltage from the VCA envelope generator. Once again, the envelope generator can be controlled by the loudness parameters detected by the instrument parameter extraction system. This kind of control will give the instrument-controlled synthesizer a realistic set of loudness dynamics responsive to the input signal.

Not all instrument-controlled synthesizers have this degree of sophistication and dynamic interaction with the controlling musical instrument. The essential elements of instrument-controlled synthesis systems, as described, are the basis for all product designs that have appeared.

USING AN INSTRUMENT- CONTROLLED SYNTHESIZER

At a recent NAMM convention, Don Tavel of Musico (makers of the Resynator Instrument Controlled Synthesizer) made a comment which sums up the essence of the proper use of instrument-controlled synthesizers. "A guy comes up to play the Resynator, which is set up to sound like a trumpet. He is playing it with the guitar, but instead of playing with trumpet dynamics and trumpet phrasing, he is using guitar dynamics and guitar phrasing. How does he expect it to sound like a trumpet unless he plays it like a trumpet? "That is a very subtle statement, actually. It means that you want to play "through" the whole system with the end product in mind. It implies a knowledge and "pre-visualization" of the final sound and articulation you want to convey. Most important, it calls for a musical technique which en- compasses the characteristics and limitations of both your native instrument and your instrument-controlled synthesizer. Guidelines for playing instrument-controlled synthesizers are very similar to guidelines for playing any musical instrument, once you understand that your musical instrument's output (rather than your fingers) is playing the synthesizer. Playing "clean" and "distinctly" will yield the best results. You really, at first, have to "tell" the instrument exactly what you want. If you are a wind player, remember that breath noise and indistinct note transitions will provide misleading information to the pitch and envelope-follower circuits. If you are a guitar or keyboard player, remember that you need to play one note at a time. This may seem easy; but you should realize that strings can vibrate from harmonic excitation, or shocks to the musical instrument, or imprecise playing. Any of these additional "noises" will throw off even the best pitch-to-voltage converter. On the guitar, you can learn to "damp" strings with the fleshy parts of either hand, allowing only the melody string to vibrate. Pickups and tone controls on the instrument should be adjusted to provide maximum volume and a strong fundamental. On a guitar, the center pickup is usually best. Sometimes, on an instrument with a low output level, preamplification can help. Each available instrument-controlled synthesizer has one or more "sensitivity" or input controls. Finding the optimum settings of your instrument's controls and the synthesizer controls will pay for the time spent. Once you master the techniques to make the synthesizer track your instrument optimally, you will be ready to explore the subtleties of the synthesizer. Many differences exist in the form and layout of the "synthesizer" controls on different instrument-controlled synthesizers. In fact, some notable advancements in synthesis technology have appeared in these products. In the next article, I will discuss some of the commercially available products with the objective of exploring their different approaches to synthesis techniques. I hope your interest in these new products has been stimulated, and that you will help bring them into the musician's repertoire of performing instruments.

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