the inputs to a sound spatialization system. Thus, initially at least, source generation and spatial synthesis would remain as functionally separate components of an integrated acoustic display system. While there would necessarily be some overhead cost in controlling separate devices, the advantage is that each component can be developed, upgraded, and utilized as stand-alone components so that systems are not locked into an outmoded technology.

Synthesis Techniques

Many of the algorithms likely to be useful for generating nonspeech sounds will be based on techniques originally developed for the synthesis of music as well as speech. The main goal in speech synthesis is the production of intelligible (and natural sounding) speech waveforms. To do this, one must accurately synthesize the output of a specific type of instrument, the human voice. Also, in speech synthesis, the final acoustic output can be rated according to the measurable amount of information conveyed and the naturalness of the speech. In synthesizing music, typically the goals are not as specific or restricted: they are defined in terms of some subjective criteria of the composer. Usually, the goal is to produce an acoustic waveform with specific perceptual qualities: either to simulate some traditional, physical acoustic source or to produce some new, unique sound with appropriate attributes.

Because the aims of synthesized music are more diverse than those of speech synthesis, there are a number of different, acceptable methods for its synthesis. Choice of the method depends on the specific goals, knowledge, and resources of the composer. In any synthesis method, the composer controls some set of time-varying parameters to produce an acoustic waveform whose perceptual qualities vary accordingly. These computer-controlled parameters may be related to a physical parameter in a modeled instrument, to the shape or spectrum of the acoustic waveform, or to the perceptual quality desired for the sound. Often, these varying techniques are combined to get a specific effect. Some of the most common techniques are described below.

One method used for computer-controlled synthesis is known as additive synthesis. In this method, a synthesized voice (or instrumental line) is generated by the addition of simple sine waves using short-time Fourier methods. One of the problems with this method is that the number of parameters needed to generate an acoustic signal with specific qualities is large, reflecting the fact that many important music percepts are far removed from the short-time Fourier parameters. Thus, synthesizing a particular sound quality can be cumbersome. Often, additive synthesis is used to simulate known sounds by first analyzing the desired sound and



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