Equipment

Synthesizers : sound basics

Harmonic frequencies

When natural tonal instruments' sounds are analyzed in the frequency domain, the spectra of tonal instruments exhibit amplitude peaks at the harmonics. These harmonics' frequencies are primarily located close to the integer multiples of the tone's fundamental frequency.

Percussives and rasps usually lack harmonics, and exhibit spectra that are comprised mainly of noise shaped by the resonant frequencies of the structures that produce the sounds. The resonant properties of the instruments (the spectral peaks of which are also referred to as formants) also shape the spectra of string, wind, voice and other natural instruments.

Conventional synthesizers

In most conventional synthesizers, for purposes of resynthesis, recordings of real instruments can be thought to be composed of several components.

These component sounds represent the acoustic responses of different parts of the instrument, the sounds produced by the instrument during different parts of a performance, or the behavior of the instrument under different playing conditions (pitch, intensity of playing, fingering, etc...) The distinctive timbre, intonation and attack of a real instrument can therefore be created by mixing together these components in such a way as resembles the natural behaviour of the real instrument. Nomenclature varies by synthesizer methodology and manufacturer, but the components are often referred to as oscillators or partials. A higher fidelity reproduction of a natural instrument can typically be achieved using more oscillators, but increased computational power and human programming is required, and most synthesizers use between one and four oscillators by default.

Amplitude envelope

One of the most salient aspects of any sound is its amplitude envelope. This envelope determines whether the sound is percussive, like a snare drum, or persistent, like a violin string. Most often, this shaping of the sound's amplitude profile is realized with an "ADSR" (Attack Decay Sustain Release) envelope model applied to control oscillator volumes. Each of these stages is modeled by a change in volume (typically exponential). The attack is the initial run-up of the sound level. The decay is the run down immediately after the attack. Sustain is the volume when the note is held. The release is the volume profile when the note is released. Exponential rates are commonly used because they closely model real physical vibrations, which usually rise or decay exponentially.

Although the oscillations in real instruments also change frequency, most instruments can be modeled well without this refinement. This refinement is necessary to generate a vibrato.

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