2.3 Synchronicity of granular synthesis

There are two distinct ways of using granular synthesis. These methods are quite different from each another and produce different textural effects.

They are:

Synchronous granular synthesis
Asynchronous granular synthesis

Synchronous granular synthesis is very closely related to Formant Synthesis, more commonly known as FOF synthesis.¹ FOF synthesis uses the addition of sinusoidal (FOF) grains to create FOF textures. It is considered basic synchronous granular synthesis, because of it's use of FOF grains which are similar to sine grains. The main difference though is the way they are used. FOF grains have very strict parameters. They use only sinusoidal contents, and the envelopes have a very strict structure. It is the parameters of the envelope which change the sound created, in conjunction with the FOF grain density. This method is very measured and predictive, therefore desired results are easier to achieve. A change in grain density affects the sense of pitch when using FOF grains (Roads 1996a: 299). This rule also applies when using synchronous granular synthesis, although it is not as predictive, due to the variety of grains that can be produced.

Synchronous granular synthesis uses a number of control parameters which are:
 

Amplitude
Grain Density
Pitch
Inter-onset time
                   - also attack and decay times.

2.3.1.2 Grain Density

The Grain Density is the single most important factor in creating granular textures. Initially, it determines how densely the grains are distributed within the texture, but the density affects a few different parts of the texture. Grain density is measured as the number of grains per second. There is really no limit on grain density, although the denser it is, the longer it takes to process the sound. There are many side effects of grain density which will increase as the density does.  The Grain density affects the timbre, the amplitude and the pitch of the texture. A sparse density of less than 30 grains per second will create a rhythmic effect. As the density increases the distinction of any rhythm will be lost and will give way to a pitch sensation determined by the grains. The higher the density the greater the spectral complexity of the timbre. Denser textures will yield more partials, formants and side bands in the texture making them richer. The amplitude will increase as the density increases. The pitch will rise as the density rises.

There is a direct relationship between grain density and grain duration. The longer the grains are, the more they will overlap making the texture denser at lower grain density rates.
 

2.3.1.3 Pitch

The sense of pitch can be set from the texture level. This can be done by playing the grains at different sampling rates. Normally the sense of pitch would be changed at the grain level, but it is possible to control it from a higher level as part of a change in frequency. As mentioned above the sense of pitch can be increased by increasing the grain density.
 

2.3.1.4 Inter-onset time

This function is associated very closely with the grain density and the synchronicity. It could be considered an obsolete parameter, because it can be fully determined by the density and the synchronicity. It is used mainly as a measurement, rather than a parameter. The inter-onset time is the duration between two adjacent grains.
 

2.3.2 Asynchronous granular synthesis

With asynchronous granular synthesis the grains are added to the texture without the strict linear relationship of synchronous granular synthesis. The relationship will contain random elements or there will be a very indistinct relationship. Asynchronous textures can be created by randomly spreading grains across the texture without worrying about timing.

The control parameters in asynchronous granular synthesis are:

Amplitude
Grain Density
Pitch
Inter-onset time

2.3.2.2 Grain density

As with synchronous granular synthesis, a very low density will cause a rhythmic effect. Once the density reaches a certain level the rhythmic effect will become a pitched effect. The pitch does not change as the density increases. The density is simply a parameter within which the grains are dispersed. For example if the density is specified at 200 g/s then 200 grains will be randomly (or statistically) distributed throughout that second.

2.3.2.3 Pitch

Initially this is the same as synchronous granular synthesis except that it plays a less noticeable role and it is not affected by density.
 

2.3.2.4 Inter-onset time

This is a measurement of the duration between the beginnings of adjacent grains and as such would change constantly. Asynchronicity can be achieved by setting up a random or statistical inter-onset parameter.