100
D. Rocchesso: Sound Processing
fortissimo
control signals
control rate
Temporal envelopes
Attack - Decay - Sustain -
Release
Low-Frequency Oscillators
LFO
vibrato
tremolo
grains
granular synthesis
asynchronous granular
synthesis
Sampling fortissimo notes and obtaining lower intensities by dynamic
filtering (usually lowpass).
In wavetable sampling synthesis, the control signals are extremely important
to achieve a natural sound behavior. The control signals are tied to the evolution
of the musical gesture, thus evolving much more slowly than audio signals.
Therefore, a control rate can be used to generate signals for
· Temporal envelopes (e.g., Attack - Decay - Sustain - Release);
· Low-Frequency Oscillators (LFO) for vibrato and tremolo;
· Dynamic control of filters.
5.2.4
Granular synthesis
(with Giovanni De Poli)
Short wavetables can be read at different speeds and the resulting sound grains
can be concatenated and overlapped in time. This time-domain approach to
sound synthesis is called granular synthesis. Granular synthesis starts from the
idea of analyzing sounds in the time domain by representing them as sequences
of short elements called "grains". The parameters of this technique are the
waveform of the grain g
k
(·), its temporal location l
k
and amplitude a
k
s
g
(n) =
k
a
k
g
k
(n - l
k
) .
(14)
A complex and dynamic acoustic event can be constructed starting from a large
quantity of grains. The features of the grains and their temporal locations de-
termine the sound timbre. We can see it as being similar to cinema, where a
rapid sequence of static images gives the impression of objects in movement.
The initial idea of granular synthesis dates back to Gabor [26], while in music it
arises from early experiences of tape electronic music. The choice of parameters
can be via various criteria driven by interpretation models. In general, granular
synthesis is not a single synthesis model but a way of realizing many different
models using waveforms that are locally defined. The choice of the interpreta-
tion model implies operational processes that may affect the sonic material in
various ways.
The most important and classic type of granular synthesis (asynchronous
granular synthesis) distributes grains irregularly on the time-frequency plane in
form of clouds [77]. The grain waveform is
g
k
(i) = w
d
(i) cos(2f
k
T
s
i) ,
(15)
where w
d
(i) is a window of length d samples, that controls the time span and the
spectral bandwidth around f
k
. For example, randomly scattered grains within
a mask, which delimits a particular frequency/amplitude/time region, result
in a sound cloud or musical texture that varies over time. The density of the
grains within the mask can be controlled. As a result, articulated sounds cane
be modeled and, wherever there is no interest in controlling the microstructure
exactly, problems involving the detailed control of the temporal characteristics
of the grains can be avoided. Another peculiarity of granular synthesis is that
it eases the design of sound events as parts of a larger temporal architecture.