Stria is a piece fully generated by the computer, with
the creation of all the parameters needed to play each
sound, starting by a certain number of input sessions,
elaborated by algorithms.
Using the division of the audio spectrum,
Chowning generated all the sounds with a unique
instrument: starting with the input parameters, the algorithms generated 30 parameters for each
instrument. The whole piece can be intended as played
by a 26 instruments orchestra: each instrument
generates one sound every time it is called to play; the
sound played is different at every call, having different
parameters used in the call itself. The basic scheme
used for the instrument is the FM modulator, with
double modulator: all the oscillators used were sine
functions, and the modulators were summed to the
carrier frequency. The sound was then shaped in time:
Chowning applied envelope generators to the
amplitudes of all the oscillators, thus varying the
amplitude of the signals, and changing the spectral
content of the sound in time. A light deviation (called
skew) was added to the frequencies of both the carrier
and the modulators in proportional way, to obtain a
major liveliness and reality. The two modulators
allowed Chowning to increase the spectral density
without using large indexes. (Matteo Meneghini,STRIA, BY JOHN CHOWNING: ANALYSIS OF THE COMPOSITIONAL PROCESS)