Source systems

Source systems generate three kinds of signal, shown in the Table.

1. The simple sinusoid is a continuous sine wave; it can only be approximated by real signals, which are always complex to some degree.
2. Complex signals can always be modelled as a sum of simple signals (Fourier's Theorem):
   1. Tones are sums of simple signals which are harmonically related, Tones are typically sonorants, particularly vowels, in speech.
   2. Noises are sums of simple signals which are not harmonically related. Noises are typically obstruents in speech.
   3. Superimposed tones and noises are typically the voiced obstruents in speech, in which the voicing component has the property of being a tone, and the obstruent component has the quality of being a noise.

Fourier's Theorem states that any signal (under certain idealisations) can be decomposed into a series of simple sinusoid signals, and any operation over a complex signal can be decomposed into operations over the simpler signals, if required. This property is often made use of in filtering signals in the frequency domain, by operations over particular sections of the frequency spectrum.