__Digital
signals spectra____ __

To determine the spectrum of a digital signal, the formula used is the discrete Fourier transform. This digital Fourier transform is deduced from the analogue expression mainly by replacing the integral by a discrete sum S.

__Fourier transform of a discrete signal
(FTDS____)__

This expression is a periodic continuous function of the
frequency f. This expression recalls the expression of the Fourier series
development, with the frequency and the time having "exchanged their roles". It
is a consequence of the **time-frequency duality**

__Inverse Fourier transform of a discrete signal
(IFTDS)__

This expression also looks like the definition of the cn coefficients of the Fourier series development. It is again a consequence of the time-frequency duality.