Until now we have dealt with signals represented in the time domain, that is to say, we have described
the signal by means of its value on the time axis. It is possible to use another representation for the same
signal: that of the frequency domain. Any signal may be described as a continuum of sine waves having
different amplitudes and phases. The frequency representation describes the signals by means of the
amplitudes and phases of the sine waves. The transformation between the two representations is given
by the Fourier transform (FT):