The function ftrep generates a three dimensional graph of one dimensional signals where the x axis represents the frequency of each signal, the y axis represents the duration of the signals and the z axis measures theirs magnitudes, the graph illustrate the concept of two independent variables (f,t) and explains the results we get when we calculate the Discrete Fourier transform of signals. However from theoretical view point, the time frequency uncertainty principle is not respected because the number of measurement is set to N=200 points and when we project on x axis we see pure discrete spectra which corresponds to infinite number of observations.

1. Projecting on y axis, we visualize a signal which is a superposition of K=length(f) sine waves.

2. Projecting on the (y,x) plane, we get the spectrogram, which obviously must consist of simple lines, in other words, no frequency change over time.

Demo: y=ftrep([10 15 33]);