> What makes a signal weak/strong periodic ?

By an exactly periodic signal I mean something like sin(f*t). Such a
signal produces a very narrow peak of a characteristic shape in an
FFT, and so can be recognized even in the presence of quite strong
noise. If your signal is only quasi-periodic - perhaps the frequency
is a slowly-varying function of time - you'll have a much broader
peak, which will be much lower for the same input signal amplitude,
and hence more difficult to distinguish from noise. If your signal is
only quasi-periodic, or comes and goes in the data, you may want to do
a series of FFTs on short, overlapping pieces of the data, so you can
look at time evolution of the signal's spectral properties.


In my opinion this is not quite so. Periodic signal, as you rightly pointed out,
is a sinusoidal signal. Quasi-periodic signal behaves like a periodic one, even though
it does not satisfy the periodic condition x(t) = x (t+To), where To is the period. Best known
examples are when you add two sinusoidal signals with frequencies that are not a fractional integer of each other.
For example: sin(2pi f t)+sin(2pi^2 f t). You would still see a "spike" in the frequency domain, but quasi-periodicity
definitely does not relate to low frequency.

Ivo