A method for forming a global spectrum (Sg) of an analogue signal (A) to be digitized, in which: the analogue signal (A) is sampled with a first analogue-digital converter (21, 22) of determined Shannon frequency (Fs) so as to obtain an aliasing spectrum (Sr), the analogue signal (A) not having been previously filtered by an anti-aliasing filter; a base spectrum (Sb) is subtracted from the aliasing spectrum (Sr) so as to obtain an aliased spectrum (Sre), the base spectrum (Sb) corresponding to an aliasing-free spectrum of the said analogue signal; a dealiased spectrum (Sd) is computed on the basis of the aliased spectrum (Sre) as a function of the Shannon frequency (Fs) of the converter (21, 22); the base spectrum (Sb) is concatenated with the dealiased spectrum (Sde) so as to form the global spectrum (Sg) of the analogue signal (A).