Method of generating a footprint for a useful signal, wherein the useful signal represents the evolution of a spectrum comprising useful signal frequencies, for example audio frequencies, over time, which allows automatic detection of identical or similar useful signals in a cost-efficient way and where the footprint is robust against modifications of the useful signal not perceptible to human users, wherein at least one data set comprising a part of the useful signal is processed by an analyzer according to a predetermined analyzing instruction, where the analyzer outputs as a result of the processing a footprint data vector depending on and identifying the processed data set.