Abstract: A digital signal is transformed in at least two different frequency bands according to at least two different resolutions. The signal is divided into first blocks all having the same predetermined first number of samples. Each of the first blocks is transformed into a plurality of second blocks, any second block under consideration having a second respective number of samples which depends on the resolution of the second block under consideration, and containing samples selected according to their frequency. Second blocks issuing from the transformation of different first blocks are grouped in order to form third blocks all having the same predetermined third number of samples which is at least equal to the largest of the second numbers.