The indexing method comprises forming a set of tracks of pulse positions, restraining the positions of the non-zero-amplitude pulses of the combinations of the codebook in accordance with the set of tracks of pulse positions, and indexing in the codebook each non-zero-amplitude pulse of the combinations at least in relation to the position of the in the corresponding track, the amplitude of the pulse, and the number of pulse positions in said corresponding track. For indexing the position(s) of one and two non-zero amplitude pulse(s) in one track, procedures code.sub.--1 pulse and code.sub.--2 pulse are respectively used. When the positions of a number X of non-zero-amplitude pulses are located in one track, X.gtoreq.3, subindices of these X pulses are calculated using the procedures code.sub.--1 pulse and code.sub.--2 pulse, and a global index is calculated by combining these subindices.