CELP coding with two-stage search over displaced segments of a one-dimensional codebook

A decoder comprising

• an input at which an optimal excitation sequence is received, the optimal excitation sequence having been produced according to the following operations:
• deriving a plurality of excitation sequences that are parts of a main sequence, the parts being mutually displaced over a plurality of positions,
• selecting an excitation sequence comprising:
• deriving at least one further excitation sequence that is a part of the main sequence, the at least one further excitation sequence being displaced with respect to the selected sequence by a distance smaller than the distance between the plurality of positions;
• selecting the optimal excitation sequence from the selected excitation sequence and the at least one further excitation sequence, the optimal excitation sequence being that one which minimises an error between a synthetic signal derived from the further excitation sequence and a target signal derived from an original signal;
• in response to the selecting of the optimal excitation sequence, transmitting the optimal excitation sequence;
• a unit comprising
• an excitation signal generator for deriving the optimal excitation sequence from a signal representing the optimal excitation sequence, and
• a synthesis filter for deriving a synthetic signal from the signal representing the optimal excitation sequence.