1. A method of encoding speech according to a source-filter model whereby speech is modeled to comprise a source signal filtered by a time-varying filter, the method comprising: receiving a speech signal; from the speech signal, deriving a spectral envelope signal representative of the modeled filter and a first remaining signal representative of the modeled source signal; at each of a plurality of intervals during the encoding, determining a period between portions of the first remaining signal having a degree of repetition and determining a correlation between said portions based on said period effective to produce a respective vector of the correlation for each interval, each vector comprising a plurality of parameters derived from the respective correlation; once every number of said intervals, selecting a codebook from a plurality of codebooks for quantizing said vectors, quantizing the vectors of that number of intervals according to the selected codebook, and transmitting the quantized vectors along with an indication of the selected codebook over a transmission medium as part of an encoded signal representative of said speech signal.