11. The method of claim 1 , wherein said plurality of encoder modes comprises a NELP encoder mode, wherein the speech signal is represented by a residual signal generated by filtering the speech signal with a Linear Predictive Coding (LPC) analysis filter, and wherein said encoding comprises: (i) estimating the energy of the residual signal, and (ii) selecting a codevector from a first codebook, wherein said codevector approximates said estimated energy; and wherein decoding comprises: (i) generating a random vector, (ii) retrieving said codevector from a second codebook, (iii) scaling said random vector based on said codevector, such that the energy of said scaled random vector approximates said estimated energy, and (iv) filtering said scaled random vector with a LPC synthesis filter, wherein said filtered scaled random vector forms said synthesized speech signal.
12. The method of claim 11 , wherein the speech signal is divided into frames, wherein each of said frames comprises two or more subframes, wherein estimating the energy comprises estimating the energy of the residual signal for each of said subframes, and wherein said codevector comprises a value approximating said estimated energy for each of said subframes.