LSF quantisation for pitch synchronous speech coders

The quality of low bit-rate speech coders is reduced at transitions where speech spectral characteristics vary significantly, as usual speech parameter interpolation assumptions fail to correctly model such variations. This paper presents a joint quantisation-interpolation algorithm for coding of LPC parameters in pitch synchronous speech coders to model the rapidly evolving parameters. In this technique a number of sets of pitch synchronous LPC parameters, corresponding to a frame of speech, are jointly coded by coding two reference sets of LSF´s and an interpolation trajectory. Coding an interpolation function allows the parameters to vary within the set. The proposed joint quantisation-interpolation coding of the pitch synchronous LSF is evaluated by comparison with time synchronous extraction and linear interpolation. It is also compared with linear interpolation between sets of pitch synchronous LSF´s. Comparison results show that the joint quantisation-interpolation method reduces the average spectral distortion when compared to fixed interpolation. The proposed quantiser was included in to the PS-SBLPC coder and informal listening tests carried out. The synthesised speech was found to be of better quality when joint quantisation-interpolation is used.