New innovations in multi-pulse speech coding for bit rates below 8 kb/s

Recently, MP-LPC [1] and other similar analysis-bysynthesis (A-by-S) coding schemes have proved that it is possible to achieve very high quality coded speech down to around 8 kb/s. In this paper we report on simulation results of a multi-pulse coder with pitch prediction at bit rates of 8 kb/s and below. The effects of various long term prediction configurations have been evaluated both subjectively and objectively. In order to reduce the bit rate and maintain the output quality, effi.cient quantisation of the MP-LPC parameters are required. Since in a multi-pulse coding scheme most of the bits are used to code the pulses. we have investigated various methods of' adaptive pulse amplitude quantisation. Finally we report on a new joint quantisation scheme of the pulse amplitudes. which resulted in reductions of up to 1000 bits/sec with no impairment in the speech quality.