A study on short-time phase and multipulse LPC

Multipulse LPC, as is often designated the model proposed by Atal and Remae, has been directed towards 9.6 kb/s speech coding. However, at such bit rate, the speech quality is not yet generally acceptable. This paper has a double purpose - one is to investigate the role of short-time phase in multipulse LPC. The other is to look for different modelling structures to be used with this method. This study was prompted by direct observation of the structure of the excitation, where patterns of pulses may be found which are associated with phase-correcting mechanisms of the LPC impulse response. Consequently, a new multipulse technique was developed, based on a special ARMA model formed by cascading an all-pole with an all-pass network. This new model will be referred to as the MAPAP (Multipulse All-Pole All-Pass) method. Another one was tried in which the synthetic speech is formed by combination of several MAPAP signals. We therefore denoted it "multichannel multipulse" method. The potential advantages of both single and multichannel models seem rather promising.