A mixed prototype waveform/CELP coder for sub 3kb/s
Lee Burnett, R.J. Holbeche · International Conference on Acoustics, Speech, and Signal Processing · 1993
CELP Analysis-by-Synthesis speech coders do not make a distinction between voiced and unvoiced speech frames. For sub-3kb/s coding it is necessary to separate unvoiced and voiced frames and code voiced speech using an inherently periodic scheme. This paper addresses these problems by using a prototype waveform coder for voiced frames, while retaining a CELP algorithm for unvoiced frames. For voiced speech a single 'residual prototype' is selected to represent a section of 25ms. Prototypes are interpolated across the frame to provide a smooth evolution of amplitude and harmonic content. Two coding schemes for the prototypes are discussed; a pitch harmonic scheme operating in the DFT domain, and an impulsive codebook time domain technique. Unvoiced frames are coded using a standard CELP architecture excluding the adaptive codebook search. The overall bit rate using either of the voiced frame coding algorithms is shown to be sub 3kb/s for good communications quality speech.