MULTIBAND CODE-EXCITED LINEAR PREDICTION (MBCELP) FOR SPEECH CODING

This paper presents a new speech coding model targeted at the bit-rate above 4 kbit/s, referred to as multiband code-excited linear prediction (MBCELP). The analysis and synthesis of speech are accomplished in the time domain by comparing the original to the synthetic speech while a perceptual criterion is used. A usual short-term linear predictive filter is employed as the synthesis filter; the excitation signal is modelled as a linear combination of a long-term predictive excitation, periodic multiband excitations and a noise-like excitation; no voiced/unvoiced decision is required. The periodic multiband excitation is produced by convoluting a periodic impulse sequence with a sinc function corresponding to a frequency band; the noise-like excitation is represented by a codebook. We estimate a pitch which is appropriate not only to the long-term predictive filter but also to the periodic multiband excitations and to the 'pitch' prefilter in the decoder. Several CELP vocoders are developed as a reference to test the property of the MBCELP vocoder. Listening tests clearly indicate that this vocoder reconstructed very high quality speech without 'buzziness' or 'hoarseness' for both clean and noisy speech. A 4.8 kbit/s MBCELP vocoder is shown as an example. Its perceptual quality is virtually identical to the original 8 kbit/s CELP vocoder and the improved 7.2 kbit/s CELP vocoder. Since less subframes are used for the MBCELP vocoders, their complexity is not greater than that of usual CELP vocoders with the same type of codebook. A lot of techniques used to simplify CELP coding can be also adopted for the MBCELP coding.