A 4 kb/s hybrid MELP/CELP coder with alignment phase encoding and zero-phase equalization

The paper describes a hybrid multi-modal codec with MELP and CELP coders used for different speech regions. Three modes are used: strongly voiced, weakly voiced, and unvoiced. The weakly voiced mode includes transitions and plosives; it is used when neither strong voicing nor unvoiced region are clearly identified. In the strongly voiced mode the MELP coder is used, while in the weakly voiced and unvoiced modes the CELP coder is employed. To limit switching artifacts between the coders, alignment phase is estimated and transmitted in the MELP mode making the original and MELP-synthesized speech time-synchronous. Additionally, in zero-phase equalization, the phase component of the CELP target signal is removed making the target waveform more similar to the MELP-synthesized speech. These two techniques, alignment-phase encoding and zero-phase equalization, greatly reduce switching artifacts in MELP/CELP transition regions. Formal listening test results of the 4 kb/s hybrid coder show that it can achieve speech quality equivalent to 32 kb/s ADPCM