Pitch synchronous innovation code excited linear prediction (PSI‐CELP)

This paper proposes a new speech coding method pitch synchronous innovation code excited linear predictor (PSI‐CELP). This method is based on CELP but adds pitch synchronous innovation. This results in even random codevectors being adaptively converted to have pitch periodicity for voiced frames. This scheme can improve the synthesized speech quality of voiced frames in the low bit‐rate CELP without increasing either computational complexity or bit rate. In addition to pitch synchronous innovation, this paper also proposes four other methods in which the quality of synthesized speech is effectively improved or the computational complexity is reduced for low bit‐rate CELP: (1) the perceptual weighting filter is an MA‐type and uses nonquantized LPC parameters; (2) the fixed codebook is prepared as part of the adaptive codebook for nonperiodic frames; (3) the random codebook consists of two‐channel conjugate sub‐codebooks; and (4) codebook searches are made using delayed decision. In a speech coding system with pitch synchronization of random codevectors, improvements in ratio SNR, segmental SNR, and opinion equivalent Q value are respectively 1.6 dB, 1.0 dB, and 0.5 dB. These basic methods were adopted in the standard codec for Japanese half‐rate digital mobile telephone service.