Implementation and performance of an 8-kbit/s conjugate structure CELP speech coder

This paper presents a high-quality 8-kbit/s speech coder (conjugate structure CELP: CS-CELP) that is a candidate for standardization by the ITU-T (formerly CCITT). To achieve high-quality for two types of speech (IRS and non-IRS (flat) speech) and real-time implementation, CS-CELP has been revised by two novel schemes. To handle two types of speech, the LSP parameters are quantized by multistage VQ with fourth-order interframe MA prediction. This scheme has little spectrum distortion, even if the two types of speech have many variations of the LSP parameters. The computational complexity of the implementation is reduced for adaptive and fixed-shape codebooks without degrading the speech quality. Multistage selection is adopted in the adaptive codebook; this selection uses a truncated impulse response. Improved pre-selection is proposed in the fixed-shape codebook. Subjective testing indicates that the quality of CS-CELP is equivalent to that of the 32-kbit/s ADPCM under error-free conditions for IRS and non-IRS speech. It also operates in real time using fixed-point DSP chips