Variable bit-rate CELP coding of speech with phonetic classification

A variable bit-rate speech coder intended for digital cellular applications is described. A voice activity detection algorithm is used to distinguish active speech from background noise. Each frame of active speech is further classified to distinguish between three phonetic categories: voiced, unvoiced, and onset. Each input frame is assigned one of five bit rates according to voice activity and phonetic classification and coded using an analysis-by-synthesis algorithm tailored to the needs of the class that it belongs to. The resulting coder, called Variable Rate Phonetic Segmentation, produces good quality speech at an average bit-rate below 3 kbit/s when operating with a voice activity factor of 0.5. Informal subjective quality assessment for speech in clean and noisy backgrounds indicates a performance that is comparable to the TIA standard QCELP algorithm while operating at a 25% to 40% lower average bit rate.