A variable rate speech coding algorithm for cellular networks

An improved version of the variable rate phonetic seg mentation (VRPS) algorithm for speech coding, applicable to CDMA digital cellular systems is presented. The coder performs activity detection to distinguish speech from background noise. Each frame of active speech is classifted into one of four phonetic categories. A distinct coding configuration and bit rate is applied to each category. The perceptual quality of the coded speech has been improved through the use of a new class-dependent adaptive postfiltering which attenuates the noise in the valleys of the speech spectrum. The robustness of the phonetic classffication algorithm in the presence of back ground vehicle noise has also been improved, by adapting classilier parameters using properties of the noise. These two modifications combine to improve the perceptual quality of the coder, while reducing the average rate by 300-450 b/s.