Enhancement of speech coding in background noise for low-rate speech coder

Claim 1

In a method of low-bit-rate speech coding of input speech occurring in a noisy environment, for a system which employs linear predictive coding (LPC) analysis of input speech frames to generate reflection coefficients, conversion of the reflection coefficients to vectors representing spectral parameters of the input speech frames, and matching of the spectral parameter vectors against reference vectors of a vocabulary of codewords generated in a training sequence in order to select the corresponding index of an optimally matching codeword for transmission, the improvement comprising the steps of: selecting a set of at least two features which are characterized by a probability distribution which is not strongly affected in the noisy environment and which allow discrimination between voiced and unvoiced input speech, wherein said selected features include the feature of zero-crossing counts which are based on average noise energy; measuring the selected features for input speech frames; and using said feature measurements to make voiced/unvoiced speech decisions in order to select the voice/unvoiced excitation for speech synthesis in the receiver; using noise estimates to update the reference vectors of the vocabulary of codewords, wherein new reference vectors are generated corresponding to said vocabulary of codewords in the noisy environment, said noise estimates including noise amplitude and noise reflection coefficients, wherein said noise estimate for speech frame I is performed only if the ith speech frame is unvoiced and more than a given number L of continuous unvoiced speech frames are accumulated, in order to prevent using voiced or unvoiced speech in the noise estimate.