Variable bit rate speech encoder

Abstract

A method and apparatus for allocating transmission bits for use in transmitting samples of a digital signal. An aggregate allowable quantization distortion value is selected representing an allowable quantization distortion error for a frame of samples of the digital signal. A set of samples are selected from the frame of samples such that a plurality of the selected samples are greater than a noise threshold. For each sample of the set, a sample quantization distortion value is computed which represents an allowable quantization distortion error for the sample. The sum of all sample quantization distortion values is approximately equal to the aggregate allowable quantization distortion value. For each sample of the set, a quantization step size is selected which yields a quantization distortion error approximately equal to the sample's corresponding quantization distortion value. Each sample is then quantized using its quantization step size. An estimation signal is prepared which is representative of the digital signal but has fewer samples than the digital signal. A signal index is prepared which represents, for at least one sample of the estimation signal, the magnitude of a signal component relative to the magnitude of a noise component. Based on the signal index, samples of the digital signal are selected which have a sufficiently large signal component. The selected samples of the digital signal and the samples of the digital estimation signal are both transmitted to a remote device. The remote device reconstructs the digital signal from the transmitted selected samples and estimation samples.

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