Picturetel Corporation

Antony Henry Crossman

Edmund S Thompson

Brant Martin Helf

The invention features a method of transmitting a digital signal which includes frames of frequency coefficients arranged in bands. For each frame, the method includes generating a spectral estimate of the energy in each of the bands, a class value that corresponds to the energy in a band relative to the average energy in the other bands for each of the bands, and a step size for each of the frequency coefficients based on the spectral estimate for the corresponding band. The step size varies with the energy in the band so that the step size for a frequency coefficient in a band having relatively high energy is larger than the step size for a frequency component in a band having relatively low energy. The frequency coefficients are then quantized, based on the relationship between the value of the frequency coefficient, the corresponding class value, and the corresponding step size, to produce quantized frequency coefficients. These quantized frequency coefficients are then encoded based on the class values to produce encoded quantized frequency coefficients. Thereafter, the class values are iteratively modified and the steps of generating class values and step sizes and quantizing and encoding the frequency coefficients are repeated until the number of bits that represent the encoded quantized frequency coefficients is optimized. Finally, the encoded quantized frequency coefficients are transmitted.

Fixed bit rate speech encoder/decoder

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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Swampscott, MA, United States of America

Antony Henry Crossman