Communications Satellite Corporation

Communications Satellite Corporation (comsat)

Comsat Corporation

Intelsat Corporation

Henri G Suyderhoud

Michael Onufry, Jr

Krishnamoorthy Virupaksha

Samuel Joseph Campanella

Yohtaro Yatsuzuka

Jack H Rieser

Vaikunth Gupta

Vaikunth N Gupta

Eric R Kauffman

K Virupaksha

Hans Dodel

A low-rate encoding/digital speech interpolation (LRE/DSI) system architecture is organized to eliminate the LRE mistracking problem normally associated with DSI-induced speech clipping, to reduce the number of LRE codes required and to provide a mechanism for gradually introducing overload.

Low-rate encoding/digital speech interpolation system

In an adaptive differential PCM (ADPCM) system transmitter, three quantizers provide, respectively, 3, 4, and 5-bit quantization at the signal sampling rate. Each quantizer produces, in response to an input signal sample, a coded signal I(n) in the form of a bit sequence the number of bits in the coded signal from each quantizer being different. The number of bits comprising the coded signal is proportional to the bit rate. The bit rate can thus be altered by switching the codec output to selected ones of the adaptive quantizers. To eliminate quantizing distortion during bit rate transitions, one adaptive quantizer is caused to operate in the dominant mode, to receive the input signal sample and produce the coded signal I(n). At the same time, the other adaptive quantizers operate in a secondary mode to update their step-size .DELTA.(n) on the basis of the quantized signal produced by the dominant mode quantizer. When a secondary mode quantizer is switched to the dominant mode, minimal time is required for tracking the input signal thereby reducing the quantization distortion during bit rate transitions.

Variable bit rate speech codec with backward-type prediction and

An adaptive gain control is provided at the input of a Variable Rate Nearly Instantaneous Companding (VRNIC) processor to adjust the gain of low level speech signals so that the transmission bit rate and SNR will remain substantially the same for low signal levels as for high signal levels.

Adaptive gain variable bit rate NIC processor

A robust ADPCM encoder with a modified leaky quantizer and a pair of parallel predictors is provided. A large leakage is initially inserted into the quantization scale adaptation by utilizing a number of most recent multipliers in a modified leaky Jayant quantizer. Using a dual predictor feedback loop, inside predictor coefficients are updated by the step size having unit variance instead of by the quantized prediction error.

Adaptive quantizer with instantaneous error robustness

A nearly instantaneous companding processor uses variable precision block quantization for reducing the digital transmission rate of speech signals while at the same time maintaining a high quality signal in a digital communication system. The variable number of bits which are used to encode each sample is controlled as a function of the maximum sample magnitude in a block of N samples. This technique takes advantage of the amplitude probability density function of a speech signal to thereby reduce the quantizing noise without increasing the transmission rate.

NIC Processor using variable precision block quantization

An echo suppressor for use in the send path of the four-wire part of a long distance telephone circuit employs frequency-selective center clipping. The bandwidth of the channel in which echo occurs is subdivided in contiguous bands or subchannels by appropriate filtering, and each resulting signal is center clipped by a center clipper. Threshold control voltages are predicted from knowledge of the average speech amplitude distribution characteristic as a function of frequency. This characteristic is subdivided into the same frequency bands as that used by the center clippers, and the ratios of energies in the bands are used to equalize the ratios of the control voltages that determine the center clipper thresholds. This technique is implemented through the use of a peak rectifier, the input of which is coupled to the receive path of the echo suppressor and the output of which is connected to each of the filtered subchannels by a resistive dividing network. The values of the resistances in each dividing network are determined so that the ratios of the various voltages appearing at these networks are proportional to the ratios of the average energies of the telephone speech in the respective frequency bands. These voltages are then applied to the center clippers to regulate each clipper threshold so that it is proportional to the applied voltages.

Echo suppressor using frequency-selective center clipping

An automatic H-register clear system for the prevention of erroneous convolution in adaptive digital echo cancellers is disclosed. Incoming signals from the H-register are accumulated and compared to a predetermined threshold value and when the value is exceeded, the H-register is cleared with the occurrence of the internal housekeeping cycle of the echo canceller. The clearing of the H-register allows a new impulse response model to be generated from new sampled data. In one embodiment using analog implementation, the accumulation takes place using a sequence of signal processing of digital-to-analog conversion, full wave rectification followed by integration of the signal. In another preferred embodiment using a counter detection system, each word is compared to a threshold and if this value is exceeded, a counter is indexed. If the value of the count in the counter exceeds a predetermined number during the sampling interval, the H-register is cleared. In this latter embodiment, either analog or digital implementation can be used.

Automatic H-register clear for prevention of erroneous convolution in

A low cost digital echo suppressor having break-in logic for overriding send-line interrupt operation. The send and receive signals are multiplexed and digitized. The digitized received signals are summed over a period of time to determine interrupt operation. The difference over a period of time of the receive and send line signals controls break in logic for overriding the interrupt operation.

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Potomac, MD, United States of America

Henri G Suyderhoud