Nortel Networks Corporation

Northern Telecom Limited

David Dal Farra

Heping Ding

Scott McClennon

Scott Mcclennon

The invention relates to an acoustic echo canceller particularly well suited for use in a hands-free terminal or teleconference system. The echo canceller is designed to assess the inherent signal attenuation occurring in the analog loop that connects the terminal to the switch mechanism of the PSTN domain or the PBX domain. The echo canceller is then adjusted in accordance with the degree of signal attenuation manifested by the loop. For high signal attenuation levels, the degree of echo control at the terminal is reduced since the signal attenuation occurring in the loop itself will compensate for the reduction, allowing to reach the echo control target at the far end of the loop (switch side). This feature avoids overly attenuating the echo at the terminal by taking advantage of the inherent echo control provided by the local loop conductors.

Method and apparatus for echo control in a communication system

The invention is an audio compression system for reducing distortion arising primarily from excessive loudspeaker excursion. For a given input audio signal, the maximum signal magnitude (or envelope, energy or power) level above which the loudspeaker will begin to distort is dependent upon the frequency content of the signal. In this invention, the input audio signal is filtered and the magnitude (or envelope, energy or power) level of the filtered signal is compared to a single, constant threshold level. The input audio signal is subsequently compressed only if the magnitude (or envelope, energy or power) level of the filtered audio signal exceeds the threshold value. In one embodiment, the filter has a frequency response which models the excursion response of the loudspeaker in question, and which can be obtained analytically for various types of loudspeakers. In another embodiment, the filter has a frequency response which is related to an experimental curve describing the maximum voltage leading to just acceptable distortion as a function of frequency. By attenuating only those signals which cause distortion (due primarily to excessive loudspeaker excursion), efficiency of the audio compression system is improved.

Loudspeaker-dependent audio compression

An improved echo canceller capable of reducing a magnitude of an echo signal propagating in a return channel of a communication device. The echo canceller includes an adaptive filter that models the echo path to generate an estimate of the echo. That estimate is subtracted from the echo-corrupted signal. The residual echo signal (error-signal) is then attenuated in a switching loss device. The switching loss device manifests a variable attenuation level selected in accordance with certain characteristics of the residual echo signal. These characteristics are the strength of the residual echo signal and its duration. For residual echo signals of very short duration, the attenuation level is set lower because those signals are perceived by the human ear as being weaker. For signals having a longer duration, the attenuation level is raised to inhibit the residual echo to acceptable levels.

Method and apparatus for inhibiting echo in a channel of a communication system

A system and method for dynamically optimizing the analog gain for a microphone in a telephone terminal operable in handsfree mode. The system makes use of a process whereby a relatively inexpensive analog to digital converter is used to meet gain requirements so as to accommodate the loudest echo through transmit path while maintaining an acceptable signal to noise ratio for the quietest talker. The system digital signal processor in conjunction with speech activity detection means is used to provide an optimum microphone gain under all circumstances.

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Nepean, Canada

David Dal Farra