Echo controller with compensation for variable delay networks

Details Echo controller with compensation for variable delay networks Echo controller with compensation for variable delay networks

1998-09-10

2001-11-27

Cangialosi, Salvatore (Department: 2661)

Multiplex communications

Duplex

Transmit/receive interaction control

C370S289000

Reexamination Certificate

active

06324170

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the controlling of echo in telephone communications over variable delay networks. Specifically, the method and system of the present invention provide for the cancellation of echo when using hands-free telephones for communicating voice over a LAN/WAN network.
BACKGROUND OF THE INVENTION
Telephone connections in which one or more hands-free devices, such as speakerphones, are connected over conventional telephone networks are faced with a classic problem of controlling acoustic echo. Echo is often introduced at the hands-free devices by virtue of the close proximity of a microphone and a speaker and results in the received acoustic signal to be transmitted on the return path. This acoustic “feedback”, however, is delayed by a finite amount of time, usually on the order of milliseconds. This acoustic signal is received and heard by the speaking party as an objectionable echo of the speaking party's voice.
Voice-switching, also known as switched-loss, is a commonly used technique to reduce echo. In this technique, the echo control unit inserts loss into either or both paths of the circuit connection, depending on which party is determined to be speaking at that particular time. In a speakerphone, the loss is inserted into the transmit path when the far-end user is talking (the speakerphone receive signal is declared as active). When the near-end user is talking, the loss is placed in the receive path and if neither is talking the loss may be in either path or split between them. In all cases, the loss contributes directly to control of the echo signal. When both parties are talking—such as the case where one user attempts to interrupt the other—voice-switching results in the clipping or attenuation of one or both user's speech signal(s). In effect, the connection is only half-duplex. Some hands-free devices offer full-duplex connections by using acoustic echo cancellers within the echo control unit with algorithms to model the delay path and subtract out the echo signals such that the signals transmitted from the devices do not contain a significant amount of echo. Such algorithms typically provide up to 25-35 dB of echo control with little or no supplementary voice-switching or loss due to non-linear processing which might otherwise render the call half-duplex. This amount of echo control is adequate in most conventional wireline (vs. wireless) telephone connections.
As an alternative to conventional circuit-switched networks, LAN/WAN networks and other IP networks have been employed increasingly to communicate voice and to provide for telephone connections. Accordingly, a connection over a LAN/WAN network may include one or more hands-free devices, such as speakerphones, which can potentially introduce objectionable echo into the LAN/WAN telephone connections. The round-trip delay introduced in these networks varies, and when significant, places more severe requirements for control of such echo to prevent it from being objectionable to users. In fact, for aroundtrip delays up to a few 100s of milliseconds, the requirement for echo control rises commensurate with the amount of delay in the call. In cases of high delay, greater than 100 msec for example, the required amount of echo control will be beyond the capabilities of today's acoustic echo cancellers—without the addition of adequate voice-switching or non-linear processing loss which will render such calls half-duplex. This supplementary (voice-)switched or non-linear processing loss will be denoted as signal-dependent loss.
The strategy in current high-delay voice calls, such as those made over digital wireless connections or voice-over-IP, is to add the necessary loss, via voice-switching or some non-linear processing, to achieve the necessary echo control for the worst-case expected round-trip delay. In the process, full-duplex operation is sacrificed on all calls. In LAN/WAN networks, however, the amount of delay in a call varies depending on network conditions and types. Thus the echo control strategy designed for worst-case expected delay tends to penalize call quality (“full-duplex-ness”) on those calls with low delay.
SUMMARY OF THE INVENTION
Having identified the above described problems, the inventors have developed the following solution, which is embodied in the present invention. As described herein, the present invention provides a method of and system for minimizing the amount of signal-dependent loss employed to control echo in telephone connections over variable delay networks such as LAN/WAN networks. This has the advantage of maintaining fill-duplex operation wherever possible (under low-delay conditions) while avoiding objectionable echo levels (under high-delay conditions).
The method of the present invention includes first estimating the round-trip user-to-user delay in the telephone connection. This estimation can be made in a number of different ways, including using timestamps in existing data packets according to a Real-Time Protocol (RTP) or a Real-Time Control Protocol (RTCP) protocol and by sending timestamps in data packets according to an Internet Control Message Protocol (ICMP). The method further includes determining an echo control target for the telephone connection based upon the estimated user-to-user delay. This target represents the total amount of echo control required to limit the probability of objectionable echo to less than a predetermined threshold. The amount of echo control required increases as the length of the estimated user-to-user delay increases. The method further includes determining the amount of signal-dependent loss required in addition to the acoustic echo cancellation in order to reach the determined echo control target. Then, in the method of the present invention, the determined amount of signal-dependent loss is applied to the telephone connection to effectively reduce echo based upon the estimated user-to-user delay.
An echo control unit of the present invention is connected to or combined with user equipment, such as a hands-free telephone, and uses the method of the invention to control and effectively cancel echo in a telephone connection over a variable delay network while minimizing the amount of signal-dependent loss applied to the connection. The echo control unit includes a delay estimator for estimating user-to-user delay in the telephone connection, a signal dependent loss unit for determining the echo control target based upon the user-to-user delay, for determining the required amount of signal-dependent loss to be applied such that the target is reached and for applying that amount of loss to the transmit path and/or the receive path of the telephone connection.
It is a primary object of the present invention to improve the voice quality of telephone connections over variable delay networks such as LAN/WAN networks.
It is also an object of the present invention to make practical the connection of one or more hands-free devices to telephone links over variable delay networks such as LAN/WAN networks.
It is a further object of the present invention to facilitate full-duplex operation of such hands-free devices on variable delay networks during periods of relatively low delay.
It is yet a farther object of the present invention to facilitate effective echo control in telephone links with one or more hands-free devices on variable delay networks during periods of relatively high delay.
It is another object of the present invention to provide dynamic echo control over the course of a telephone call carried over a variable delay network.
Other features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the

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