Telephonic communications – Substation or terminal circuitry – For loudspeaking terminal
Reexamination Certificate
1998-07-07
2001-08-14
Chan, Wing F. (Department: 2742)
Telephonic communications
Substation or terminal circuitry
For loudspeaking terminal
C379S202010, C381S092000
Reexamination Certificate
active
06275580
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to full duplex telecommunication systems. More particularly, the present invention relates to a teleconferencing device having an acoustic configuration employing destructive cancellation of loudspeaker signals in the vicinity of an audio detection transducer to improve the acoustic echo return loss (AERL) of the device.
BACKGROUND OF THE INVENTION
Whether incorporated in video conferencing systems or standard speakerphones, the ability to communicate high quality Near-End signals while simultaneously receiving Far-End signals (i.e. full duplex communication) has proven to be a basic requirement of telecommunication systems.
An undesirable phenomenon inherent in full duplex, “hands-free”, teleconferencing systems is that of signal echo caused by the acoustic coupling between a communication terminal's transducers. The echo in audio conferencing results from the re-transmission of a Far-End signal by the Near-End terminal of a communication system.
In speakerphones, the echo is caused by reflected Far-End voice transmissions which are coupled to the Near-End communication terminal's microphone section via the Near-End terminal's loudspeaker. Echo occurs with such audio conferencing systems because of the close physical proximity of the loudspeaker and microphone elements. The change in level of the echo as it is coupled from the Near-End's loudspeaker to the microphone is known as the Acoustic Echo Return Loss (AERL).
In order to reduce echo signals present in full-duplex conversation, several signal processing alternatives have been developed. Those alternatives include analog voice switching, echo suppression, and digital adaptive echo cancellation techniques.
High quality communication terminals often employ digital signal processing such as adaptive echo cancellation circuitry which predicts and synthesizes an expected feedback signal, and then subtracts the expected feedback signal from the Near-End microphone signal. Although adaptive echo cancellation provides significant reduction in echo signal levels, it does not eliminate echo signals. Moreover, such elaborate and often costly techniques are not economically feasible for all applications.
Even in applications utilizing echo cancellation circuitry, the performance of the adaptive echo cancellation circuitry is often affected by the strong coupling between the loudspeaker(s) and microphone. The coupling dominates the control process within the internal adaptive filter used by these communication devices, reducing the performance by limiting the maximum loudspeaker and microphone levels in order to reduce the acoustic echo to acceptable levels. Often times, users of such devices attempt to compensate this condition by increasing the volume beyond a limit which causes the device's software to revert to a “semi full-duplex” or half-duplex mode.
Therefore, it would be desirable to further reduce AERL prior to the electronic processing of the Near-End signal by a communication terminal to decrease the reliance on complex signal processing circuitry. In addition, it is also desired to reduce the level of feedback available to a communication device employing echo cancellation circuitry to offset limitations in the dynamic range of the adaptive filters of the circuitry.
SUMMARY OF THE INVENTION
In accordance with the present invention, a teleconferencing apparatus for electronic communication is provided. In one embodiment, the teleconferencing apparatus includes at least two loudspeakers and a uni-directional microphone all disposed in a single housing.
The loudspeakers are mounted in the housing along a placement axis with their mouths facing outwardly thereof. The loudspeakers have central axes directed away from the housing surface and from the unidirectional microphone. The loudspeakers are “matched” (i.e., they have essentially equal impedance characteristics) and are connected in phase opposition across the near-end output path of the teleconferencing apparatus. The angular positioning of the loudspeakers results in an overlap of their sound intensity dispersion patterns in the vicinity of the microphone's axis of maximum sensitivity. The overlap portion of each individual phase opposing loudspeaker intensity pattern being essentially identical with respect to the other such that equal but opposite signal transmissions result in destructive cancellation in the overlap region.
The uni-directional microphone is mounted in the housing with its sound-responsive element facing outwardly thereof. The uni-directional microphone is disposed symmetrically relative to the placement of the loudspeakers such that the microphone's axis of maximum sensitivity is positioned in a region where the destructive cancellation of sounds emanating from the loudspeakers occurs. By virtue of the aforesaid arrangement, acoustic coupling between the loudspeaker and the microphones is substantially reduced.
REFERENCES:
patent: 3889058 (1975-06-01), Gabr
patent: 4346261 (1982-08-01), Hestad et al.
patent: 5787183 (1998-07-01), Chu et al.
patent: 5808663 (1998-09-01), Okaya
Faraci Philip
Lang Philip
Berryhill John B.
Chan Wing F.
Dann Dorfman Herrell & Skillman
Tellabs Operations Inc.
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