Abstract:
A teleconferencing terminal for hands-free full duplex audio telecommunication systems is provided which limits the amount of acoustic coupling between the terminal&#39;s microphone and loudspeaker. A directional microphone is positioned equidistantly from two loudspeakers such that audio signals emanating from the loudspeakers are acoustically coupled by the microphone at substantially equivalent audio levels. The terminal loudspeakers are impedance matched and connected in phase opposition with respect to each other so that signals emanating from the loudspeakers in the vicinity of the microphone are destructively canceled.

Description:
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&#39;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&#39;s microphone section via the Near-End terminal&#39;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&#39;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&#39;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&#39;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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other benefits and advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed specification and related drawings, wherein 
     FIG. 1 is an exploded section elevation view of a teleconferencing device according to one embodiment of the present invention; 
     FIG. 1A is a front elevation view of the teleconferencing device of FIG. 1 as viewed along line  1 A— 1 A; 
     FIG. 2 is a functional block diagram of the communications circuit of the teleconferencing device of FIG. 1; 
     FIG. 3 is a side view of the teleconferencing device of FIG. 1 showing the sound intensity dispersion patterns of the loudspeakers; and 
     FIG. 4 is a schematic view of an alternative embodiment of the present invention using a single loudspeaker. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Microphones can be broadly categorized as omni-directional or directional. Omni-directional microphones are substantially equally sensitive to sound waves arriving at the microphone from any direction. Directional microphones exhibit a greater degree of sensitivity to sounds arriving from certain directions than to sounds arriving from other directions. Bi-directional microphones, for example, are characterized by maximum sensitivity in two directions, usually separated by about 180°. Uni-directional microphones are characterized by maximum sensitivity in a single direction. 
     One type of uni-directional microphone is a cardioid microphone, wherein the sensitivity pattern resembles a cardioid, or “heart shape”, which has at least one direction of minimum sensitivity. The direction of minimum sensitivity of a cardioid microphone often referred to as the “shadow” of the microphone, is ordinarily at an angle of 180° from the direction of maximum sensitivity. 
     Cardioid microphones include super-cardioids and hyper-cardioids, which may have two minima separated by angles of ±120° to ±140° from the direction of maximum sensitivity. The actual response pattern that is obtained in a practical setting also depends upon the acoustic environment of the microphone. 
     It is noted that the term “sensitivity” is often defined in the acoustic arts as the inverse proportion of the electrical response produced by a microphone relative to an incident sound pressure level. For clarity of explanation, the term “sensitivity” is used herein to refer to the proportionate electrical response relative to the pressure of the incident sound wave. Hence, as used herein, the term “greater sensitivity” refers to a larger electrical response for a given sound pressure level, relative to a reference sensitivity level. 
     The term loudspeaker is used herein to refer to loudspeaking transducers, loudspeaker is not used to differentiate a transducer of a particular power rating, but to avoid usage of the term “speaker.” In telecommunications technology “speaker” is often confused with a “person who is talking” (who are usually called talkers for this purpose). 
     Referring now to FIGS. 1-1A, there is shown a teleconferencing terminal  5  in accordance with the present invention. The teleconferencing terminal  5  includes a pair of matched loudspeakers  9 A and  9 B connected in phase opposition, a unidirectional microphone  7 , a power terminal  13 , a communication terminal  15 , and a communications circuit  18 . Cover  16  is easily removable for access to microphone  7  and loudspeakers  9 A and  9 B, the cover serving to protect the fragile sound responsive elements of the acoustic devices from accidental damage or puncture. 
     Those components are all contained in a housing  12  having a front surface  14 , a back cover  20 , and a front cover  16 . A communication terminal  15  and power terminal  13  are conveniently disposed on a sidewall of housing  12  or can be disposed on the back cover thereof. Suitable terminals for such a configuration are well known. Communications terminal  15  is preferably embodied as an RJ11U telephone jack for connecting the terminal with the communication interface circuitry of a personal computer or workstation. The power terminal  13  is preferably embodied as a DC power jack for connecting the teleconferencing terminal  5  to a source of electrical power. 
     Communication circuit  18  includes amplification means for boosting the level of far-end and near-end transmissions. Additionally, the communications circuit  18  includes echo cancellation circuitry for reducing the acoustic echo. 
     Referring now to FIG. 2, the two loudspeakers  9 A,  9 B have essentially the same, preferably identical, sound propagation patterns and impedance characteristics (i.e., they are matched). The loudspeakers  9 A and  9 B are mounted behind the front surface  14  of housing  12  and connected in phase opposition with respect to the near-end output interface  50  of communications circuit  18 . In this way, upon proper positioning of loudspeakers  9 A and  9 B, phase opposing signals will be equally coupled in the vicinity of the microphone  7 . 
     Referring again to FIGS. 1-1A, the orientation of loudspeakers  9 A and  9 B is such that the mouths of the loudspeakers face outwardly from the front surface  14 . Loudspeakers  9 A and  9 B have central axes  25 A and  25 B respectively, which define the direction of sound propagation for each loudspeaker. As shown in FIG. 1A, loudspeakers  9 A and  9 B and microphone  7  are aligned along a common axis  35 , such that the axes  25 A,  25 B and  27  are aligned in a common plane. 
     The uni-directional microphone  7  is disposed in the central portion of front surface  14  and supported by the microphone stalk  32  extending away from surface  14 . The microphone  7  is a uni-directional microphone, preferably a cardioid microphone. 
     The orientation of microphone  7  is such that the sound responsive element of microphone  7  is equally sensitive to corresponding portions of the equal phase opposing sound intensity dispersion patterns of loudspeakers  9 A and  9 B. A microphone axis  27  defines the center of microphone  7 . Thus, microphone  7  is oriented such that corresponding portions of the sound intensity dispersion patterns of loudspeakers  9 A and  9 B are symmetrical with respect to the microphone axis  27 . The portions of the sound intensity dispersion patterns have substantially identical but phase opposing amplitudes. The microphone  7  functions as a summing junction acting to cancel these symmetrical sound intensity dispersion pattern portions. 
     In the embodiment shown, the microphone  7  is also aligned along loudspeaker axis  35 , however such alignment is not necessary to practice the invention. The microphone  7  extends forward of housing front surface  14  such that the microphone  7  is everywhere equidistant from the mouths of loudspeakers  9 A and  9 B. Microphone  7  has a microphone axis  27  and the direction of maximum sensitivity of the microphone is centered along microphone axis  27 . Microphone  7  is preferably positioned forward of the front surface  14  such that it is within a region common to the sound intensity dispersion patterns of both loudspeakers. In an alternative embodiment, the microphone  7  can be mounted directly to the front surface  14  without a stalk when the common region of the intensity loudspeaker region is configured in close proximity to front surface  14 . 
     In the embodiment shown in FIG. 1, loudspeakers  9 A and  9 B have their central axes,  25 A and  25 B respectively, directed away from microphone axis  27  at an angle of about 35°. It is desirable to have the central axes  25 A and  25 B directed away from the microphone axis  27  as a preferred way to reduce the intensity of the acoustic energy in the vicinity of microphone  7 . However, alternative configurations including parallel arrangement of central axes  25 A and  25 B is within the scope of the invention. In all embodiments, the central axes of the loudspeakers  9 A and  9 B are oriented to symmetrically place congruent phase opposing portions of their respective intensity patterns such that the portions envelope the microphone axis  27 . In the preferred embodiment, the symmetrically placed phase portions overlap in defined areas, destructively cancelling loudspeaker signals in the vicinity of microphone  7 . 
     Referring now to FIG. 3 the acoustic environment of the oriented transducers of the device  5  is shown. The loudspeakers  9 A and  9 B have sound intensity dispersion patterns  29 A and  29 B, respectively. The outer limits of these patterns are defined by the 3 dB line (i.e., half power limit). In areas outside the 3 dB line the sound intensity dispersion decreases exponentially (roll-off) as the distance from the 3 dB line is increased. As such, areas outside the 3 dB line of a transducers range (i.e., loudspeakers and microphone) are not significant relative to the efficacy of the acoustic arrangement described herein. 
     The sound intensity dispersion patterns  29 A and  29 B partially overlap to form a zone of loudspeaker cancellation  24 . The cancellation zone  24  results from the phase opposing nature of the acoustic signals in the overlap area of the sound intensity dispersion patterns  29 A and  29 B of loudspeakers  9 A and  9 B. The size of the zone  24  depends upon the spacing between loudspeakers  9 A and  9 B from microphone  7  along placement axis  35 . In the overlap zone  24  sounds emanating from the loudspeakers  9 A and  9 B are subject to destructive cancellation. Microphone  7  has its sound responsive element positioned within cancellation zone  24  and has its center axis  27  positioned to bisect the zone of cancellation  24 . In this way, the zone of loudspeaker cancellation is centered about microphone axis  27 , the axis coinciding with the location of the maximum sound sensitivity of microphone  7 . The requirement for and/or length of microphone stalk  32  depends upon the proximity of zone  24  to surface  14 , which is in turn dependent upon the spacing of loudspeakers  9 A and  9 B along placement axis  35 . The result of this arrangement is that acoustic coupling between the loudspeaker  9 A and uni-directional microphone  7  is essentially canceled because of the phase opposing acoustic energy generated by loudspeaker  9 B. Similarly acoustic coupling between the loudspeaker  9 B and uni-directional microphone  7  is essentially canceled because of the phase opposing acoustic energy generated by loudspeaker  9 A. Therefore, sounds emanating from loudspeakers  9 A and  9 B are destructively canceled in the region of the microphones maximum sensitivity. 
     Thus, as FIG. 3 illustrates, sounds which are not applied to the microphone  7  at even, phase opposing levels with respect to one another will result in an output signal from the microphone  7  through communication circuit  18 . Multipath arrival of loudspeaker signals caused by room acoustics will be negligible due to the limited sensitivity of the microphone to sounds emanating from outside cancellation zone  24  and beyond the microphone 3 dB line. Additionally, the teleconferencing device  5  is preferably positioned away from reflective surfaces in the immediate vicinity thereof. For example, AERL levels in communication such as that from a two-way telephone conversation are reduced due to the cancellation zone created between the microphone&#39;s sound responsive element and the microphone&#39;s 3 DB line. The echo from loudspeakers  9 A and  9 B caused by far-end speech is destructively canceled in zone  24 . 
     Referring now to FIG. 4, an alternative embodiment of a teleconferencing unit  10  according to this invention is shown. The sound is produced in the device  10  by means of a single loudspeaker device  22 . Loudspeaker  22  is disposed with its mouth in the plane of intersection of two acoustic channels  45 A and  45 B. The acoustic channels are formed in the housing  12 . The loudspeaker  22  propagates sound energy upward and away from the housing  12  through channels  45 A and  45 B. The loudspeaker  22  is positioned at an equal distance from the mouths of channels  45 A and  45 B. The movement of the sound responsive element of loudspeaker  22  produces an acoustic “push” or “pull” on the appropriate side of the sound responsive element. The result of such loudspeaker positioning produces phase opposing acoustic energy exiting the mouths of channels  45 A and  45 B. The sound intensity dispersion patterns of this arrangement are equivalent to the previously described embodiment in that phase opposing portions of the intensity patterns overlap in the vicinity of the microphone axis  29 . Thus, sound detected between the loudspeaker channel  45 A and uni-directional microphone  47  is essentially equal in magnitude to the acoustic coupling between loudspeaker channel  45 B and uni-directional microphone  47 . Therefore, sounds emanating from the channels  45 A and  45 B will be destructively canceled in the region of the microphones maximum sensitivity which is located about the microphone axis  29 . 
     In the foregoing description some of the several novel features of the teleconferencing device according to the present invention have been described. The combined effects of the teleconferencing terminal&#39;s acoustic configuration, namely the symmetrical placement of the loudspeakers relative to the microphone and phase opposing operation of the loudspeakers, provides a reduction in overall feedback relative to what would traditionally be expected in a compact speakerphone device. 
     The terms and expressions which have been employed are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized, however, that various modifications are possible within the scope of the invention as claimed.