| US4932063A | 1990-06-05 | |||
| US4908855A | 1990-03-13 | |||
| US5027393A | 1991-06-25 |
| 1. | A duplex handsfree communication apparatus, comprising: speaker means for transmitting a signal received from a remote source: first and second microphone means for transducing received acoustical signals wherein said first microphone means is positioned significantly closer to said speaker means than said second microphone means so that said first microphone means receives significantly more acoustical signal strength from said speaker means than said second microphone means; first amplification means connected to said first microphone means and second amplification means connected to said second microphone means for providing said second amplification means with a gain relative to a gain of said first amplification means such that the output signals of said first and second amplification means are generally equal with respect to signals received by way of said speaker means; and filter means connected to the outputs of said first and second amplification means for filtering transduced signals from said speaker means thereby preventing transmission to the remote source of essentially all of said signals from said speaker means. |
| 2. | The apparatus as set forth in claim 1, further including a high pass filter connected to the output of said filter means and a low pass filter connected to the output of said high pass filter for eliminating the low frequency resonating characteristics of said speaker means. |
| 3. | The apparatus as set forth in claim 2, further including an automatic level control circuit connected to the output of said low pass filter and a line driver circuit connected to the output of said automatic level control circuit. |
| 4. | The apparatus as set forth in claim 1, said speaker means including: a speaker: a power amplifier circuit with an output connected to said speaker for driving said speaker: and an automatic level control circuit connected to an input of said power amplifier circuit. |
| 5. | The apparatus as set forth in claim 1, said remote source including a telephone. |
| 6. | The apparatus as set forth in claim 1. said first and second microphone means including electret microphones. |
| 7. | The apparatus as set forth in claim 6 wherein said first electret microphone is smaller than said second electret microphone. |
| 8. | The apparatus as set forth in claim 1, said first microphone means being positioned on a plane parallel to and in front of a frontal plane of said speaker means. |
| 9. | The apparatus as set forth in claim 8 wherein said speaker means includes a speaker cone, said first microphone means and second microphone means are positioned such that each lies on an axis generally perpendicular to the speaker means frontal plane and passing through essentially the center of the speaker cone. |
| 10. | The apparatus as set forth in claim 9, said first microphone means being positioned less than an inch in front of the frontal plane. |
| 11. | The apparatus as set forth in claim 1, said first and second amplification means each including a differential amplifier. |
| 12. | The apparatus as set forth in claim 11, said first amplification means having a gain of approximately 100 and said second amplification means having a gain of approximately 400. |
| 13. | The apparatus as set forth in claim 1. said filter means including: a first differential amplifier having two inputs and an output wherein the inputs are connected to the output of one of said amplification means through resistive means for shifting the phase of said transduced speaker signals into alignment with said transduced speaker signals of the output of the other of said amplification means; and a second differential amplifier having two inputs and an output wherein the output of said first differential amplifier is connected to one of said second differential amplifier inputs and the output of the other of said amplification means is connected to the other of said second differential amplifier inputs thereby substantially reducing the presence of said transduced speaker signals at the output of said second differential amplifier. |
| 14. | A duplex handsfree communication apparatus, comprising: speaker means for transmitting a signal received from a remote source: first and second microphone means for transducing received acoustical signals wherein said first microphone means is positioned significantly closer to said speaker means than said second microphone means so that said first microphone means receives significantly more acoustical signal strength from said speaker means than said second microphone means; first amplification means having an input connected to said first microphone means and second amplification means having an input connected to said second microphone means for providing a gain at an output of said second amplification means which is significantly larger than a gain at an output of said first amplification means so that the output signals of said first and second amplification means are generally equal with respect to transduced speaker means signals; phase shift means having inputs connected to the output of one of said amplification means for shifting the phase of the transduced speaker means signal such that the phase of the transduced speaker means signal of said one of said amplification means is aligned with the phase of the transduced speaker means signal of the other of said amplification means; and differential amplifier means having a first input connected to the output of said phase shift means and a second input connected to the output of the other of said amplification means whereby common mode rejection of said differential amplifier means the transduced speaker means signals are essentially eliminated at an output of said differential amplifier means. |
| 15. | A method of providing simultaneous duplex handsfree voice broadcast and reception with a remote location, comprising the steps of: transducing signals with a speaker wherein the signals are received from a remote location by way of a communication link; transducing acoustical signals received by at least first and second microphones wherein said transduced microphone signals include a user generated voice signal and said transduced speaker signals and said first microphone is positioned significantly closer to said speaker than said second microphone so that said first microphone receives significantly more acoustical signal strength from said speaker than said second microphone; amplifying first and second microphone transduced signals wherein the said second microphone transduced signals have a gain relative to said first microphone transduced signals such that said amplified microphone transduced signals are generally equal with respect to signals received by way of said speaker; and filtering said transduced speaker signals with electrical circuits thereby preventing the transmission of essentially all of said speaker signals and transmitting said user generated voice signal. |
| 16. | The method as set forth in claim 15 said filtering step including the steps of: phase shifting one of said amplified microphone transduced signals such that the phase of the transduced speaker signal of said one of said amplified microphone transduced signals is aligned with the phase of the transduced speaker signal of the other of said amplified microphone transduced signals; and suppressing the transduced speaker signal using common mode rejection of a differential amplifier thereby essentially eliminating the transduced speaker signal at an output of said differential amplifier. |
| 17. | A duplex handsfree communication apparatus, comprising: a transducer means for transducing signals received from a remote location by way of a communication link and for transducing acoustical signals received from a user wherein the user signals are to be transmitted to the remote location; an impedance matching means for matching the impedance of said transducer means wherein said remote signals are applied essentially equally to said transducer means and said impedance matching means; and filter means connected to said transducer means and said impedance matching means for filtering said remote signals thereby preventing said remote signals from being transmitted back to said remote location and for transmitting said user signals to said remote location. |
| 18. | The apparatus as set forth in claim 17, further including buffer means for isolating the signals applied to said transducer means and said impedance matching means from each other. |
| 19. | The apparatus as set forth in claim 17 wherein said transducer means is worn in a user's ear. |
| 20. | The apparatus as set forth in claim 19 wherein said transducer means includes a bonevibration transducer. |
| 21. | The apparatus as set forth in claim 17 wherein said transducer means includes a piezoelectric transducer. |
| 22. | The apparatus as set forth in claim 17 wherein said remote signal is passed through an automatic level control circuit and then a power amplifier circuit before being applied to said transducer means and said impedance matching means. |
| 23. | The apparatus as set forth in claim 17 wherein said impedance matching means includes a second transducer means. |
| 24. | The apparatus as set forth in claim 23 wherein said second transducer means includes a bonevibration transducer. |
| 25. | The apparatus as set forth in claim 23 wherein said first and second transducer means each have a positive and a negative port, said positive port of one of said transducer means is connected to said filter means and said remote signal and the negative port of the other of said transducer means is connected to said filter means and said remote signal. |
| 26. | The apparatus as set forth in claim 17 said filter means including: first and second differential amplifier means, said amplifier means each having an inverting and a noninverting input terminal wherein signals from said transducer means and said remote signals are applied to the inverting input terminal of said first differential amplifier means and to the noninverting input terminal of said second amplifier means and signals from said impedance matching means and said remote signals are applied to the noninverting input terminal of said first differential amplifier means and to the inverting input terminal of said second differential amplifier means for substantially reducing said remote signals through common mode rejection of said first and second differential amplifier means; phase shift means having inputs connected to the output of one of said differential amplifier means for shifting the phase of the reduced remote signal such that the phase of the reduced remote signal of said one of said amplification means is aligned with the phase of the reduced remote signal of the other of said differential amplifier means; and third differential amplifier means having a first input connected to the output of said phase shift means and a second input connected to the output of said other of said differential amplifier means whereby common mode rejection of said third differential amplifier means the remote signals are essentially eliminated at an output of said third differential amplifier means. |
| 27. | A duplex handsfree communication apparatus, comprising: a transducer means for transducing signals received from a remote location by way of a communication link and for transducing signals received from a user wherein the user signals are to be transmitted to the remote location; an impedance matching means for matching the impedance of said transducer means wherein said remote signals are applied essentially equally to said transducer means and said impedance matching means; first differential amplifier means for receiving said remote and user signals having an inverting input connected to said transducer means and a noninverting input connected to said impedance matching means; second differential amplifier means for receiving said remote and user signals having an inverting input connected to said impedance matching means and a noninverting input connected to said transducer means; phase shift means connected to the output of one of said differential amplifier means for shifting the phase of the remaining remote signal such that the phase is aligned with the phase of the remote signal of the other of said differential amplifier means; and third differential amplifier means connected to the output of said phase shift means and to the output of said other of said differential amplifier means for essentially eliminating through common mode rejection of said third differential amplifier means said remote signals and for transmitting said user signals to the remote location. |
| 28. | A method of providing simultaneous duplex handsfree voice broadcast and reception, comprising the steps of: transducing with a transducer suitable for simultaneous duplex transmission signals received from a remote location by way of a communication link and transducing signals received from a user wherein the user signals are to be transmitted to the remote location; matching the impedance of said transducer with an equivalent circuit; applying said remote signals essentially equally to said transducer and said equivalent circuit; and filtering said remote signals with electrical circuits thereby preventing said remote signals from being transmitted to said remote location and for transmitting said user signals to said remote location. |
| 29. | The method as set forth in claim 28 said filtering step further including the steps of: applying signals from said transducer and said remote signals to an inverting input terminal of a first differential amplifier and to a noninverting input terminal of a second differential amplifier and signals from said impedance matching circuit and said remote signals are applied to a noninverting input terminal of said first differential amplifier means and to the inverting input terminal of said second differential amplifier means whereby through common mode rejection of said differential amplifiers said remote signals are substantially reduced at outputs of said differential amplifiers; phase shifting the phase of the reduced remote signal of one of said differential amplifiers with a phase shifting circuit such that the phase of the reduced remote signal of said one of said differential amplifiers is aligned with the phase of the reduced remote signal of the other of said differential amplifiers: and applying the phase shifted reduced remote signal to one of an inverting and a noninverting input terminal of a third differential amplifier and applying the reduced remote signal of said other of said first and second differential amplifiers to the other of said inverting and noninverting input terminals of said third differential amplifier whereby common mode rejection of said third differential amplifier means the remote signals are essentially eliminated at an output of said third differential amplifier. |
RECEPTION
Background of the Invention
1. Field of the Invention
The present invention relates generally to a device for providing duplex hands-free communication and more particularly to providing duplex communication as a speaker telephone.
2. Description of the Prior Art
The use of speaker phones for hands-free communication is well known. These speaker phones typically use fast switching circuitry to alternate between broadcasting and receiving. If this switching did not take place then an annoying feedback squeal would occur. Other approaches employing a speaker and one or two microphones have been tried using various cancellation techniques but all the attempts have thus far been unsatisfactory because of phase changes occurring between cancellation signals when connected to different transmission paths or return losses are not great enough to permit a satisfactory speaker acoustical level without inducing loop oscillations.
Another approach to duplex systems has been to use an echo cancellation scheme wherein the system generates an estimate of the echo and subtracts the estimate from the signal containing the echo. The estimate of the echo, however, is not always accurate and under certain conditions a feedback squeal may still be generated. Summary of the Invention
The problems outlined above are solved and an advance in the state of the art provided by the duplex communication devices of the present invention. More particularly, circuits which are relatively inexpensive and highly effective are provided which in a preferred embodiment uses a speaker in combination with two microphones
and in another embodiment a single transducer is used for duplex transmission.
A preferred duplex hands-free communication device includes a speaker for transmitting a signal received from a remote source, first and second microphones for transducing received acoustical signals wherein the first microphone is positioned significantly closer to the speaker than the second microphone so that the first microphone receives significantly more acoustical signal strength from the speaker than the second microphone. A first amplifier is connected to the first microphone and a second amplifier is connected to the second microphone wherein the second amplifier has a gain relative to a gain of the first amplifier such that the output signals of the first and second amplifiers are generally equal with respect to signals received by way of the speaker. A filter is then connected to the outputs of the first and second amplifiers to prevent the transmission to the remote source of essentially all of the signals from the speaker.
Another preferred duplex hands-free communication device includes a transducer for transducing signals received from a remote location by way of a communication link and for transducing acoustical signals received from a user wherein the user signals are to be transmitted to the remote location and an impedance matching circuit for matching the impedance of the transducer wherein the remote signals are applied essentially equally to the transducer and the impedance matching circuit. A filter is connected to the transducer and the impedance matching circuit to prevent the remote signals from being transmitted back to the remote location and to allow the transmission of the user signals to the remote location.
Brief Description of the Drawings
FIG. 1 is a schematic diagram of a preferred embodiment in accordance with the present invention.
FIG. 2 is a schematic representation of a physical construction of the preferred embodiment of FIG. 1.
FIG. 3 is a schematic diagram of a second embodiment in accordance with the present invention. Detailed Description of the Preferred Embodiment
A schematic diagram of a preferred embodiment of the present invention is shown
in FIG. 1. A duplex transducer system 100 includes microphones 102 and 104. speaker 106, gain circuits 108 and 110, phase shifter 112. differential amplifier 114, high pass filter 116, low pass filter 118, automatic level controls 120 and 122. line driver 124 and speaker driver 126.
In general, duplex transducer system 100 allows simultaneous broadcast and reception of acoustical signals in a hands-free configuration without annoying feedback. Microphone 102 is positioned at a distance relative to microphone 104 such that microphone 102 transduces more acoustical signal strength from speaker 106. The transduced signals from microphones 102 and 104 are then applied to gain circuits 108 and 110 respectively, with gain circuit 110 having a higher gain than gain circuit 108 such that the amplitudes of the transduced signals from speaker 106 are essentially equal at the outputs of gain circuits 108 and 110. A fixed phase difference between the acoustical signals transduced by microphones 108 and 110 exists because of the distance separating the microphones.
In order to bring the transduced speaker signals into phase the output of gain circuit 108 is applied to phase shifter 112 so that at the outputs of phase shifter 112 and gain circuit 110 the transduced speaker signals are equal in amplitude and phase. Next, the phase shifter 112 output and the gain circuit 110 output are applied to inverting and noninverting input terminals of differential amplifier 114, respectively. Through common mode rejection of differential amplifier 114 the transduced speaker signals are then substantially reduced. As those skilled in the art will realize, by reducing the transduced speaker signals that are broadcast unwanted feedback squeal is prevented from occurring.
A user's voice is picked up essentially equally by microphones 102 and 104 therefore, the transduced user voice signals at the output of gain circuit 110 will be significantly larger than at the output of gain circuit 108. Phase shifter 112 introduces a further difference between the transduced user voice signals at the outputs of phase shifter 112 and gain circuit 110 but not enough for the signals to be in differential mode. Therefore, at the output of differential amplifier 114 the transduced user signals will be slightly larger than at the output of gain circuit 110. It should be readily apparent that acoustic signals other than those from speaker 106 which microphones 102 and 104 transduce are broadcast by duplex transducer system 100.
The high pass filter 116 and the low pass filter 118 eliminate high and low
resonant frequencies and the automatic level controls 120 and 122 maintain the signal strengths of the outgoing and incoming signals respectively, at an essentially constant level. Line driver 124 matches the voltage of the communication line to which the transducer device is to be attached and speaker driver 126 drives speaker 106.
More particularly, microphone 102, preferably an electret type microphone, has a positive and a negative terminal one of which is grounded and the other connected to one side of resistor Rl (lkΩ) and to the positive side of capacitor Cl (lμf) of gain circuit 108. The other side of resistor Rl is connected to +5 v.d.c. of an appropriate power supply (not shown). Similarly, microphone 104, which is also preferably an electret type microphone only larger than microphone 102, has one of its terminals grounded and the other connected to one side of resistor R2 (lkΩ) and to one side of capacitor C2 (lμf) of gain circuit 110. The connections of microphones 102 and 104 should correspond with respect to their positive and negative terminals in order to keep the transduced signals in phase.
Gain circuit 108 is a single supply inverting amplifier circuit. The other side of capacitor Cl is connected to one side of a resistor R3 (lOOkΩ) and the other side of R3 is connected to one side of a resistor R4 (IMΩ) and an inverting input terminal of an operational amplifier (op amp) 128, which is preferably a TL084 Bifet op amp from Texas Instruments. The other side of R4 is connected to one side of resistors R5 (lOkΩ) and R6 (lOOkΩ) with the other side of R5 grounded and connected to the other side of R6 connected to an output terminal of op amp 128. This configuration gives gain circuit 108 a gain of approximately 100. One side of a resistor R7 (lOOkΩ) is connected to a noninverting input terminal of op amp 128 and the other side of R7 is grounded.
The configuration of gain circuit 110 resistors R8-R12 (lOOkΩ, IMΩ, 2.2kΩ, 470kΩ and lOOkΩ. respectively) is the same as that of gain circuit 108 resistors R3-R7 except the resistor values are such that a gain of approximately 400 is achieved at an output terminal of op amp 130 (TL084). Also, one side of a potentiometer 132 (lOkΩ) is connected to the ground side of resistor R10 and the other side of potentiometer is grounded. Potentiometer 132 provides a one time adjustment to equalize the magnitudes of the transduced signals from speaker 106. At the output terminals of op amps 128 and 130 the transduced signals from speaker 106 are now equal in magnitude but there is a phase difference caused by the distance between microphones 102 and 104.
Preferably, microphone 102 is less than an inch in front of and centered with respect to a cone (not shown) of speaker 06 and microphone 104 is placed five to seven inches from the front of speaker 106 and on the same axis with the cone and microphone 102, as shown in FIG. 2. The preferred physical construction of duplex transducer system 100 is designated by the numeral 200 in FIG. 2. An acoustical attenuator 202 such as polystyrene is placed between microphones 102 and 104 to provide further isolation of microphone 104 from the signals generated by speaker 106. Duplex transducer system 100 is preferably completely surround by an acoustical grill 204 which allows microphones 102 and 104 to receive a user's voice equally. A lighted push on-off switch 206 is provided for connection to the communication line.
To compensate for the phase difference between the transduced speaker signals at the output terminals of op amps 128 and 130 phase shifter 112 is connected to one of the output terminals of gain circuits 108 and 110. FIG. 1 shows phase shifter 112 connected to gain circuit 108 although it could be connected to gain circuit 110 with equally satisfactory results. Phase shifter 112 includes a resistor R13 (lOOkΩ) one side of which is connected to the output terminal of op amp 128 and to one side of a potentiometer 134 (50KΩ). Potentiometer 134 provides a one time adjustment to enable the phase of the transduced speaker signals at the outputs of gain circuit 110 and phase shifter 112 to be aligned precisely. The other side of R13 is connected to one side of a resistor R14 (lOOkΩ) and an inverting input terminal of an op amp 136 (TL084) and the other side of potentiometer 134 is connected to one side of a capacitor C3 (0.0 Iμf) and the noninverting input terminal of op amp 136. The other side of R14 is connected to an output of op amp 136 and the other side of C3 is grounded. The amount of phase shift necessary will vary but should be approximately 20-30 degrees.
The signals from phase shifter 112 and gain circuit 110 are then applied to differential amplifier 114. One side of a resistor R15 (lOOkΩ) of differential amplifier 114 is connected to the output terminal of op amp 136 and the other side of R15 is connected to one side of a resistor R16 (100kΩ) and an inverting input terminal of an op amp 138 (TL084). The output terminal of op amp 130 is connected to one side of a resistor R17 (lOOkΩ) and the other side of R17 is connected to one side of a resistor R18 (lOOkΩ) and a noninverting input terminal of op amp 138. The other side of R18 is grounded and connected to the other side of R16 is connected to an output terminal of
op amp 138 and one side of a capacitor C4 (O.Olμf) of high pass filter 116. The now equal and in phase transduced speaker signals are fed into differential amplifier 114 so that at the output of op amp 138 the transduced speaker signals are virtually eliminated due to the common mode rejection of op amp 138. Any other acoustic signals which have been transduced will be present at the output of op amp 138 because of the large gain of gain circuit 110. It should now be apparent to those skilled in the art that the annoying squeal caused from feedback associated with acoustical signals generated by speaker 106 is eliminated by duplex transducer system 100.
The other side of capacitor C4 is connected to one side of a resistor R19 (56KΩ) and one side of a capacitor C5 (O.Olμf). The other side of C5 is connected to one side of a resistor R20 and a noninverting input terminal of an op amp 140 (TL084) and the other side of R20 is grounded. An output terminal of op amp 140 is connected to the other side of R19, one side of a resistor R21 (47kΩ) and one side of a resistor R23 (4.7kΩ) of low pass filter 118. A resistor R22 is grounded and connected to the other side of R21 and to an inverting input terminal of op amp 140. High pass filter 116 eliminates unwanted low frequency resonation and low pass filter 118 described below eliminates high frequency resonation.
One side of a resistor R24 (4.7kΩ) of low pass filter 118 is connected to the other side of R23 and one side of a capacitor C6 (O.Olμf). The other side of R24 is connected to one side of a capacitor C7 and a noninverting input terminal of an op amp 142 (TL084) and the other side of C6 is connected to an output terminal of op amp 142. A resistor R25 (56kΩ) on one side is connected to ground and on the other side to one side of a resistor R26 (47kΩ) and to an inverting input terminal of op amp 142. The other side of R26 is connected to the output terminal of op amp 142.
Automatic level control (ALC) 120 is a conventional ALC using a Signetics Compander NE570. This exact circuit is described in the Signetics data book for the NE570. ALC 120 includes one side of capacitors C8 (2.2μf) and C9 (2.2μf) connected to the output terminal of op amp 142. The other side of C8 is connected to pin 11 of a compander 144 and the other side of C9 is connected to pin 15 of compander 144 and one side of a potentiometer 146. Ground is connected to the other side of potentiometer 146. Potentiometer 146 provides a "presence" control for duplex transducer system 100 bv which the amount of background noise the user wants to be transmitted is controlled
by adjusting potentiometer 146. The more background noise transmitted the greater the "presence." Other connections to compander 144 include capacitors C10-C14 (200pf, lμf, 2.2μf, lOμf and 2.2μf, respectively) and resistors R27 and R28 (33kΩ each) as shown in FIG. 1.
One side of C14 is connected to one side of a potentiometer 148 (50kΩ) which provides a one time adjustment for line driver 124 in order to match the voltage of the communication line. The adjustment arm of potentiometer 148 is connected to a noninverting input terminal of an op amp 150 and an inverting input terminal of op amp 150 is connected to an output terminal of op amp 150. The output terminal of op amp 150 is then connected to a conventional hybrid circuit (not shown) for connection with a communication line such as a telephone line.
A signal is received from the hybrid circuit above and connected to an ALC 122 which is the identical to ALC 120 described above. ALC 122 includes compander 144, capacitors C15-C21, potentiometer 152 and resistors R29 and R30 with all the values corresponding to those of ALC 120.
A potentiometer 154 of speaker driver 126, which provides volume control for speaker 106 is connected to C21, a noninverting input terminal of a power amplifier 156 (preferably an LM386 from National Semiconductor) and an inverting input terminal of power amplifier 156. The inverting input terminal of power amplifier 156 is also connected to -5 v.d.c. and one side of a capacitor C22 (0.05μf). The other side of C22 is connected to one side of a resistor R31 (10Ω) and the other side of R31 is connected to an output terminal of power amplifier 156 and one side of a capacitor C23 (50μf). Speaker 106 is connected to the other side of C23.
COMMON TRANSDUCER EMBODIMENT
Referring now to FIG. 3 a common transducer system 300 is shown. System 300 includes an automatic level control (ALC) 302, a transducer driver 304. a duplex transducer 306 and an equivalent impedance circuit 308 (shown here as a second duplex transducer), buffers 310, differential amplifiers 312 and 314, phase shifter 316, differential amplifier 318 and line driver 320.
In general, system 300 receives signals at ALC 302 which maintains the incoming signals at an essentially constant level and then the received signal is applied to transducer driver 304. Transducer driver 304 applies the received signals equally across
duplex transducer 306 and its equivalent impedance circuit 308 to produce acoustical signals and also to the four buffers 310 which provide isolation for the signals applied to buffers 310.
Buffers 310 also receive user generated signals from duplex transducer 306 and possibly from equivalent impedance circuit 308 if circuit 308 is an second duplex transducer. User generated signals must be received at transducer 306 and circuit 308 in phase, i.e. at the same time. The received signals are applied to differential amplifiers 312 and 314 in common mode but any transduced user signals are applied in differential mode because circuit 308 is connected to buffer circuits with a polarity opposite of transducer 306. This results in the received signals being greatly reduced and the user signals being increased at the outputs of differential amplifiers 312 and 314 compared to the signals at the inputs to the amplifiers.
Even though the received signals are greatly reduced another stage of filtering is required in order to obtain user signals which are sufficiently large with respect to the received signals. Therefore, phase shifter 316 is connected to the output of one of differential amplifiers 312 and 314 in order to bring the received signals present at the output terminals of differential amplifiers 312 and 314 into phase with respect to each other. Phase shifter 316 is shown in FIG. 3 connected to the output terminal of differential amplifier 312 but it could be connected to differential amplifier 314 and achieve equally satisfactory results. The output terminals of phase shifter 316 and differential amplifier 314 are connected to differential amplifier 318 and the received signals are even further reduced because they are again applied in common mode.
The user signals, on the other hand, will still essentially be in differential mode and therefore larger at the output terminal of differential amplifier 318 than at the input terminals. The user signals are then applied to line driver 320 and transmitted across a communication line.
In more detail, ALC 302 is a conventional ALC using a Signetics Compander NE570. This exact circuit is described in the Signetics data book for the NE570. ALC 302 includes one side of capacitors C30 (2.2μf ) and C31 (2.2μf) connected to an incoming communication line. The other side of C30 is connected to pin 11 of a compander 322 and the other side of C31 is connected to pin 15 of compander 322 and one side of a potentiometer 324. Potentiometer 324 provides a "presence" control for common
transducer system 300 by controlling the amount of background noise which the user wants to be transmitted. The more background noise transmitted the greater the "presence." Other connections to compander 322 include capacitors C32-C38 (lμf, 200pf. 2.2μf, lOμf and 2.2μf, respectively) and resistors R40 and R41 (33kΩ each) as shown in FIG. 3.
A transducer driver 304 is connected to ALC 302 at one side of a resistor R42 (27kΩ). The other side of R42 is connected to a potentiometer 326 which controls the volume of the received signals. Potentiometer 326 is also connected to one side of a resistor R43 (lOOkΩ). The other side of R43 is connected to one side of a resistor R44 (220kΩ) and an inverting input terminal of a power amplifier 328 (preferably, an LM386 from National Semiconductor). Power amplifier 328 provides the amplification necessary to drive duplex transducer 306 and equivalent impedance circuit, if necessary. A noninverting input terminal of power amplifier circuit 328 is grounded and the other side of R44 is connected to an output terminal of power amplifier 328 and to one side of a capacitor C39 (lμf). The other side of C39 is connected to a potentiometer 330 which in turn is connected to duplex transducer 306, equivalent impedance circuit 308 and buffer circuits 310, as shown in FIG. 3. Potentiometer 330 is adjusted to ensure that the received signals are applied equally to buffer circuits 310.
If equivalent impedance circuit 308 is a second duplex transducer then it needs to be connected to potentiometer 330 electrically opposite with respect to duplex transducer 306. This is so that user signals are transduced by circuit 308 and transducer 306 180 degrees out of phase with respect to each other, which allows the transduced user signals to be applied to differential amplifier circuits in differential mode. Duplex transducer 306 is preferably a conventional bone conduction transducer which is placed in a user's ear or could also be a device such as a flat plane piezo transducer.
Buffers 310 are conventional signal isolation circuits to prevent interference from occurring with the signals applied to differential amplifiers 312 and 314. All the buffers include op amps 332 (TL084) and capacitors C40 (lμf) with an inverting input terminal of op amps 332 connected to an output terminal of op amps 332. Noninverting input terminals of two of buffers 332 are connected to duplex transducer 306 and noninverting terminals of the other two buffers 332 are connected to equivalent impedance circuit 308. The output terminal of one of the buffers connected to circuit 308 is connected to a
noninverting input terminal of differential amplifier 312 and the other buffer connected to circuit 308 is connected to an inverting input terminal of differential amplifier 314. One of the output terminals of one- of the buffers connected to duplex transducer 306 is connected to an inverting input terminal of differential amplifier 312. The other output terminal of the buffers connected to duplex transducer 306 is connected to a potentiometer 334.
Potentiometer 334 is also connected to a noninverting input terminal of differential amplifier 314 and is adjusted to reduce the magnitude of the signals from the output of buffer 310 in order to allow the signals applied to the inverting input terminal of differential amplifier 314 to control the output of differential amplifier 314. Differential amplifiers 312 and 314 are identical, each have gains of approximately 470 and include resistors R45-R50 (lOOkΩ, lOOkΩ, lOOkΩ, IMΩ.470kΩ and lOkΩ, respectively) and an op amp 336 (TL084) connected as shown in FIG. 3. As explained above, the received signals are applied to differential amplifiers 312 and 314 in common mode and therefore are greatly reduced while any user signals from transducer 306 and equivalent impedance circuit 308 will be in differential mode and therefore amplified. It is noted that if equivalent impedance circuit 308 is not a transducer but is instead an impedance network to match the impedance of transducer 306 then the signals from transducer 306 will be passed through differential amplifiers 312 and 314 without being increased by any signals from circuit 308.
Though the user signals have been amplified and the received signals reduced there still needs to be further filtering of the received signals to achieve a sufficiently large user signal compared to the receive signal. This is because a received signal for a user to hear comfortably with a bone conduction transducer must be on the order of 50 to 100 millivolts but the user signal will only be approximately 1 millivolt at the output of the bone conduction transducer. Therefore, phase shifter 316 is connected to differential amplifier 312 to bring the remaining received signals of differential amplifier 312 into phase with the received signals of differential amplifier 314. This phase shift is approximately 3-5 degrees which allows the user signals to remain in essentially differential mode so that the user signals are again amplified and the received signals reduced after passing through differential amplifier 318.
Phase shifter 316 includes a resistor R51 (lOOkΩ) one side of which is connected
to the output terminal of differential amplifier circuit 312 and to a potentiometer 338. The other side of R51 is connected to. one side of a resistor R52 (lOOkΩ) and an inverting input terminal of an op amp 340 (TL084). Potentiometer 338, which provides a one time adjustment to allow the phase to be adjusted precisely, is also connected to one side of a capacitor C41 and a noninverting input terminal of op amp 340. The other side of C41 is grounded and the other side of R52 is connected to an output terminal of op amp 340 and to an inverting input terminal of differential amplifier circuit 318.
Differential amplifier 318 includes resistors R53-R56 (each lOOkΩ), an op amp 342 (TL084) and a capacitor C42 (lμf) connected as shown in FIG. 3. A potentiometer 344 of line driver 320 is connected to one side of C42 and is adjusted to match the communication line voltage. Line driver 320 further includes one side of a resistor R57 (lOOkΩ) connected to potentiometer 344 and the other side to a resistor R58 (220kΩ) and an inverting input terminal of an op amp 346 (TL084). The other side of R58 is connected to an output terminal of op amp 346 and one side of a capacitor C43. A noninverting input terminal of op amp 346 is grounded and the other side of C43 is connected to the communication line.
As those skilled in the art will appreciate, it is noted that substitutions may be made for the preferred embodiments and equivalents employed herein without departing from the scope of the present invention as recited in the claims. For example, microphone 104 could be replaced with two or more microphones in order to receive more user signal at gain circuit 110. Also, other configurations of microphone 104 with respect to speaker 106 could be employed such as having microphone 104 positioned adjacent speaker 106. In addition, potentiometer 334 could be eliminated which would result in phase shift circuit 316 needing to shift the phase of the received signals 20-30 degrees.
Having thus described the preferred embodiment of the present invention, the following is claimed as new and desired to be secured by Letters Patent: