GB2295300A - Hands-free Duplex radio transceiver apparatus and method of training - Google Patents
Hands-free Duplex radio transceiver apparatus and method of training Download PDFInfo
- Publication number
- GB2295300A GB2295300A GB9423348A GB9423348A GB2295300A GB 2295300 A GB2295300 A GB 2295300A GB 9423348 A GB9423348 A GB 9423348A GB 9423348 A GB9423348 A GB 9423348A GB 2295300 A GB2295300 A GB 2295300A
- Authority
- GB
- United Kingdom
- Prior art keywords
- signal
- echo cancelling
- hands
- radio transceiver
- echo
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M9/00—Arrangements for interconnection not involving centralised switching
- H04M9/08—Two-way loud-speaking telephone systems with means for conditioning the signal, e.g. for suppressing echoes for one or both directions of traffic
- H04M9/082—Two-way loud-speaking telephone systems with means for conditioning the signal, e.g. for suppressing echoes for one or both directions of traffic using echo cancellers
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B3/00—Line transmission systems
- H04B3/02—Details
- H04B3/20—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other
- H04B3/23—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers
Landscapes
- Engineering & Computer Science (AREA)
- Signal Processing (AREA)
- Computer Networks & Wireless Communication (AREA)
- Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
Abstract
A hands-free duplex radio transceiver is described having a transmitting part (33), a receiving part (34), a microphone (31) and a speaker (20). The transmitting part (33) has a single frequency synthesizer (25) which produces a modulated signal (24) which is used as an injection signal in the receiver part. A double echo canceller is provided in a digital signal processor (32) to cancel from the voice signal from the microphone (31) acoustic feedback arising from the speaker (20) and to cancel from the baseband receive signal (16) from the receiver part (34) any feedback arising from the modulated signal (24) from the frequency synthesizer (25). A method of training the double echo canceller is also described. Applications is to a vehicle speakerphone. <IMAGE>
Description
Hands-free Duplex Radio Transceiver Apparatus and Method of Training
Field of the Invention
This invention relates to the field of hands-free duplex radio transceiver apparatus. Separately and in addition it relates to a method of training an echo canceller.
Background of the Invention
Modern full duplex mobile radio transceivers offer the possibility to operate in a hands-free mode without using a handset. When intended for installation in a vehicle, such equipment is sometimes referred to as a vehicular speakerphone (VSP).
From European Patent EP0204718B1, invented by Puhl et al. and assigned to the same assignee as the present invention, some hands-free techniques used for mobile radio transceivers are known. An adaptive double echo canceller arrangement is described that cancels the speaker to microphone coupling and whatever echo comes from the hybrid (i.e. the fixed end telephone switch) and the remainder of the communications system.
In British Patent Application GB2267015A invented by Schwartzman et al. and assigned to the same assignee as the present invention, a cost effective design is described that uses only a single frequency generator for duplex mobile radio transceivers. With that known technique a handset is required because the feedback of the modulated local oscillator is suppressed by approximately 20 dB only. In that application the feedback is considered as an advantage for handset users.
The applicant has found that the arrangement of that British
Application is not suitable for handsfree operations because an separation of at least 30 dB for suppression of audio regeneration and howling would be required.
Additionally it is a problem that known echo cancellers use adaptation techniques that are processor-intensive.
In the field of voice conference systems, it is known to train an echo canceller by feeding a training signal to the loudspeaker. There is a need for an improved echo canceller for use in a VSP.
Summarv of the Invention
According to a first aspect of the present invention a hands-free duplex radio transceiver apparatus is provided comprising a transmitting part, a receiving part, a microphone and a speaker. The transmitting part has frequency generating and modulating means for generating a transmit frequency signal modulated with a voice signal received by the microphone. The receiving part has a mixer and means for injecting the modulated signal into the mixer for downmixing the received radio signal to provide an intermediate frequency signal.The transceiver apparatus further comprises demodulating means coupled to the speaker to demodulate the intermediate frequency signal to provide a baseband receive signal and to reproduce acoustically the baseband receive signal at the speaker; first echo cancelling means to cancel from the voice signal acoustic feedback arising from the speaker and second echo cancelling means to cancel from the baseband receive signal feedback arising from the modulated signal.
In this manner, the influence of the modulated injection signal upon the receive path can be cancelled by the echo canceller.
It has been found that with the use of an echo canceller the feedback via the modulated local oscillator can be reduced by approximately 20 dB.
With this novel design the applicant has achieved 40 dB overall TX/RX separation thus allowing handsfree operation.
The arrangement has the advantage of requiring only a single frequency generator (e.g. single voltage controlled oscillator or single synthesizer), which is a compact and inexpensive arrangement.
In accordance with a second aspect of the invention, a training method is provided for setting filter coefficients of an echo cancelling means in a hands-free duplex radio transceiver apparatus having a transmitting part, a receiving part, a speaker and a microphone, the microphone providing the transmitter part with a received voice signal, and the receiving part having an output to provide a baseband receive signal, the output being coupled to the speaker to reproduce acoustically the baseband receive signal; wherein the echo cancelling means are arranged to cancel from the voice signal acoustic feedback arising from the speaker. The method comprises the steps of: switching off the baseband receive signal; providing the echo cancelling means and the speaker with a test signal; minimizing with the echo cancelling means the acoustic feedback between the speaker and the microphone.
It is preferred that the echo cancelling filter coefficients are stored in a non-volatile memory after training. By training the echo canceller, later adaptation is rapid and less intensive on processor power. By storing the coefficients in non-volatile memory, training is not necessary on repeated occurrences of use of the echo canceller.
In accordance with a further aspect of the invention, a training method is provided for setting filter coefficients of an echo cancelling means in a hands-free duplex radio transceiver apparatus having a transmitting part, a receiving part and echo cancelling means. The method comprises the steps of: providing the transmitting part with a modulator input and modulating means for generating a modulated transmit frequency signal, providing the receiving part with a mixer, demodulating means and means for injecting the modulated transmit frequency signal into the mixer for downmixing a received radio signal to provide an intermediate frequency signal, receiving with the hands-free duplex radio transceiver a first test signal provided by test transmitting means independent from the hands-free duplex radio transceiver apparatus; providing the modulator input with a second test signal; demodulating the intermediate frequency to provide a baseband receive signal; and minimizing with the echo cancelling means feedback arising in the baseband receive signal originating from the modulated transmit frequency signal.
In accordance with a further aspect of the invention, a method is provided for setting filter coefficients of a first and second echo cancelling means in a hands-free duplex radio transceiver apparatus having a transmitting part with a modulator input, a receiving part, a speaker, and a microphone, the microphone providing the modulator input with a received voice signal thereby generating a modulated transmit frequency signal, the receiving part having a mixer, demodulating means and means for injecting the modulated transmit frequency signal into the mixer for downmixing the received radio signal to provide an intermediate frequency signal, the demodulating means providing a demodulated baseband receive signal, the first echo cancelling means being arranged to cancel from the voice signal acoustic feedback arising from the speaker, and the second echo cancelling means being arranged to cancel from the demodulated baseband receive signal feedback arising from the modulated transmit frequency signal. The method comprises the steps of: switching off the feedback cancelled baseband receive signal; switching off the echo cancelled received voice signal; providing the first echo cancelling means and the speaker with a second test signal; minimizing with the first echo cancelling means the acoustical feedback between speaker and microphone; receiving with the hands-free duplex radio transceiver a first test signal provided by test transmitting means independent from the hands-free duplex radio transceiver apparatus; providing the second echo cancelling means and the modulator input with a third test signal; and minimizing with the second echo cancelling means the feedback arising from the modulated transmit frequency signal.
Brief Description of the Drawings
FIG. 1 shows a full duplex radio transceiver with a single frequency generator and a double echo canceller according to the preferred embodiment of the present invention and
FIG. 2 serves to illustrate operation of the circuit of FIG. 1 during a training stage of operation.
Detailed description of the drawings
Referring to FIG. 1 there is shown a full duplex radio transceiver with a double echo canceller. The transceiver comprises an antenna 10 connected to a duplexer 11, connected in turn to a receive path and a transmit path. The receive path comprises a receiver front end 12, a mixer 13 and a demodulator 15 connected via a summing stage 17 to an audio amplifier 19 and a loudspeaker 20. The transmit path comprises a radio frequency (r.f.) power amplifier 23 connected to a frequency generating and modulating means 25 having a modulator input 26 connected to an audio amplifier 30 via a summing stage 28. The frequency generating and modulating means 25 are preferably a synthesizer having a VCO locked to a reference source (not shown). A microphone 31 is connected to the input of the audio amplifier 30.
The receiver front end 12, mixer 13 and demodulator 15 will be referred to as the receive part 34. The frequency generating and modulating means 25 and amplifier 23 will be referred to as the transmit part 33.
The summing stages 17 and 28 are parts of a double echo canceller formed in software in a digital signal processor (DSP) 32. The double echo canceller comprises first and second echo cancellers 22 and 21. The first echo canceller 22 is connected between the receive path (at the output of the summing stage 17) and the summing stage 28 and the second echo canceller 21 is connected between the transmit path (at the output of the summing stage 28) and the summing stage 17.
During operation voice is received by the microphone 31, amplified by the audio amplifier 30 and filtered by the first echo canceller 22 and the resultant echo cancelled received voice signal 27 is provided to the modulator input 26 of the transmitting part 33. A modulated transmit signal 24 is amplified by r.f. power amplifier 23 and passed via duplexer 11 to the antenna 10.
Received radio signals are picked up by the antenna 10, passed through the duplexer 11, received and demodulated by the receiving part 34 and filtered by the second echo canceller 21 and a resultant feedback cancelled baseband receive signal 18 is provided to the audio amplifier 19 for amplification and is reproduced at the speaker 20.
In the receiving part 34 the received radio signal is mixed with the modulated transmit signal 24 produced by the frequency generating and modulating means 25 of the transmitting part 33 to downmix to an intermediate frequency (i.f.) signal 14. The i.f. signal 14 has the same frequency as the transmit-receive duplex separation of the signals transmitted and received at the antenna 10. The i.f. signal 14 is demodulated by the demodulator 15 to create a demodulated baseband receive signal 16.
Acoustic feedback arising from the speaker 20 and received by the microphone 31 is cancelled from a received voice signal 29 produced by the audio amplifier 30 at the summing stage 28 by subtracting via the first echo canceller 22 the filtered feedback cancelled baseband receive signal 18, thereby providing the echo cancelled received voice signal 27.
Internal injection feedback is similarly cancelled from the demodulated baseband receive signal 16 at the summing stage 17 by subtracting via the second echo canceller 21 the filtered echo cancelled received voice signal 27.
Referring to FIG. 2 a method of training the double echo canceller is now explained. In order to avoid repetition only new elements will be described. In addition to the elements of FIG.1, a switch 35 (illustrated as a hardware element but implemented in software) allows disconnection of the demodulated baseband receive signal 16 from the summing stage 17, a switch 36 allows disconnection of the feedback cancelled baseband receive signal 18 from the first echo canceller 22 and audio amplifier 19, a switch 38 allows disconnection of the received voice signal 29 from the summing stage 28 and a switch 37 allows disconnection of the echo cancelled received voice signal 27 from the second echo canceller 21 and the modulator input 26. A non-volatile memory 39 is provided that stores the filter coefficients of the two echo cancellers.A test signal (white noise) generator 43 is connected to the first and second echo cancellers 22 and 21. A voice activity detector (VAD) 44 is connected to sample the signals at the inputs and outputs of the summing stages 17 and 27 and is connected (as shown in dashed outline) to the first and second echo cancellers 22 and 21 for control of adaptation of the echo cancellers.
A suitable VAD is known from the field of landline echo suppressors and need not be described in detail.
A test transmitter 40 e.g. a base station with connection to a local telephone terminal (and having a hybrid circuit typically giving -6bB of side tone) provides the hands-free duplex radio transceiver with an r.f. test signal 42 to be picked up at the antenna 10.
By preference the two echo cancellers 21, 22, the switches 35 to 38, the summing stages 17, 28, the test signal generator 43 and the VAD 44 are implemented in one digital signal processor 32 and for clarity reasons neither the internal connections between the memory and the echo cancellers nor the control connections within the DSP 32 to the switches 35 to 38 and echo cancellers 21, 22 are shown.
Because of the independence of the training methods the echo cancellers are trained individually once at the time of installation of a VSP into a vehicle. The training of the two echo cancellers can be carried out one after the other or simultaneously. In a non-limiting way first the training of the first echo canceller 22 will be described and later the training of the second echo canceller 21.
In a first step the DSP 32 opens the switches 35, 36 and 37, freezes via
VAD 44 the adaption of the second echo canceller 21, closes the switch 38 and causes the test signal generator 43 to inject white noise into the first echo canceller 22 and audio amplifier 19. That noise is reproduced by the speaker 20 and received by the microphone 31, thereby allowing the first echo canceller 22 to match its transfer function to the acoustics of the vehicle so that the feedback is minimized. Then the noise injection of the test signal generator 43 into the first echo canceller 22 and audio amplifier 19 is stopped.
In a second step the DSP 32 opens the switches 36, 37 and 38, freezes via VAD 44 the adaption of the first echo canceller 22, closes the switch 35 and causes the test signal generator 43 to inject white noise into the second echo canceller 21 and modulator input 26. At the same time, a test call is placed to a predetermined test telephone terminal via the test transmitter 40. The generated noise is provided via the modulator input 26 to the frequency generating and modulating means 25 and produces a modulated transmit signal 24 that is injected into the mixer 13 where it is mixed with a received r.f. test signal 42 produced by the test-transmitter 40. The r.f.
test signal 42 radiated from the test transmitter 40 is modulated with any echo arising from the white noise (because of the influence of its hybrid circuit) but is otherwise unmodulated. The received signal is downmixed to an intermediate frequency signal 14 and further demodulated by the demodulator 15 thereby producing a demodulated baseband receive signal 16. This allows the second echo canceller 21 to match its transfer function, i.e. its filter coefficients, to minimize the injection feedback.
One skilled in the art will appreciate that the number of software switches 35 - 38, the positions of these switches and the positions of injection of the (second and third) test signals, as well as the exact sequence of operations can be varied.
After having finished the training of the two echo cancellers all the switches are closed, the noise producing is stopped and the test transmitter is no longer necessary.
The filter coefficients are then stored in the memory 39. In the case where the radio transceiver has a code plug memory for storing the identity of the radio, it is advantageous to use that memory to store also the filter coefficients of the two echo cancellers.
During normal operation the echo cancellers adapt dynamically to the actual situation in the vehicle cabin. If for example a person enters the vehicle, the acoustics of the cabin are changed and the first echo canceller 22 has to adapt itself to that new situation. This is a standard adaptation technique which need not be described in detail.
In a similar way the second echo canceller 21 adapts itself to changes in the injection feedback (if any).
In order to avoid divergence it is necessary to allow only one echo canceller to adapt at a time while freezing the other one's coefficients.
This is done with the voice activity detector 44 that detects speech activity on the land side, the mobile side or both and freezes either the first echo canceller, the second canceller or both. Various schemes can be devised for freezing these coefficients, dependent on different levels of voice activity in the various signals.
Claims (17)
1. A hands-free duplex radio transceiver apparatus comprising a transmitting part, a receiving part, a microphone and a speaker;
the transmitting part having frequency generating and modulating means for generating a modulated signal comprising a transmit frequency signal modulated with a voice signal received by the microphone, and
the receiving part having a mixer and means for injecting the modulated signal into the mixer for downmixing the received radio signal to provide an intermediate frequency signal;
wherein the transceiver apparatus further comprises::
demodulating means coupled to the speaker to demodulate the intermediate frequency signal to provide a baseband receive signal and to reproduce acoustically the baseband receive signal at the speaker;
first echo cancelling means to cancel from the voice signal acoustic feedback arising from the speaker and
second echo cancelling means to cancel from the baseband receive signal feedback arising from the modulated signal.
2. A hands-free duplex radio transceiver apparatus as described in claim 1 further comprising test signal producing means for injecting a test signal in at least one of the first and second echo cancelling means for training the first and second echo cancelling means.
3. A hands-free duplex radio transceiver apparatus as described in claims 1 or 2 wherein the test signal producing means injects a test signal in each of the first and second echo cancelling means.
4. A hands-free duplex radio transceiver apparatus as described in any of the preceding claims wherein the test signal producing means produces white noise.
5. A hands-free duplex radio transceiver apparatus as described in claim 1 wherein the two echo cancelling means are implemented in one digital signal processor.
6. A hands-free duplex radio transceiver apparatus as described in claim 1 comprising a code plug memory and means for storing the echo cancelling filter coefficients in the code plug memory.
7. A hands-free duplex radio transceiver according to claim 1, wherein the first and second echo cancellers are adaptive echo cancellers, the transceiver further comprising a voice activity detector arranged to detect speech activity in the voice signal and the baseband receive signal and, responsive to the detection of voice, arranged to allow one only of said echo cancellers to adapt at a time.
8. A training method for setting filter coefficients of an echo cancelling means in a hands-free duplex radio transceiver apparatus having a transmitting part, a receiving part, a speaker and a microphone, the microphone providing the transmitter part with a received voice signal, and the receiving part having an output to provide a baseband receive signal, the output being coupled to the speaker to reproduce acoustically the baseband receive signal; wherein the echo cancelling means are arranged to cancel from the voice signal acoustic feedback arising from the speaker,
the method comprising the steps of:
switching off the baseband receive signal;
providing the echo cancelling means and the speaker with a test signal;
minimizing with the echo cancelling means the acoustic feedback between the speaker and the microphone.
9. A training method for setting filter coefficients of the echo cancelling means in a hands-free duplex radio transceiver apparatus as described in claim 8, wherein the test signal is white noise.
10. A training method as described in claim 8 or 9 comprising the step of storing the echo cancelling filter coefficients in a non-volatile memory.
11. A training method for setting filter coefficients of an echo cancelling means in a hands-free duplex radio transceiver apparatus having a transmitting part, a receiving part and echo cancelling means, the method comprising the steps of:
providing the transmitting part with a modulator input and modulating means for generating a modulated transmit frequency signal,
providing the receiving part with a mixer, demodulating means and means for injecting the modulated transmit frequency signal into the mixer for downmixing a received radio signal to provide an intermediate frequency signal,
receiving with the hands-free duplex radio transceiver a first test signal provided by test transmitting means independent from the handsfree duplex radio transceiver apparatus;
providing the modulator input with a second test signal;;
demodulating the intermediate frequency to provide a baseband receive signal; and
minimizing with the echo cancelling means feedback arising in the baseband receive signal originating from the modulated transmit frequency signal.
12. A training method for setting filter coefficients of the echo cancelling means in a hands-free duplex radio transceiver apparatus as described in claim 11, wherein the second test signal is white noise.
13. A training method for setting filter coefficients of the echo cancelling means in a hands-free duplex radio transceiver apparatus as described in claim 11, wherein the first test signal is modulated with any echo arising from the second test signal but is otherwise unmodulated.
14. A training method for setting filter coefficients of a first and second echo cancelling means in a hands-free duplex radio transceiver apparatus having a transmitting part with a modulator input, a receiving part, a speaker, a microphone, the microphone providing the modulator input with a received voice signal thereby generating a modulated transmit frequency signal, the receiving part having a mixer, demodulating means and means for injecting the modulated transmit frequency signal into the mixer for downmixing the received radio signal to provide an intermediate frequency signal, the demodulating means providing a demodulated baseband receive signal, the first echo cancelling means being arranged to cancel from the voice signal acoustic feedback arising from the speaker, and the second echo cancelling means being arranged to cancel from the demodulated baseband receive signal feedback arising from the modulated transmit frequency signal,
the method comprising the steps of:
switching off the feedback cancelled baseband receive signal;
switching off the echo cancelled received voice signal;
providing the first echo cancelling means and the speaker with a second test signal;
minimizing with the first echo cancelling means the acoustical feedback between speaker and microphone;
receiving with the hands-free duplex radio transceiver a first test signal provided by a test transmitting means independent from the handsfree duplex radio transceiver apparatus;
providing the second echo cancelling means and the modulator input with a third test signal; and
minimizing with the second echo cancelling means the feedback arising from the modulated transmit frequency signal.
15. A training method for setting filter coefficients of first and second echo cancelling means in a hands-free duplex radio transceiver apparatus as described in claim 14, wherein both echo cancelling means are trained independently at the same time.
16. A training method for setting filter coefficients of first and second echo cancelling means in a hands-free duplex radio transceiver apparatus
as described in claim 14, wherein the second and third test signals are
white noise.
17. A training method for setting filter coefficients of a second echo cancelling means in a hands-free duplex radio transceiver apparatus as described in claim 14, wherein the first test signal is modulated with any echo arising from the third test signal but is otherwise unmodulated.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9423348A GB2295300A (en) | 1994-11-18 | 1994-11-18 | Hands-free Duplex radio transceiver apparatus and method of training |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9423348A GB2295300A (en) | 1994-11-18 | 1994-11-18 | Hands-free Duplex radio transceiver apparatus and method of training |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9423348D0 GB9423348D0 (en) | 1995-01-11 |
| GB2295300A true GB2295300A (en) | 1996-05-22 |
Family
ID=10764650
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9423348A Withdrawn GB2295300A (en) | 1994-11-18 | 1994-11-18 | Hands-free Duplex radio transceiver apparatus and method of training |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2295300A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1999035812A1 (en) * | 1998-01-09 | 1999-07-15 | Ericsson Inc. | Methods and apparatus for controlling echo suppression in communications systems |
| GB2344500A (en) * | 1998-12-02 | 2000-06-07 | Mitel Corp | Speakerphone echo cancellers |
Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1986003912A1 (en) * | 1984-12-14 | 1986-07-03 | Motorola, Inc. | Full duplex speakerphone for radio and landline telephones |
| GB2220546A (en) * | 1988-06-20 | 1990-01-10 | Nec Corp | Voice conference system using echo cancellers |
| US5008923A (en) * | 1989-04-19 | 1991-04-16 | Hitachi, Ltd. | Testable echo cancelling method and device |
| GB2267015A (en) * | 1992-05-16 | 1993-11-17 | Motorola Israel Ltd | Interference cancelling a radio transceiver |
-
1994
- 1994-11-18 GB GB9423348A patent/GB2295300A/en not_active Withdrawn
Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1986003912A1 (en) * | 1984-12-14 | 1986-07-03 | Motorola, Inc. | Full duplex speakerphone for radio and landline telephones |
| GB2220546A (en) * | 1988-06-20 | 1990-01-10 | Nec Corp | Voice conference system using echo cancellers |
| US5008923A (en) * | 1989-04-19 | 1991-04-16 | Hitachi, Ltd. | Testable echo cancelling method and device |
| GB2267015A (en) * | 1992-05-16 | 1993-11-17 | Motorola Israel Ltd | Interference cancelling a radio transceiver |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1999035812A1 (en) * | 1998-01-09 | 1999-07-15 | Ericsson Inc. | Methods and apparatus for controlling echo suppression in communications systems |
| US6148078A (en) * | 1998-01-09 | 2000-11-14 | Ericsson Inc. | Methods and apparatus for controlling echo suppression in communications systems |
| GB2344500A (en) * | 1998-12-02 | 2000-06-07 | Mitel Corp | Speakerphone echo cancellers |
| GB2344500B (en) * | 1998-12-02 | 2003-06-25 | Mitel Corp | Improved method of operating a full duplex speakerphone echo canceller |
| US6768723B1 (en) | 1998-12-02 | 2004-07-27 | Zarlink Semiconductor Inc. | Method of operating a full duplex speakerphone echo canceller |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9423348D0 (en) | 1995-01-11 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |