CA2229424C - Circuit to improve transducer separation in handsfree telephone - Google Patents
Circuit to improve transducer separation in handsfree telephone Download PDFInfo
- Publication number
- CA2229424C CA2229424C CA002229424A CA2229424A CA2229424C CA 2229424 C CA2229424 C CA 2229424C CA 002229424 A CA002229424 A CA 002229424A CA 2229424 A CA2229424 A CA 2229424A CA 2229424 C CA2229424 C CA 2229424C
- Authority
- CA
- Canada
- Prior art keywords
- microphone
- speaker
- enclosure
- sound
- housing
- 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.)
- Expired - Fee Related
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M1/00—Substation equipment, e.g. for use by subscribers
- H04M1/60—Substation equipment, e.g. for use by subscribers including speech amplifiers
- H04M1/6033—Substation equipment, e.g. for use by subscribers including speech amplifiers for providing handsfree use or a loudspeaker mode in telephone sets
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M1/00—Substation equipment, e.g. for use by subscribers
- H04M1/02—Constructional features of telephone sets
- H04M1/19—Arrangements of transmitters, receivers, or complete sets to prevent eavesdropping, to attenuate local noise or to prevent undesired transmission; Mouthpieces or receivers specially adapted therefor
Landscapes
- Engineering & Computer Science (AREA)
- Signal Processing (AREA)
- Telephone Function (AREA)
- Circuit For Audible Band Transducer (AREA)
- Telephone Set Structure (AREA)
Abstract
In a device having a sound source within an enclosure and a first microphone for picking up sounds outside the enclosure, a system for improving acoustic separation between the sound source and first microphone comprising an additional microphone located approximately equidistant from the sound as the first microphone for picking up sounds within the enclosure, and a circuit connected to the firstmicrophone and the additional microphone for subtracting the sounds within the enclosure from the sounds outside the enclosure.
Description
Circuit to Improve Transducer Separation in Handsfree Telephone Field of the Invention The present invention relates in general to devices incorporating a microphone and a sound or noise source in a single enclosure, such as handsfree or speakerphone systems, telephones, and more particularly to a circuit for improved separation between microphone and speaker.
0 ~ k~round of the Invention It is a requirement of a handsfree or speakerphone design to provide sufficient acoustic separation between the microphone and the speaker. For systems where the microphone and speaker are to be in the same enclosure this requirement can present a challenge. This is a problem for both half-duplex and full-duplex systems, where the acoustic coupling of the speaker output to the microphone input must be minimi7ed to m~int~in stability in the system.
One component of this coupling is mechanical (i.e. through the structure of the enclosure). The other component is transmitted via air. This coupling is usually greater at or about the resonance frequency of the speaker.
Much acoustic, industrial and mechanical design effort can be expended in design of the enclosure in order to minimi7.~ the separation, but this does not usually 2s occur because of cost and the limited time within which to supply a product to market. Sometimes the acoustic aspect of an enclosure are dealt with after otheraspects of the enclosure are fin~li7e-1, thus reducing the opportunity to minimi7e couplmg.
The use of unidirectional microphones can reduce coupling, but it is difficult to design unidirectional microphones into a telephone type of enclosure because they require the sound to be present at the front and back of the microphone. Also, a unidirectional microphone restricts the user to one position in front of the microphone.
It is an object of an aspect of the present invention to provide a simple and inexpensive means for providing improved separation between the speaker and s microphone in a handsfree or speakerphone system where both transducers are located in the same enclosure.
Summarv of the Invention o According to the present invention, improved acoustic separation is provided in an handsfree or speakerphone enclosure by means of providing a second inexpensive omnidirectional microphone (referred to herein as the speaker microphone) which is placed inside the enclosure to primarily pick up sound from the speaker, and a circuit for subtracting this sound from the sound picked up by the voice microphor e which is positioned to pick up sound occurr.ng outside of the enclosure.
Brief Introduction to the Drawin~s A detailed description of the pr~relled embodiment is provided herein below, with reference to the following drawings, in which:
Figure 1 shows placement of a speaker microphone within a handsfree telephone set relative to the speaker and the voice microphone, according to an aspect of the invention; and Figure 2 is a schematic diagram of a circuit for subtracting the sound picked up by the speaker microphone from the sound picked up by the voice microphone.
Detailed De- i~lion of the Preferred Embodiment Turning to Figure 1, a handsfree telephone set enclosure 1 is shown comprising a plastic cover 3 mounted to a plastic base 5. A speaker 7 is provided 5 within the enclosure 1 for co,l"l,u"icating sound to a user through a grill 8 in the plastic cover 3. To facilitate handsfree telephone communications in conjunction with the speaker 7, a voice microphone 9 is provided in the enclosure l for picking up sounds via an aperture 10 in the plastic cover 3. A printed circuit board 1 l is provided in the base S for housing the microphone 9 and additional ch~;uill.y required for o operation of the telephone.
According to the present invention, a speaker microphone 13 is inside the enclosure 1 for picking up the sound emitted by the speaker 7. The enclosure 1 sigIuficantly attenuates sounds from outside of the enclosure, such that the sounds 5 picked up by the speaker microphone 13 may be subtracted from the sounds picked up by the voice microphone 9 which picks up sounds from outside of the enclosure, including sounds emitted by the speaker 7. As shown in Figure 1, the microphones 9 and 13 are positioned so as to be equidistant from the speaker 7, and are mechanically connected to the enclosure 1 in a similar fashion. Preferably, the microphones 9 and 20 13 are the same model of microphone but need not be perfectly matched, provided that the sounds from speaker 7 are picked up by both microphones 9 and 13 subst~nti~lly in phase.
A subtracting circuit is illustrated in Figure 2, according to one embodiment of25 the present invention. The subtracting circuit is preferably designed for optimum performance in the frequency band in which the voice microphone 9 has maximum coupling to speaker 7 (i.e. typically a band around the resonance frequency of speaker 7). Each of the voice microphone and speaker microphone 13 are connected to a respective input of an operational amplifier 15 via pull-up resistors 17 and 19, each 30 having a predetennined resistance (Rbias) and connected to a source of bias voltage (Vbias). Voice microphone 9 is connected to an inverting input of amplifier 15 via a seriesconnectionofcapacitor21 andresistor23. Speakermicrophone 13 is connected to a non-inverting input of amplifier 15 via a series connection of capacitor 25 and resistor 27. A feeclback resislor 29 connects the output of amplifier 15 to the inverting input thereof, and a resistor 31 connects the non~inverting input of amplifier 15 to ground.
The gains and poles for the signal paths in the circuit embodiment of Figure 2 are useful for a situation where the microphone outputs are in phase and the output of speaker microphone 13 is twice that of voice microphone 9. Specifically, Each ofo resistors 23, 29 and 31 has resistance "R", capacitor 25 has a capacitance of"C" and capacitor 21 has a capacitance of "4C"', while resistor 27 has a resistance of "3R".
The gain of the voice microphone 9 is -R/R=l, while the gain of the speaker microphone 13 is R/(R+3R)x(l+R/R)=1/2. Since the frequency response is the same for both microphones, in-phase signals the speaker sounds picked up by voice 15 microphone 9 are canceled by the speaker sounds picked up by the speaker microphone 13, leaving only the desired voice sounds at the output of the circuit (Vout).
Testing of the system according to the present invention has resulted in 20 measured cancellation of 20dB with matched microphones and 12 dB with 2.5 dB
mi~m~tt~h between the microphones 9 and 13.
Alternatives and modifications of the invention are possible. All such embodiments and modifications are believed to be within the sphere and scope of the 25 invention as set forth in the claims appended hereto. For example, the principles of the present invention may be implemented in digital form (e.g. via a digital signal processor (DSP)). Moreover, it is contemplated that the principles of the invention may be employed in any device co~ti~ g a microphone and a sound or noise source within a single enclosure, such as video conferencing systems, cellular hands free 30 devices and portable tape machines. [n the case of a tape machine, the source is noise and vibration from the tape drive, whereas video conferencing and cellular handsfree phones are applications of the speaker phone technology discussed in connection with the preferred embodiment.
0 ~ k~round of the Invention It is a requirement of a handsfree or speakerphone design to provide sufficient acoustic separation between the microphone and the speaker. For systems where the microphone and speaker are to be in the same enclosure this requirement can present a challenge. This is a problem for both half-duplex and full-duplex systems, where the acoustic coupling of the speaker output to the microphone input must be minimi7ed to m~int~in stability in the system.
One component of this coupling is mechanical (i.e. through the structure of the enclosure). The other component is transmitted via air. This coupling is usually greater at or about the resonance frequency of the speaker.
Much acoustic, industrial and mechanical design effort can be expended in design of the enclosure in order to minimi7.~ the separation, but this does not usually 2s occur because of cost and the limited time within which to supply a product to market. Sometimes the acoustic aspect of an enclosure are dealt with after otheraspects of the enclosure are fin~li7e-1, thus reducing the opportunity to minimi7e couplmg.
The use of unidirectional microphones can reduce coupling, but it is difficult to design unidirectional microphones into a telephone type of enclosure because they require the sound to be present at the front and back of the microphone. Also, a unidirectional microphone restricts the user to one position in front of the microphone.
It is an object of an aspect of the present invention to provide a simple and inexpensive means for providing improved separation between the speaker and s microphone in a handsfree or speakerphone system where both transducers are located in the same enclosure.
Summarv of the Invention o According to the present invention, improved acoustic separation is provided in an handsfree or speakerphone enclosure by means of providing a second inexpensive omnidirectional microphone (referred to herein as the speaker microphone) which is placed inside the enclosure to primarily pick up sound from the speaker, and a circuit for subtracting this sound from the sound picked up by the voice microphor e which is positioned to pick up sound occurr.ng outside of the enclosure.
Brief Introduction to the Drawin~s A detailed description of the pr~relled embodiment is provided herein below, with reference to the following drawings, in which:
Figure 1 shows placement of a speaker microphone within a handsfree telephone set relative to the speaker and the voice microphone, according to an aspect of the invention; and Figure 2 is a schematic diagram of a circuit for subtracting the sound picked up by the speaker microphone from the sound picked up by the voice microphone.
Detailed De- i~lion of the Preferred Embodiment Turning to Figure 1, a handsfree telephone set enclosure 1 is shown comprising a plastic cover 3 mounted to a plastic base 5. A speaker 7 is provided 5 within the enclosure 1 for co,l"l,u"icating sound to a user through a grill 8 in the plastic cover 3. To facilitate handsfree telephone communications in conjunction with the speaker 7, a voice microphone 9 is provided in the enclosure l for picking up sounds via an aperture 10 in the plastic cover 3. A printed circuit board 1 l is provided in the base S for housing the microphone 9 and additional ch~;uill.y required for o operation of the telephone.
According to the present invention, a speaker microphone 13 is inside the enclosure 1 for picking up the sound emitted by the speaker 7. The enclosure 1 sigIuficantly attenuates sounds from outside of the enclosure, such that the sounds 5 picked up by the speaker microphone 13 may be subtracted from the sounds picked up by the voice microphone 9 which picks up sounds from outside of the enclosure, including sounds emitted by the speaker 7. As shown in Figure 1, the microphones 9 and 13 are positioned so as to be equidistant from the speaker 7, and are mechanically connected to the enclosure 1 in a similar fashion. Preferably, the microphones 9 and 20 13 are the same model of microphone but need not be perfectly matched, provided that the sounds from speaker 7 are picked up by both microphones 9 and 13 subst~nti~lly in phase.
A subtracting circuit is illustrated in Figure 2, according to one embodiment of25 the present invention. The subtracting circuit is preferably designed for optimum performance in the frequency band in which the voice microphone 9 has maximum coupling to speaker 7 (i.e. typically a band around the resonance frequency of speaker 7). Each of the voice microphone and speaker microphone 13 are connected to a respective input of an operational amplifier 15 via pull-up resistors 17 and 19, each 30 having a predetennined resistance (Rbias) and connected to a source of bias voltage (Vbias). Voice microphone 9 is connected to an inverting input of amplifier 15 via a seriesconnectionofcapacitor21 andresistor23. Speakermicrophone 13 is connected to a non-inverting input of amplifier 15 via a series connection of capacitor 25 and resistor 27. A feeclback resislor 29 connects the output of amplifier 15 to the inverting input thereof, and a resistor 31 connects the non~inverting input of amplifier 15 to ground.
The gains and poles for the signal paths in the circuit embodiment of Figure 2 are useful for a situation where the microphone outputs are in phase and the output of speaker microphone 13 is twice that of voice microphone 9. Specifically, Each ofo resistors 23, 29 and 31 has resistance "R", capacitor 25 has a capacitance of"C" and capacitor 21 has a capacitance of "4C"', while resistor 27 has a resistance of "3R".
The gain of the voice microphone 9 is -R/R=l, while the gain of the speaker microphone 13 is R/(R+3R)x(l+R/R)=1/2. Since the frequency response is the same for both microphones, in-phase signals the speaker sounds picked up by voice 15 microphone 9 are canceled by the speaker sounds picked up by the speaker microphone 13, leaving only the desired voice sounds at the output of the circuit (Vout).
Testing of the system according to the present invention has resulted in 20 measured cancellation of 20dB with matched microphones and 12 dB with 2.5 dB
mi~m~tt~h between the microphones 9 and 13.
Alternatives and modifications of the invention are possible. All such embodiments and modifications are believed to be within the sphere and scope of the 25 invention as set forth in the claims appended hereto. For example, the principles of the present invention may be implemented in digital form (e.g. via a digital signal processor (DSP)). Moreover, it is contemplated that the principles of the invention may be employed in any device co~ti~ g a microphone and a sound or noise source within a single enclosure, such as video conferencing systems, cellular hands free 30 devices and portable tape machines. [n the case of a tape machine, the source is noise and vibration from the tape drive, whereas video conferencing and cellular handsfree phones are applications of the speaker phone technology discussed in connection with the preferred embodiment.
Claims (11)
1. In a device having a sound source within an enclosure and a first microphone for picking up sounds outside said enclosure, a system for improving acoustic separation between said sound source and said first microphone, comprising an additional microphone located approximately equidistant from said sound source as said first microphone for picking up sounds within said enclosure due to said sound source, and a circuit connected to said first microphone and said additional microphone for subtracting said sounds within said enclosure from said sounds outside said enclosure, wherein said enclosure has an opening directly corresponding to said first microphone but does not have an opening directly corresponding to said additional microphone.
2. The system of claim 1, wherein said circuit is characterized by an optimized frequency response in a band around the resonance frequency of said sound source.
3. The system of claim 1, wherein said circuit includes an amplifier for amplifying said sounds outside said enclosure by a first gain and for amplifying said sounds inside said enclosure by a second gain, wherein said first gain and said second gain are chosen such that sound from said sound source is canceled by said circuit.
4. The system of claim 1, 2 or 3 wherein said device is a speakerphone, said first microphone is a voice microphone, said sound source is a speaker and said additional microphone is acoustically matched to said voice microphone.
5. A speakerphone apparatus having improved acoustic separation between a speaker and a microphone, comprising:
a speaker within a housing, the housing having an opening adjacent said speaker;
a speaker microphone within the housing to detect sound internal to the housing due to said speaker's output, the housing not having an opening directly corresponding to said speaker microphone;
6a a voice microphone within the housing but receiving sound external to the housing via an opening in the housing adjacent said voice microphone; and a subtraction circuit coupled to said voice microphone and said speaker microphone, said subtraction circuit subtracts the sound internal to the housing detected by said speaker microphone from the sound external to the housing detected by said voice microphone.
a speaker within a housing, the housing having an opening adjacent said speaker;
a speaker microphone within the housing to detect sound internal to the housing due to said speaker's output, the housing not having an opening directly corresponding to said speaker microphone;
6a a voice microphone within the housing but receiving sound external to the housing via an opening in the housing adjacent said voice microphone; and a subtraction circuit coupled to said voice microphone and said speaker microphone, said subtraction circuit subtracts the sound internal to the housing detected by said speaker microphone from the sound external to the housing detected by said voice microphone.
6. A speakerphone apparatus as recited in claim 5, wherein said speaker microphone and said voice microphone are positioned within the housing approximately equidistant from said speaker.
7. A speakerphone apparatus as recited in claim 5, wherein sound emitted from said speaker is detected by said speaker microphone and said voice microphone substantially in phase.
8. A speakerphone apparatus as recited in claim 5, wherein said subtraction circuit comprises:
an amplifier for amplifying sound from said speaker microphone by a first gain and for amplifying sound from said voice microphone by a second gain, the first gain being less than the second gain.
an amplifier for amplifying sound from said speaker microphone by a first gain and for amplifying sound from said voice microphone by a second gain, the first gain being less than the second gain.
9. A speakerphone apparatus as recited in claim 8 wherein sound emitted from said speaker is detected by said speaker microphone and said voice microphone substantially in phase.
10. A speakerphone apparatus as recited in claim 5, wherein said speaker microphone and said voice microphone are acoustically matched.
11. A speakerphone apparatus as recited in claim 5, wherein said subtraction circuit has an optimized frequency response in a band around the resonance frequency of said speaker.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/802,001 US6438240B1 (en) | 1997-02-18 | 1997-02-18 | Circuit to improve transducer separation in handsfree telephone |
| US08/802,001 | 1997-02-18 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2229424A1 CA2229424A1 (en) | 1998-08-18 |
| CA2229424C true CA2229424C (en) | 2001-04-24 |
Family
ID=25182583
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002229424A Expired - Fee Related CA2229424C (en) | 1997-02-18 | 1998-02-13 | Circuit to improve transducer separation in handsfree telephone |
Country Status (3)
| Country | Link |
|---|---|
| US (1) | US6438240B1 (en) |
| CA (1) | CA2229424C (en) |
| GB (1) | GB2322257B (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6266410B1 (en) * | 1997-06-27 | 2001-07-24 | Kabushiki Kaisha Toshiba | Speakerphone and microphone case for the same |
| GB0024350D0 (en) | 2000-10-05 | 2000-11-22 | Mitel Corp | Use of handset microphone to enhance speakerphone loudspeaker performance |
| US20060188089A1 (en) * | 2005-02-18 | 2006-08-24 | Diethorn Eric J | Reduction in acoustic coupling in communication systems and appliances using multiple microphones |
| US7925307B2 (en) | 2006-10-31 | 2011-04-12 | Palm, Inc. | Audio output using multiple speakers |
| US8311590B2 (en) | 2006-12-05 | 2012-11-13 | Hewlett-Packard Development Company, L.P. | System and method for improved loudspeaker functionality |
| US7720216B1 (en) | 2008-11-20 | 2010-05-18 | Microsoft Corporation | Acoustic pass through for handsets |
Family Cites Families (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2309109A (en) | 1937-06-04 | 1943-01-26 | Rca Corp | Microphone |
| GB589031A (en) | 1943-03-26 | 1947-06-10 | Ernest Dennision Fear | Improvements in or relating to the manufacture of ice cream and the like |
| GB859031A (en) * | 1958-10-01 | 1961-01-18 | Lustraphone Ltd | Improvements in or relating to microphones |
| JPS5852780Y2 (en) * | 1980-07-19 | 1983-12-01 | パイオニア株式会社 | microphone |
| US5251263A (en) * | 1992-05-22 | 1993-10-05 | Andrea Electronics Corporation | Adaptive noise cancellation and speech enhancement system and apparatus therefor |
| US5732143A (en) * | 1992-10-29 | 1998-03-24 | Andrea Electronics Corp. | Noise cancellation apparatus |
| US5673325A (en) | 1992-10-29 | 1997-09-30 | Andrea Electronics Corporation | Noise cancellation apparatus |
| JP2839815B2 (en) | 1993-02-26 | 1998-12-16 | 松下電器産業株式会社 | Sound pickup device for video camera |
| US5323458A (en) * | 1993-10-25 | 1994-06-21 | Motorola Inc. | Echo cancellation in a full-duplex speakerphone |
| JPH07202998A (en) * | 1993-12-29 | 1995-08-04 | Nec Corp | Telephone set provided with peripheral noise eliminating function |
| CA2269027A1 (en) * | 1996-10-17 | 1998-04-23 | Andrea Electronics Corporation | Noise cancelling acoustical improvement to wireless telephone or cellular phone |
| US5787165A (en) * | 1996-12-13 | 1998-07-28 | Ericsson Inc. | Handsfree radiotelephone having dynamic path attenuation control for echo suppression |
-
1997
- 1997-02-18 US US08/802,001 patent/US6438240B1/en not_active Expired - Fee Related
-
1998
- 1998-02-13 CA CA002229424A patent/CA2229424C/en not_active Expired - Fee Related
- 1998-02-17 GB GB9803352A patent/GB2322257B/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| GB9803352D0 (en) | 1998-04-15 |
| GB2322257B (en) | 1999-04-28 |
| US6438240B1 (en) | 2002-08-20 |
| CA2229424A1 (en) | 1998-08-18 |
| GB2322257A (en) | 1998-08-19 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| MKLA | Lapsed |