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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a noise cancellation apparatus and, more
particularly, to an apparatus for canceling or reducing background
acoustic noise for use with a telephone handset or a boom microphone
device or the like.
2. Description of the Prior Art
As is to be appreciated, in numerous situations, the presence of background
acoustic noise is undesirable. As an example, consider the situation in
which an operator is attempting to conduct a telephone conversation from a
telephone or such similar device located in a noisy area. In this
situation, loud acoustic background noise is received by a microphone in
the handset of the telephone and converted to an electrical signal which
is supplied to the telephone(s) of the person(s) having the conversation
with the operator and is converted thereat to an acoustic signal. As a
result, the person to whom the operator is communicating constantly hears
the loud background noise. Further, when the person is speaking, such
speech is combined with the background noise and, as such, may be
difficult for the other person(s) to understand. As a result, the operator
may have to shout into the microphone of the telephone. Furthermore, the
signal representing the background noise is also supplied from the
microphone in the operator's handset to the speaker in the operator's
handset. Thus, the operator also constantly hears the background noise
from the speaker in the operator's handset and, when the other person is
speaking, may impair the understanding thereof.
As another example, consider the situation in which a pilot who is
operating a helicopter or the like wishes to communicate with another
person by way of radio frequency (RF) communication. In this situation,
the pilot typically speaks into a so-called boom microphone which is
coupled to a radio transmitting/receiving device whereupon the speech is
converted into RF signals which are transmitted to a second
receiving/transmitting device and converted therein to speech so as to be
heard by the other person(s). As with the above situation of a telephone
located in a noisy area, the loud background noise from the helicopter is
received and converted into an electrical signal by the boom microphone
device and thereafter supplied to the receiving device. As a result, the
person(s) communicating with the pilot hears the loud background noise.
This may be particularly annoying when the pilot leaves the radio
transmitting/receiving device in the "ON" position while operating the
helicopter.
In an attempt to reduce background noise so as to improve performance of a
telephone or a boom microphone located in a noisy environment or the like,
pressure gradient microphones may be utilized. Basically, a pressure
gradient microphone responds to the difference in pressure at two closely
spaced points. When used in an environment where the pressure gradient of
the background noise is isotropic, the electrical signal produced by the
pressure-gradient microphone due to such background noise is effectively
zero. However, in most actual situations, the pressure gradient of the
background noise is not isotropic and, as a result, in these situations,
the performance of the pressure-gradient microphone is adversely affected.
Additionally, since voice or speech propagates in more than one direction,
the electrical signal produced by the microphone which corresponds thereto
is often degraded. Thus, even if a pressure gradient microphone is
utilized in either a telephone handset or a boom microphone, the desired
amount of background noise cancellation may not be sufficient and the
performance may not be adequate.
Furthermore, since two opposite sides of a pressure-gradient microphone
respond to acoustic pressure, as previously mentioned, the handset of an
existing telephone would have to be substantially modified so as to enable
these two sides of the microphone to respond to the acoustic pressure.
Moreover, as a result of using such a microphone in a telephone handset,
the electrical signals produced therefrom should be amplified. Thus, to
replace the conventional microphone in a telephone handset of an existing
telephone with a pressure-gradient microphone would typically necessitate
replacing the handset with a new handset and, as such, would be relatively
expensive.
As an alternative to using pressure-gradient microphones, an acoustic
feed-back type system may be utilized. Such a system normally includes
compensation filters which are used to equalize the transfer function of
the speakers. Since the characteristics of the speakers are tightly
controlled by these filters, the cost of tile filters is relatively high.
As a result, such acoustic feed-back systems are typically relatively
expensive.
Thus, the prior art has failed to provide a relatively low-cost means for
reducing background noise to an acceptable level for use with telephones
and/or boom microphone devices or the like, and a cost-effective means for
enabling existing telephones to reduce background noise to an acceptable
level.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to provide noise reduction apparatus
which overcomes the problems associated with the prior art.
More specifically, it is an object of the present invention to provide
noise reduction apparatus which reduces background noise to an acceptable
level.
Another object of the present invention is to provide noise reduction
apparatus as aforementioned for use with a telephone or boom microphone
device or the like.
It is still another object of the present invention to provide noise
reduction apparatus as aforementioned which is relatively inexpensive.
It is yet another object of the present invention to provide a relatively
low-cost noise reduction apparatus for use with telephones which is
operable with standard available on-line power.
A still further object of the present invention is to provide a relatively
low-cost noise reduction apparatus which is readily adaptable to handsets
of existing telephones and which is operable with standard available
on-line power.
A yet further object of the present invention is to provide a relatively
low-cost noise reduction apparatus for use with telephones or which may be
readily adaptable to handsets of existing telephones which enables an
operator to selectively amplify a received signal.
In accordance with an aspect of this invention, a telephone handset
apparatus for use with a telephone operable by standard power supplied to
the telephone handset for transmitting and receiving signals representing
speech between two or more operators is provided. The apparatus includes a
housing having a first microphone means for receiving a first acoustic
signal composed of speech from the operator using the apparatus and
background noise in the vicinity of the speech and for converting the
first acoustic sound to a first signal, and a second microphone means
arranged at a predetermined angle with respect to the first microphone
means for receiving a second acoustic sound composed of substantially the
background noise and for converting the second acoustic sound to a second
signal; and a device for subtracting the second signal from the first
signal so as to obtain a signal representing substantially the speech,
Other objects, features and advantages according to the present invention
will become apparent from the following detailed description of the
illustrated embodiments when read in conjunction with the accompanying
drawings in which corresponding components are identified by the same
reference numerals,
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a telephone having a noise reduction apparatus according
to an embodiment of the present invention;
FIG. 2 is a block diagram of the noise reduction apparatus used in the
telephone of FIG. 1;
FIG. 3A is a front plan view of the receiver portion of the telephone of
FIG. 1;
FIG. 3B is a cross-sectional side view of the receiver portion of the
telephone of FIG. 1 with the cap removed;
FIG. 4 is a schematic diagram of the block diagram of FIG. 2;
FIG. 5 is another schematic diagram of the noise reduction apparatus
illustrated in FIG. 2; and
FIGS. 6A, 6B and 6C illustrate a boom microphone device utilizing a noise
reduction apparatus according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a telephone 8 which utilizes a noise reduction apparatus
in accordance with an embodiment of the present invention. As shown
therein, the telephone 8 generally includes a handset 10, having a speaker
portion 41 and a receiver portion 42, and a telephone unit 18 which may be
coupled therebetween by way of a telephone cord 30. Alternatively, the
telephone may be a cordless type telephone and, as such, the handset 10 is
coupled to the telephone unit 18 by way of RF waves. The receiver portion
42 includes first and second microphones 12 and 14, respectively, (FIG.
2), a switch 40 for adjusting the volume of a signal supplied to the
speaker portion 41, and a cap 48 having a recessed portion 44 and a mesh
portion 46.
FIG. 2 illustrates the telephone 8 in block diagram form. As shown therein,
the handset 10 generally includes first and second microphones 12 and 14,
respectively, a subtracting device 16, which in a preferred embodiment is
an operational amplifier ("op-amp"), an amplifier 20, which is preferably
an op-amp, and a speaker 22. The first and second microphones 12 and 14,
respectively, op-amp 16 and amplifier 20 are preferably contained within
the receiver portion 42 (see FIG. 1).
Acoustic signals composed of speech or the like and background noise are
supplied to the first microphone 12 and converted therein into a
corresponding electrical signal which is thereafter supplied to the plus
terminal of the op-amp 16. The background noise is supplied to the second
microphone 14 and converted therein into a corresponding electrical signal
which is thereafter supplied to the minus terminal of the op-amp 16. The
op-amp 16 is adapted to subtract the noise signal from the second
microphone 14 from the speech and noise signal from the first microphone
12 and to supply therefrom an electrical signal representing substantially
the speech to the telephone unit 18 whereupon the speech signal is
transmitted therefrom through the telephone lines to a desired telephone
or telephones. The output signal from the op-amp 16 is also combined in
the telephone unit 18 with a received signal from the telephone lines and
supplied to the amplifier 20. The op-amps 16 and 17 (see FIG. 4) are
preferably relatively low-power integrated circuits (IC's), such as
complementary metal oxide semiconductors (CMOS), and may be constructed
from either one or more CMOS IC chips. Although not shown in FIG. 2,
amplifier 20 may be selectively set by use of the switch 40 (FIG. 1) by
the operator so as to adjust the amplification of the received signal to a
desired level. The amplified signal from the amplifier 20 is supplied to
the speaker 22, whereupon the amplified signal is converted into an
acoustic signal so as to be heard by the operator.
FIGS. 3A and 3B illustrate two views of the receiving portion 42, in which
the cap 48 is removed in the view of FIG. 3A. As shown therein, the
receiving portion 42 generally includes a housing 74, a circuit board
assembly 78, the first and second microphones 12 and 14, respectively, and
the cap 48. The first and second microphones 12 and 14, respectively,
which are preferably electret microphones or similar such microphones, are
arranged or positioned as hereinafter described. These microphones are
held in place or secured by a holding member 76 which, for example, may be
constructed of a foam-like material, which, in turn, is secured to the
housing 74. The respective outputs from the first and second microphones
12 and 14 are supplied through respective wires (not shown) to the op-amp
16 which is contained on the circuit board assembly 78 which, in turn, is
attached to the housing 74. As hereinafter more fully described, the
circuit board 78 may contain additional circuit elements for processing
the signals received from the first and second microphones and for
amplifying signals for supply to the speaker 22 (FIG. 2). A cover 72 may
be utilized which is attached to the housing 74 by use of adhesives or the
like or alternatively may be sonically welded together. The cover 72 and
the housing 74 with the circuit board assembly 78, holding member 76 and
the first and second microphones 12 and 14 form an assembly 71.
The cap 48, which may be constructed from a plastic-type material such as
polycarbonate, includes an annular side member 43 and a portion 45 having
a typical thickness T which is coupled to the side member 43 and arranged
so as to be lower than the upper portion of the side member by a minimum
predetermined amount such as 0.020 of an inch, thereby creating a recessed
portion 44. The portion 45 includes a portion 46 having a thickness T'
which is less than the thickness T and which has a plurality of through
holes contained therein and may resemble a mesh-like portion. In a
preferred embodiment, the thickness T' of the portion 46 has a thickness
of less than 0.030 of an inch. Since the portion 46 represents a
relatively small amount of the portion 45, reducing the thickness therein
does not adversely affect the overall structural rigidity of the cap 48.
Alternatively, the portion 46 may be constructed from a stronger material,
for example, stainless steel or such similar material, and combined with
the portion 45. As is to be appreciated, by arranging the portions 45 and
46 so as to be recessed from the upper portion of the side member 43, even
when the receiver portion 42 is placed on a surface, the side member 43,
and not the portions 45 or 46, contact such surface. As a result, any
loads are not directly impacted on the portion 45 and/or the portion 46,
but are instead delivered to the side member 43.
The cap 48 is positioned over the assembly 71 so that the first and second
microphones 12 and 14, respectively, are arranged below the portion 46
with the first microphone positioned relatively close to the underside of
the portion 46. Thus, the speech travels a relatively short distance from
an operator, who is speaking into the receiver portion 42 from a distance
of preferably less than 1 inch, through the portion 46 to the first
microphone. As a result, acoustic distortions are minimized.
The arrangement of the first and second microphones 12 and 14,
respectively, within the receiver portion 42 is illustrated in FIGS. 3A
and 3B. More specifically, as shown in FIG. 3B, the first and second
microphones are arranged so as to have an angle .phi. therebetween, which
preferably has a value in a range between 30.degree. and 60.degree.. The
first and second microphones are further respectively arranged so as to
have an angle .THETA. and [(90-.THETA.) +.phi.] between a plane parallel
to the receiving or "sensitive" surface of the first microphone 12 and the
direction of speech from an operator, and an axis normal to the sensitive
surface of the second microphone 14 and the direction of speech, as shown
in FIG. 3B; and so as to have an angle .PSI. between the direction of
speech and the second microphone, as shown in FIG. 3A. In a preferred
embodiment, the angle .THETA. has a value of less than approximately
35.degree. and the angle .PSI. has a value of approximately 180.degree..
As a result of arranging the first and second microphones in this manner,
the first microphone 12 receives both the speech from the operator and the
background acoustic noise which is present in the vicinity, and the second
microphone 14 essentially receives only the same background acoustic noise
which is received by the first microphone.
Although, as previously mentioned, the angle .phi. has a value which is
preferably between 30.degree. and 60.degree., the first and second
microphones 12 and 14, respectively, may nevertheless operate
satisfactorily even if arranged so as to have an angle .phi. which lies
outside this range. However, as the angle .phi. becomes substantially
smaller than 30.degree. or larger than 60.degree., the performance may be
adversely affected. That is, when the angle .phi. becomes substantially
smaller than 30.degree., the second microphone 14 receives both the speech
and background noise. As a result, upon subtracting the output signal of
the second microphone 14 from the output signal of the first microphone
12, a portion or all of the speech may be canceled. On the other hand,
when the angle .phi. is substantially larger than 60.degree., the
background noise received by the second microphone 14 may not be similar
to that received by the first microphone 12. As a result, subtracting the
output signal of the second microphone 14 from the output signal of the
first microphone 12 may not adequately cancel the background noise
received by the first microphone.
In a like manner, although the angles .THETA. and .PSI. have preferred
values of less than 35.degree. and approximately 180.degree.,
respectively, as previously mentioned, the first and second microphones
may operate satisfactorily even if arranged so as to have different values
of these angles. However, as the values of the angles .THETA. and .PSI.
become substantially different from the respective preferred values, the
performance may be adversely affected. That is, when the angle .THETA.
becomes substantially larger than 35.degree., the second microphone 14 may
receive both the speech and background noise. Similarly, when the angle
.PSI. is substantially smaller or larger than 180.degree., the second
microphone 14 may receive both the speech and background noise. As a
result, in either of these situations, upon subtracting the output signal
of the second microphone 14 from the output signal of the first microphone
12, a portion or even all of the speech may be canceled.
As is to be appreciated, by using the above-described devices and materials
for the components of the receiver portion 42, the cost for constructing
such receiver portion is relatively low. Further, by using CMOS chips, as
previously described, the power consumption of the receiver portion is
kept relatively low. As a result, the receiver pollution may be powered by
the standard power available in the handset and, as such, does not require
additional power or transformers or the like. Furthermore, although the
receiver portion 42 has been described for assembly with the handset 10 of
the telephone 8, which is a new telephone, such receiver portion, or a
slight variation thereof, may be used in handsets of existing telephones.
That is, in this latter situation, the cap and microphone contained within
the handset of an existing telephone are merely replaced with the receiver
portion 42. Thus, such use of the receiver portion 42 provides a
relatively easy and low-cost means to modify a handset of an existing
telephone to include the present noise reduction apparatus.
FIG. 4 illustrates a schematic diagram of one circuit arrangement of the
telephone 8 shown in FIGS. 1 and 2. As shown in FIG. 4, the first
microphone 12 is coupled through a resistor 202, which is adapted to
function as a current limiting resistor so as to correct the bias of an
output from the first microphone, to an input terminal 200. The first
microphone 12 is further coupled through a resistor 210 to the plus
terminal of the op-amp 16 and through a resistor 212 to a variable
resistor 214. The second microphone 14 is coupled through a variable
resistor 208, which is adapted to function as a current limiting resistor
so as to correct the bias of an output of the second microphone, to an
input terminal 201, and to the minus terminal of the op-amp 16. The
limiting resistor 208 is preferably a variable current limiting resistor
which enables the level of the output signal from the second microphone to
be matched to within a predetermined value to the level of the output
signal of the first microphone 12. More specifically, the limiting
resistor 208 enables the output signal of the second microphone 14 to be
weighted such that when a signal having a similar level is outputted from
the first microphone 12, the amplitude of the difference therebetween is
minimized. The value of the current limiting resistor 208 can be selected
according to minimization criteria. An input terminal 198 is connected to
resistors 204 and 206, which are adapted to divide the voltage received at
the input terminal 198, and to the minus terminal of the op-amp 16. The
output of the op-amp 16 is coupled to capacitors 220, 222 and 226 and
resistors 224 and 228 which, in turn, is connected to a "microphone input"
terminal of the telephone unit 18. The output from the op-amp 16 is
further coupled through a variable resistor 14, a resistor 216 and a
capacitor 218 to ground. Resistors 210, 212 and 216 and variable resistor
214 provide variable gain, for example, 20 to 1 amplification, to the
output of the op-amp 16. The capacitors 218, 220 and 222 are adapted to
remove residual dc (direct current) levels which may be present in the
output signal from the op-amp 16. The resistors 224 and 228 and the
capacitor 226 are adapted to function as a low-pass filter having a break
point at a predetermined value which, for example, may be 3.7 kHz.
The telephone unit 18 is further connected to the telephone lines and is
adapted to receive signals through the microphone input terminal and to
supply these signals to the desired telephone or telephones by way of the
telephone lines. The telephone unit 18 is further adapted to receive
signals from another telephone or telephones by way of the telephone lines
and to combine such signals with those received through the microphone
input terminal, as previously described, and to supply the combined signal
to a speaker input terminal 231. The input terminal 231 is connected
through a capacitor 230, which is adapted to block dc signals, and a
resistor 232 to the minus terminal of an op-amp 17 and through a resistor
234 to a variable resistor 240. An input terminal 199 is connected to the
plus terminal of the op-amp 17. The output from the op-amp 17 is connected
through capacitors 242 and 244 and a resistor 246 to the speaker 22. The
output from the op-amp is further connected through the variable resistor
240, a resistor 238 and a capacitor 236 to ground.
The operation of tile telephone 8 shown in FIG. 4 will now be described
below.
Upon activating the handset 10, by lifting the handset 10 from the switch
hook (not shown) or the like, standard telephone line voltage is applied
to input terminals 198, 199, 200 and 201. A signal from the first
microphone 12, which has been bias corrected by the current limiting
resistor 202, is supplied through the resistor 210 to the plus terminal of
the op-amp 16. An output signal from the second microphone 14, which has
been bias corrected by the current limiting resistor 208, is supplied to
the minus terminal of the op-amp 16. The op-amp 16 subtracts the signal
received from the second microphone 14 from that received from the first
microphone 12 and outputs the resulting subtracted signal. DC levels which
may be present in the output signal are removed and the signal is
amplified. High frequency signals, such as those over 3.7 kHz, are then
removed from the amplified output signal and the resulting signal is
supplied to the telephone unit 18. Thus, a voltage signal is supplied to
the telephone unit 18 which is proportional to the difference between the
voltages generated by the first and second microphones 12 and 14,
respectively.
An output signal from the telephone unit 18, which is a combination of the
signals received through the microphone input terminal and the telephone
lines, is supplied to the input terminal 231 of the amplifier 20. The
signal from the input terminal 231 is supplied to the capacitor 230 so as
to remove any dc signals which may be present. The output from the
capacitor 230 is supplied through the resistor 232 to the minus terminal
of the op-amp 17. The op-amp 17 subtracts the signal from the telephone
unit 18 from the signal received from the input terminal 199 and supplies
a subtracted signal therefrom. Such signal may be selectively amplified,
through the use of resistors 232, 234 and 238 and variable resistor 240,
by the operator by use of the switch 40 (FIG. 1). Any dc signals which may
be present in the amplified signal are thereafter removed by the
capacitors 242, 244 and 236. The output signal from the capacitor 244 is
current limited by the resistor 246 and is thereafter supplied to the
speaker 22 so as to be converted thereat into an acoustic signal.
FIG. 5 illustrates an alternative arrangement for processing the signals
obtained from the first and second microphones 12 and 14, respectively, so
as to provide a current output for supply to the telephone unit 18 which
is proportional to the difference of the voltages generated by the first
and second microphones.
More specifically, the circuit arrangement of FIG. 5 includes a handset 10'
having a plurality of input terminals 300, 301, 370 and 390 which are each
adapted to receive standard available on-line power. The first microphone
12 is coupled through a current limiting resistor 302 to the input
terminal 300 and is further coupled to the plus terminal of a subtracting
device 316, which is preferably a CMOS op-amp. The output from the second
microphone 14 is coupled through a variable current limiting resister 308
to the input terminal 301 and is further coupled to the minus terminal of
the op-amp 316. The signal outputted from the op-amp 316 is supplied
through filtering stages 350 to the minus terminal of a subtracting device
351 which is preferably a CMOS op-amp. The filtering stages 350 are
adapted to provide a predetermined frequency response characteristic such
as a signal roll-off at a predetermined frequency. As is to be
appreciated, although two filtering stages are shown in FIG. 5 any number
of filtering stages may be utilized. The input terminal 390 is coupled to
resistors 392 and 94, which are adapted to reduce the signal supplied
thereto, and to the plus terminal of the op-amp 351. An output signal from
the op-amp 351 is supplied to the base of a transistor 366. The input
terminal 391 is connected to a Zener diode 360, a capacitor 362 and a
resistor 364 which, in turn, is connected to the collector of the
transistor 366 and to the microphone input terminal of the telephone unit
18. The emitter of the transistor 366 is coupled through resistors 367 and
368 to the minus terminal of the op-amp 351 so as to provide a feedback
loop thereto. The op-amp 351 and the associated components provide
electrical isolation between the filtering stages 350 and the transistor
366. The transistor 366 is adapted to amplify the signal supplied to the
telephone unit 18.
The output from the telephone unit 18 is coupled to the input terminal 231
(FIG. 4) and is thereafter processed in the manner previously described
with reference to the handset 10 of FIG. 4 so as to provide an acoustic
signal from the speaker 22.
The operation of the telephone 8' will now be described below.
Upon applying power to the handset 10', by lifting the handset from the
switch hook (not shown) or the like, standard telephone line voltage is
applied to input terminals 300, 301, 370, 390 and 391. A signal from the
first microphone 12, which has been bias corrected by the current limiting
resistor 302, is supplied to the plus terminal of the op-amp 316. An
output signal from the second microphone 14, which has been bias corrected
by the current limiting resistor 308, is supplied to the minus terminal of
the op-amp 316. The resistor 308 is preferably a variably current limiting
resistor which enables the level of the output signal from the second
microphone 14 to be matched to within a predetermined value to the level
of the output signal of the first microphone 12, in a manner substantially
similar to that previously described for resistor 208. The output
difference signal from the op-amp 316 is provided though the filtering
stages 350, which may include one or more RC networks or equivalent
circuits, so as to limit the upper frequency of the output signal to a
predetermined value which, for example, may be 3.7 kHz. The output signal
from the filtering stages 350 is supplied to the minus terminal of the
op-amp 351 and a voltage signal from the input terminal 390, which has
been divided to a predetermined value such as one half thereof, is
supplied to the plus terminal of the op-amp 351 which, in turn, calculates
the difference therebetween and supplies a corresponding output signal to
the base of the transistor 366. The voltage from the input terminal 391 is
supplied through the resistor 364 to the collector of the transistor 366.
As a result, an amplified signal is supplied from the handset 10' to the
telephone unit 18 for supply therefrom through the telephone lines to the
desired telephone(s) and for combining with a received signal from the
telephone(s) for supply to the input terminal 231 in a manner similar to
that previously described with reference to FIG. 4.
The individual circuit components without reference designations depicted
in FIGS. 4 and 5 are connected as shown and will not be discussed further,
since the connections and values are apparent to those skilled in the art
and are not necessary for an understanding of the present invention.
FIGS. 6A, 6B and 6C illustrate a boom microphone 100 which utilizes a noise
cancellation apparatus in accordance with an embodiment of the present
invention. More specifically, the boom microphone 100 generally includes a
housing 174, a circuit board assembly 178, first and second microphones
112 and 114, respectively, and a portion 147. The housing 174, which may
be constructed from either a plastic-like or metal-type material, includes
a circular portion 108 having a hole therethrough so as to enable a shaft
106 to be inserted therein. As a result, the boom microphone 100 may
rotate about the shaft 106 as illustrated in FIG. 6A.
The first and second microphones 112 and 114 are respectively coupled to
the circuit board assembly 178 by wires 102 and 104. The circuit board
assembly 178 contains circuitry similar to that on the circuit board
assembly 78 which, as previously described, processes the signals from the
first and second microphones 12 and 14, respectively, for supply to the
telephone unit 18 and, as such, in the interest of brevity, will not be
further described herein. Therefore, the circuit board assembly 178 is
adapted to receive a speech and background noise signal from the first
microphone 112 and to subtract therefrom the background noise signal from
the second microphone 114 so as to derive a signal which represents
substantially the speech. Such signal is supplied to a transmitting device
(not shown) so as to be converted to a RF signal and transmitted to a
remote receiving device (not shown). The first and second microphones 112
and 114, respectively, are held in place by a holding member 176 which,
for example, may be constructed of a foam-like material. A mesh-like
screen 146 which, for example, may be fabricated from a plastic-type or a
metal material or the like, is attached to the cut away portion 147 so as
to protect the first and second microphones. The mesh 146 has a
predetermined thickness which, for example, may be approximately 0.030 or
less of an inch.
The first and second microphones 112 and 114, respectively, which may be
electret microphones, are arranged in a manner similar to that of the
previously described first and second microphones 12 and 14, respectively,
of the handset 10. That is, the first and second microphones 112 and 114,
are respectively positioned so as to have an angle .THETA.' and
[(90-.THETA.')+.phi.'] between a plane parallel to the receiving or
sensitive surface of the first microphone and the direction of speech from
an operator, and between an axis normal to the sensitive surface of the
second microphone and the direction of speech, as shown in FIG. 5A.
Further, the first and second microphones 112 and 114, respectively, are
arranged so as to have an angle .phi.' therebetween, which has a preferred
value in a range between 30.degree. and 60.degree.. The first and second
microphones 112 and 114, respectively, are located in relatively close
proximity to the mesh 146 and the cut away portion 147 of the housing 174
so as not to receive acoustic sounds which have been unacceptably
distorted.
Although the above embodiments have been described as having only one first
microphone 12 (112) and one second microphone 14 (114), the invention is
not so limited and any number of microphones may be utilized for the first
microphone and/or the second microphone. For example, a receiver portion
42' (not shown) may be configured which includes two or more microphones
operating as a first microphone 12' (not shown) and two or more
microphones operating as a second microphone 14' (not shown). In this
configuration, when using multiple microphones for the first and/or second
microphones, respective variable current limiting resistors are preferably
provided for all but one microphone for the first microphone 12' and for
all microphones for the second microphone 14'. Thus, the outputs from the
first and second microphones, 12' and 14', respectively, would comprise a
weighted sum of several such microphone output voltages. The current
limiting resistors are preferably set to respective values so as to
minimize some functional of the difference of the first and second
microphones 12' and 14', respectively. The criterion for selecting the
values of the | | |
