Note: Descriptions are shown in the official language in which they were submitted.
CA 02479675 2006-02-27
1
A DIRECTIONAL CONTROLLER FOR A HEARING AID
This application is a division of Canadian Patent
Application Serial No. 2,385,812, filed June 23, 2000.
In view of division enforced by the Canadian
Intellectual Property Office the claims of this
application are directed to a directional controller.
However, for the purpose of facilitating an understanding
of all objects and features of the development which are
1o inextricably bound-up in one and the same inventive
concept as taught and claimed in the parent application,
the objects and teachings of those features claimed
in the parent Canadian Application Serial No. 2,385,812
are retained herein.
~s Accordingly, in view of enforced division required
by the Examiner in the prosecution of the aforesaid
parent application, object clauses and features have been
retained for the purposes of facilitating and
understanding of the overall development. However, the
2o retention of any clauses or features which may be more
particularly related to the parent application or a
separate divisional thereof should not be regarded as
rendering the teachings and claiming ambiguous or
inconsistent with the subject matter defined in the
25 claims of the divisional application presented herein
when seeking to interpret the scope thereof and the basis
in this disclosure for the claims recited herein.
The present invention relates to a method for
controlling the directionality of the sound receiving
3o characteristic of a hearing aid comprising spaced apart
first and second sound receiving microphones.
CA 02479675 2006-02-27
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Hearing aids having a directional sound receiving
characteristic are useful to improve speech perception in
noisy environments, where human speech may be received
simultaneously from different directions, as is the case,
e.g., in the noise environment frequently referred to as
cocktail party noise.
With a directional sound receiving characteristic,
e.g., in the shape of a cardioid or super cardioid
characteristic, the speech perception in a hearing aid is
to improved by reduced reception of sound coming from the
back of the user, while maintaining the level of sound
coming from the area in front of the user.
On the other hand, in environments with only low
noise levels or no significant speech signals, the hearing
aid user will normally prefer an omnidirectional or
spherical sound receiving characteristic, offering the
same perception of sound irrespective of the direction
from which it arrives.
As will be further explained in the following, a
2o prior art hearing aid of the kind defined above, offering
the possibility of changing the sound receiving
characteristic between an omnidirectional characteristic
and a directional characteristic of varying shape, has
been disclosed in United States Patent No. 5,757,933.
With this prior art hearing aid operating only with
an omnidirectional characteristic, the signal from the
first microphone facing the area in front of the user is
supplied to the signal processor. By manual operation of
a switch, a signal derived from the second microphone
3o facing the rear of the user is subjected to inversion,
CA 02479675 2006-02-27
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followed by adjustable phase delay and adjustable
attenuation. It is then combined via a summing node with
the signal derived from the first microphone. When the
sound receiving characteristic in a hearing aid of this
type is changed or changes from the omnidirectional to a
directional shape, the arrival time of the sound changes
during the transition. This change of phase or time delay
may become confusing for a binaural hearing aid system
that uses a pair of separate hearing aids operating with
1o independent and automatic change of the sound receiving
characteristic. When phase or arrival times change
differently in the two hearing aids, the user's ability to
locate the various sound sources in the surrounding space
decreases, and the advantage of a binaural hearing aid
system will be degraded.
Furthermore, the phase and time relationship in the
hearing aid degrades the quality of the sound perceived
by the user. It may sound like the result of a Doppler
2o effect.
At the same time, in hearing aids of this type the
amplitude characteristic will also change during
transition between the omnidirectional and a directional
characteristic, e.g., from a flat response to a response
in which the amplitudes of higher frequencies will be
increased. This increase may be in the area of about 6
dB/octave. This results in a serious problem, in that
hearing aids of this type cannot be perfectly fitted with
an optimum transfer characteristic for both the
omnidirectional and the directional characteristic.
CA 02479675 2006-02-27
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In this background, it is the object of the present
invention to provide a method of the kind defined, in
which the deficiencies of the prior art hearing aid are
addressed by effecting a change-over between the
omnidirectional characteristic and any directional
characteristic, substantially without changing the phase
relationship or time delay and the amplitude
characteristic of the signals. The change-over between
the omnidirectional characteristic and a directional
io characteristic and vice versa may be controllable or even
automatic.
The present invention describes a method for
controlling the directionality of the sound receiving
characteristic of a hearing aid, comprising spaced apart
first and second sound receiving microphone means, a
signal processor for processing signals supplied by said
microphone means and an output transducer for emission of
sound signals in response to output signals from the
signal processor, said method comprising the steps of
2o changing over said sound receiving characteristic between
an omnidirectional characteristic and a directional
characteristic and, while operating the hearing aid with
said directional characteristic, combining the signals
supplied by said first and second microphone means into
an overall combined signal, an adjustable time or phase
delay being imposed on at least one signal, wherein said
change over of the sound receiving characteristic from
the omnidirectional characteristic to the directional
characteristic and vice versa is effected by controlling
3o the attenuation and the time or phase delay of signals
CA 02479675 2006-02-27
derived from both of the signals (Xfront~ Xback) from the
first and second microphone means before forming said
overall combined signal (Y), according to an adjustable
attenuation control parameter (omni) and a delay (T),
5 whereby said overall combined signal (Y) is determined by
Y ° Xfro~t * (1 - omni * e''"T) ~' Xback * (omni - e''"T) , to
change over the hearing aid between said omnidirectional
characteristic and any desired form of said directional
characteristic as a smooth change over substantially
1o without effecting phase relationship, time delay and
amplitude characteristic of the hearing aid.
The present invention also describes a hearing
aid with controllable directionality of its sound
receiving characteristic, comprising spaced apart first
and second sound receiving microphones means, a signal
processor for processing signals supplied by said
microphone means and an output transducer for emission of
sound signals in response to output signals from the
signal processor, and further comprising change-over
2o control means for change over of the sound receiving
characteristic between an omnidirectional characteristic
and a directional characteristic and combining means for
combining of the signals from the first and second
microphone means to provide an overall combined signal
supplied to the signal processor, while operating the
hearing aid with said directional characteristic, and
adjustable time or phase delay means for producing a
phase-delayed modification of at least one signal,
wherein said change-over control means comprises
3o controllable attenuation means and controllable time or
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phase delay means acting on signals derived from the
signals (Xf=ont~ Xback) from both of the first and second
microphone means, respectively, said attenuation and
phase delay means (1-3; 14, 15, 18, 19) being controlled
for forming said overall combined signal (Y) according to
an adjustable attenuation control parameter (omni) and a
delay (T), whereby said overall combined signal (Y) is
determined by Y = Xf=ont * (1 - omni * e'~"T) + Xback * (omni
- e''"'') , to change over the hearing aid between said
omnidirectional characteristic and any desired form of
said directional characteristic as a smooth change over
substantially without affecting phase relationship, time
delay and amplitude characteristic of the hearing aid.
In an aspect of the present invention, there is
provided a directional controller for a hearing aid for
processing input signals from at least two spaced apart
microphones for producing a combined output signal for
further processing in the hearing aid, the directional
controller comprising adjustable time delay means for
2o producing a time-delayed modification of at least one
signal, and change-over control means for effecting a
change-over between an omnidirectional mode and a
directional mode, and further comprising a first
controllable attenuator for processing a signal derived
from the signal (Xpront) from the first microphone to
output a first processed signal, a second controllable
attenuator for processing a signal derived from the
signal (Xback) from the second microphone to output a
second processed signal, the adjustable time delay
3o means being adapted for delaying signals derived
from the signals from both of the first
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and the second microphones and combining means connected
to combine the processed and delayed signals with the
output signal from the first microphone to generate the
combined output signal, the change-over control means
being connected to control the controllable attenuator.
In the following the invention will be further
explained with reference to the accompanying illustrative
example drawings, in which
Fig. 1 is a schematic block diagram of the prior art
hearing aid of United States Patent No. 5,757,933,
Figs. 2 to 5 are graphic representations
illustrating variations of the sound receiving
characteristic of the hearing aid in fig. 1 between the
omnidirectional characteristic and different directional
shapes and concurrent variation of amplitude
characteristics of the front and back microphones used
therein,
Fig. 6 shows a schematic arrangement of the front
end of a hearing aid according to the present invention,
Figs. 7 to 10 are graphic representations
corresponding to the representations in figs.2 to 5 with
respect to the hearing aid shown in Fig. 6,
Fig. 11 shows a schematic arrangement of a hearing
aid according to the present invention with a change-over
controller,
Fig. 12 shows a similar schematic arrangement of an
embodiment of the present invention,
Fig. 13 schematically shows a schematic arrangement
providing a further improvement of the arrangement shown
3o in Fig. 6, and
Fig. 14 shows a schematic arrangement providing a
further development of a hearing aid according to the
invention.
CA 02479675 2006-02-27
s
In the prior art hearing aid . shown in Fig. 1, two
non-directional microphone circuits include a front
microphone MICF and a back microphone MICB. Whereas the
output signal from the front microphone MICF is supplied
directly to the hearing aid signal processor via a
summing node SN, the signal from the back microphone is
supplied to the summing node SN (i.e., via an inverter, an
adjustable phase delay circuit, and an attenuator with
adjustable gain) only by closure of a manually operated
1o switch SW, whereby the sound receiving characteristic of
the hearing is changed from the omnidirectional
characteristic of front microphone MICF to a directional
characteristic of varying shape.
The combined signal Y formed at the summing node SN
1s with switch SW closed and supplied to the signal
processor will thus be related to the signals Xfront and
Xback from front and back microphones MICF and MICE,
respectively, by the relation Y = Xfront " Xback * omni * e'
j~''. Here, the adjustable parameter omni represents the
2o adjustable gain of the attenuator, whereas T represents
the adjustable time delay corresponding to the difference
in arrival time for sound signals received by the front
and back microphones MICF and MICB, respectively.
The graphic representations in figs. 2
25 and 3 illustrate examples of the variation of the sound
receiving characteristic of the hearing aid in figure
1 from the omnidirectional shape ND and various
directional shapes D1 to D10, ranging from weak cardioid
to super cardioid form for values of the adjustable
3o parameter omni ranging from 0 to 1, measured at lkHz and
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100 Hz, respectively. The graphic representations
in figs. 4 and 5 show the variation in the amplitude
characteristics of the signals received from the areas in
front and back of the hearing aid, respectively, for
correspondingly varying values of the parameter omni. As
will appear from these representations for this prior art
hearing aid, the change-over between the omnidirectional
characteristic and the various shapes of directional
characteristic results in the desired gradual reduction in
1o gain or amplitude response for the signals received from
the area behind the user. However, this is also
accompanied by a significant change in gain or amplitude
response for the signals received from the area in front
of the user. Consequently, adjustment or fitting of the
hearing aid to compensate for a user's specific hearing
impairment in quiet surroundings (i.e., where use of the
omnidirectional characteristic is preferred) will not
provide optimum compensation when a change-over is made to
a directional characteristic, (i.e., where use of the
2p hearing aid in a more noisy sound environment, such as a
party, is preferred).
Fig. 6, shows the front end of a hearing
aid, including a change-over controller for
controlling change of the directionality of
sound receiving characteristic of the hearing aid from the
omnidirectional characteristic to a directional charac-
CA 02479675 2006-02-27
1~
teristic, and vice versa. This change may be effected as
a switch-over, or as a gradual and smooth change-over.
The front end of the hearing aid comprises at least
two microphone circuits (e.g., a front microphone Fmic and
s a back microphone Bmic) and possibly optional
preprocessing circuits for the electrical output signals
from the microphones. The distance between the two
microphones may be as small as about 1 man or as wide as
about a few cm.
1o The front end further contains at~ least two
controllable amplifiers or attenuators l and 2, at least
one time or phase delay device 3, and at least three
combining circuits 4, 5, and 6. It is to be understood
that the combining circuits may contain positive as well
is as negative input terminals, so as to form adding or
subtraction operations or combinations thereof.
In the structure, the back microphone Bmic is
connected to the controllable amplifier or attenuator 1
and to a first adding circuit 4.
2o The front microphone Fmic is connected directly to
the controllable amplifier or attenuator 2 and to a
second adding circuit 6. The output of the controllable
amplifier or attenuator 2 is further connected directly
to a second input of the first adding circuit 4, whereas
2s the output of the controllable amplifier 1 is directly
connected to a positive input of a subtraction circuit
5.
Preferably, a controllable delay device 3 is included
between the output of the first adding circuit 4 and the
3o negative input of the subtraction circuit 5.
In the following description the adding and
subtracting circuits will generally be referred to as
combining circuits.
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In operation, sound from the environment of the
hearing aid is picked up by both the front microphone
Gmic and the back microphone Bmic. The distance between
the two microphones may be as small as about 1 mm and as
s wide as about a few cm.
The output signal of the front microphone F~nic is
supplied to the combining circuit 6. The output signal of
the back microphone Bmic is supplied to the controllable
attenuator or controllable amplifier 1, and the gain can
io be controllably changed from zero to about one, i.e., from
no amplification to substantially full amplification.
This change-over may be effected as a switch-over or as a
controlled gradual change. This means that amplification
between zero and about one can be controllably achieved.
i5 Any output signal from the front microphone EYnic is
also supplied to a controllable attenuator or amplifier 2,
and the amplification may controllably be changed from
zero to about one, i.e., from no amplification to
substantially full amplification. Also in this case, the
2o change-over may be effected as a switch-over or as a
gradual controlled change. This means that amplification
between zero and about one may be achieved.
Any output signal from the controllable attenuator or
amplifier 2 is supplied to a second input of the combining
2s circuit 4. Any output signal of combining circuit 4 is
supplied to the controllable delay device 3, and the delay
may be controlled from as small as about 1 us up to about
1000 us or more.
Any output signal of delay device 3 is supplied to
3o the negative input of combining circuit 5, and the output
is supplied to the second input of the combining circuit
6.
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Thereby, the output signal of the front microphone
F~nic rnay be attenuated in the attenuator or controllable
amplifier 2 before it is added to the undelayed output
signal of the back microphone Bmic in the combining
s circuit 4, the output signal of which is then delayed in
delay device 3 before being supplied to the combining
circuit 5. The controllable delay of delay device 3 will
usually have the same value as the acoustical delay
between the arrival times of sounds at the front
~o microphone Fmic and at the back microphone Bmic.
Preferably this delay is also adjustable and/or
controllable.
Additionally, the output signal of the attenuator
or controllable amplifier 1 is supplied to the positive
1s input of the combining circuit 5. In this combining
circuit the delayed output signal of delay device 3 is
subtracted from the attenuated output signal of
amplifier or attenuator 1. The output signal of the
combining circuit 5 is supplied as a processed signal
2o to the combining circuit 6. The output signal of the
combining circuit 6 is then used as an input signal for
further processing in the remaining components of the
hearing aid such as the signal processor, which need not
be described here.
2s The remaining parts of the hearing aid may, as
known in the art, comprise more than one signal
processing channel having either a common change-over
controller or a separate controller for each channel.
As further known in the art, the output signals of
3o both microphones Fmic and Bmic may advantageously be
converted into one or more digital representations before
being supplied to the change-over controller with its
components 1 to 6.
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The function of the circuit in fig. 6 is as
follows:
For the directional mode of operation, the signal
transfer of the controllable attenuators 1 and 2 is set at
s zero, i.e., no signal is transferred.
The output signal of the front microphone Fmic is
directly supplied to the second adding circuit 6. The
output signal of the back microphone Bmic is supplied
via the first adding circuit 4 and the delay device 3 to
1o the negative input of the subtraction circuit 5, where the
signal changes its polarity. The output signal of the
subtraction circuit 5 is then supplied to a second input
of the second adding circuit 6. Thus, the delayed signal
from the back microphone Bmic is subtracted from the
is undelayed output signal of the front microphone Fmic.
The directional front characteristic may then be
created by adjusting the delay T of the delay device to
be the same as the acoustical delay A between the back
microphone Bmic and the front microphone F~nic. With this
2o delay, the signals that are first received at the back
microphone Bmic and are later received at the front
microphone Fmic are then suppressed in the adding
circuit 6, where the delayed signal of the back
microphone is subtracted from the output signal of the
as front microphone.
This mode of operation provides an output signal from
adding circuit 6 by subtraction of the delayed output
signal of the back microphone Bmic from the output signal
of the front microphone Fmic, thus substantially cancelling
3fl sound coming directly from the back of the user.
CA 02479675 2006-02-27
By adjusting T < A, sound coming partly from the side
of the user is substantially cancelled, and the direction
of the canceling effect is controlled by the ratio of T/A.
For the omnidirectional mode of operation, both
s attenuators 1 and 2 are set for a full signal transfer.
The output signals from the front microphone Fmic
and the back microphone Bmic are supplied to the first
adding circuit 4, where they are combined and supplied
via delay device 3 to the subtraction circuit 5. Then
io the combined and delayed signal is subtracted from the
output signal of the back microphone Bmic.
The output signal of the subtraction circuit 5 is
then supplied to the second adding circuit 6, where it
is combined with the undelayed output signal of the
15 front microphone Fmic. The addition of these signals
creates the omnidirectional characteristic. This mode of
operation provides an output signal from the adding circuit
6 by addition of the signals from the front and back
microphones from which the delayed front and back
2o microphone signals are subtracted.
The arrival times of sound signals at the two
microphones differ because the distances from the source
to the two respective microphones differs.
This difference is the acoustical delay A, and the
relationship between the sound signals Xfront and Xback
received at the front and back microphones,
respectively, may be generally expressed as
* - j (D
Xback - Xfront a
CA 02479675 2006-02-27
where e-i~ is the acoustical delay for the actual direc-
tion to the sound source.
The combined signal Y from adding circuit 6 is
5 Y = Xfrcnc * ( 1 - omni * e-'~'T ) + Xback * ( omni - e-j'~'~ )
where omni is an adjustable parameter controlling
attenuators 1 and 2, having a preferred value in the range
from 0 to about 1 (i.e., the lower limit amni = 0 means no
io signal transfers through attenuators 1 and 2, whereas the
upper limit omni - 1 means maximum signal transfers
through attenuators 1 and 2).
Although the invention is not limited thereto, the
parameter omni should preferably be substantially the
is same for both attenuators 1 and 2.
If the full directional mode of operation is chosen
with omni = 0, then the combined signal Y becomes
Y = X * (1-e-j~,cA.r~ )
front
If the delay T is selected equal to the delay A
directly from the back microphone to the front
microphone in the directional mode of operation, then
the part of the sound signal X coming directly from the
2s back of the user is substantially suppressed and a
directional characteristic known as a cardioid
characteristic is provided.
The signal process described so far is preferably
performed as a digital process in the time or frequency
3o domain. If processing in the frequency domain is
employed, it is advantageous to use microphone circuits,
which are capable of generating a delayed microphone
output signal in combination with a non-delayed
CA 02479675 2006-02-27
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microphone output signal.
Figs. 7 to 10 are graphic representations of sound
receiving characteristics and amplitude response of a
hearing aid embodying the front end part shown in Fig. 6,
corresponding to the representations in figs. 2 to 5 and
using the same reference designations as in these figures.
As will appear from figs. 7 and 8, the part of the sound
receiving characteristic representing the area in front of
the user is substantially unaffected by the change over
1o between the omnidirectional characteristic ND and the
various directional shapes D1 to D10. As illustrated by
fig. 10, the amplitude response of signals received from
the area in front of the user is substantially unaffected
by the change over and remains substantially the same
i r r a s p a c t i v a o f c h a n g a s t o
coming from the area behind the user. Thereby, the
adjustment or fitting of the hearing aid to compensate
for the user's hearing impairment in quiet surroundings
where the omnidirectional characteristic is used will also
2o provide improved listening performance when the hearing aid
is used in a more noisy environment using a directional
shape of the sound receiving characteristic.
The circuit in fig. 11 is similar to the circuit in
fig. 6 and includes a change-over controller with
components 1 to 6. Similar components have been assigned
the same reference numerals.
Additionally, signal processing units 7 and 8 are
placed at the outputs of the at least two microphones,
e.g., the front microphone Fmic and the back microphone
Bmic. The processed output signals of the two signal
CA 02479675 2006-02-27
processing units 7 and 8 are then supplied to the change-
over controller with components 1 to 6. The signal
processing units 7 and 8 may perform an equalizing
function on the two output signals of the two microphones,
and/or may contain various filters (e. g., band pass
filters). With the use of band pass filters, the
microphone signals may be split up onto several bands,
each equipped with its own change-over controller. The
respective output signals from the adding circuits 6 in
1o the various bands or channels may then be combined into
a composite combined signal to be further processed in
the remaining stages of the hearing aid.
Fig. 12 shows a similar circuit diagram of the
controller; for the same components the same reference
numerals are used. In this circuit, the time delay for
the output signals of the two microphones Fmic
and Bmic is effected in separate delay units 3a and 3b
representing the delay device 3. Otherwise, the function
is similar to the function of the circuits of figs. 6 and
11. Furthermore, a control unit 9 is shown which may
control the attenuation of the controllable attenuators
1 and 2 as well as the delays of delay units 3a and 3b.
This embodiment of the invention is useful in combination
with microphone input circuits that are capable of
supplying a delayed microphone signal together with an
undelayed microphone signal for a hearing aid.
In the change-over controller the amplitude response
as well as the time and phase of the
CA 02479675 2006-02-27
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audio signals are not changed when their directivity
changes.
Fig. 13 schematically shows a further improvement
of the front end circuit of a hearing aid including a
s change-over controller as described so far with
reference to fig. 6. Similar components have been
_ designated with the same reference numerals as before.
Because of the technique used in combining the
output signals of the two microphones Fmic and Bmic, the
io resulting amplitude response of the output signals of
the adding means 6 will in the relevant frequency range
rise with 6 dB per octave compared to the amplitude
response of a single microphone.
This behavior may be observed in substantially all
is systems in which a delayed version of the output signal
from the back microphone is subtracted from the undelayed
output signal from the front microphone while achieving a
directional effect.
However, in most cases, it is desirable to
2o compensate for this change in the amplitude response by
adding a filter at the output of the front end of the
hearing aid (i.e., at the adding circuit 6). Such a
filter means a reduction of 6 dB per octave in the
relevant frequency range, but also means that more circuit
2s components, time, and power would be required.
However, the change-over controller of the present
invention can also be adapted to perform this compensation
3o filtering without the above-noted drawback. Therefore,
there will be no need to add a filter at the output of the
adding circuit 6.
For this purpose, an additional subtraction circuit
is arranged between the adding circuit 4 and the
CA 02479675 2006-02-27
i
19
delay device 3, and the output signal of the adding
circuit 6 is directly supplied to the negative input of
adding means 10 in a feedback loop.
This new arrangement has the desired effect.
s
It may be preferable to include in the feedback
loop a controllable amplifier or attenuator 11.
Thus, the output signal of the change-over
controller is fed back from the adding circuit 6 via the
io controllable attenuator 11 to the negative input of
subtraction circuit 10. Thus, the output signal of
attenuator 11 is subtracted in the subtraction circuit
from the output signal of adding circuit 4.
The resulting, output signal of subtraction circuit
is 10 is supplied to the delay device 3 and hence to the
negative input of the subtraction circuit 5, the
positive input of which is connected to the output of
the controllable attenuator 1.
In the schematic arrangements according to the
2o present invention in figs. 6 and 11 to 13, subtraction
circuit 5 and adding circuit 6 can also be combined into
a single combining circuit, provided this has, in every
respect, the same properties as the two separate adding
means 5 and 6.
25 Ideally, the gain factor of attenuator 11 should be
one or unity for the filtering, being able to perform the
6 dB per octave fall at very low frequencies. However,
this would probably result in a loop gain of unity so that
the circuit might become unstable. Therefore, it is
3o preferred to have the gain of the amplifier or attenuator
11 set to a little less than one or unity.
In fig. 14 a further schematic arrangement of a
hearing aid according to the present invention is shown.
The controllable attenuation and phase delay operations to
CA 02479675 2006-02-27
.~
which the signals from the front and back microphones
Fmic and Bmic are subjected before forming the overall
combined signal are implemented by a different circuit
structure as represented by the relationship stated in the
s foregoing, i.e.,
X:ront * ( 1 - OIflIll * a j~) + Xback * ( 0!17111 - a 7t~~ ~ .
are implemented by a different circuit structure.
In this case, the change-over means comprises a
first adding circuit 12 connected with the front and
io back microphones Fmic and Bmic and a first~subtraction
circuit 13 having a positive input connected with the
front microphone Fmic and a negative input connected
with the back microphone Bmic. First and second phase
delay devices 14 and 15 are connected with the first
is subtraction and adding circuits 13 and 12, respectively.
A second adding circuit 16 is connected with the first
subtraction circuit 13 and the first phase delay device
l4,and a second subtracting circuit 17 has its positive
input connected with the first adding circuit 12 and its
2o negative input connected with second phase delay device
15. A first controllable attenuator 18 acts on the
signal from the second adding circuit 16 for attenuation
of this signal by a factor (1 - omni)/2, and a second
controllable attenuator 19 acts on the signal from the
2s second subtraction circuit 17 for attenuation of this
signal by a factor (1 + omni)/2, whereas a third adding
circuit 20 is connected with the first and second
attenuators 18 and 19 for addition of the signals
therefrom to provide the overall combined signal to be
3o supplied to the signal processor.
The microphones used in the description above are
preferably omnidirectional microphones.
CA 02479675 2006-02-27
AW'
21
when two microphones are used in the omni-
directional mode of operation, both microphones generate
an electrical noise signal N. These two noise signals
have a similar power:
C Nback ( ' ~ Nfront ~ ~
where Nback is the noise signal from the back microphone
Bmic, and Nfront is the noise signal from the front
to microphone Fmic.
The noise signals N are random signals. Therefore,
the resulting signal amplitude is less than twice the
single amplitude. Thus, a 3 dB-noise reduction results.
The total noise signal can be calculated as:
INIz-INfrontl2*,1'OI11I11*e jmT~z+INpacklZ*I1-OIflIll*e ~~TI2
~N~°~N:ro,t~*2°.s11_omn.i*e~~''~)
2o It has been shown that with the new front end of a
hearing aid comprising a change-over controller in
accordance with the invention a great variety of
directional characteristics patterns may be controllably
realized.
