Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PROVIDING THE TRANSMISION CHARACTERISTICS OF A
MICROPHONE ARRANGEMENT AND MICROPHONE ARRANGEMENT
The present invention relates to a microphone arrangement and to a method
for establishing a desired transfer characteristic which converts an
acoustical
input signal impinging on the microphone arrangement into an electric output
signal as a function of the angle at which said acoustical input signals
impinge
on said microphone arrangement.
There is frequently a need in the technology for the
reception and processing of acoustic signals to
implement microphone arrangements having a transmission
characteristic that generate the electric output signal
as a prescribed or prescribable function of the
direction of incidence of the acoustic signals. In
particular, there is a need in,this case to implement
microphone arrangements having a characteristic
directed in a prescribed or prescribable fashion, in
the case of which arrangements acoustic signals from
prescribed directional ranges act more strongly, while
those from other directional ranges act less strongly
on the output signal, up to arrangements with a
reception characteristic focused virtually in one
direction.
Multifarious modes of procedure are known for
implementing such transmission characteristics. Purely
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by way of example, reference may be made in this regard to PCT application
published under publication number W099/04598 (cp multiplication), or to PCT
application published under publication number W099/09786 (cp filtering) of
the
same applicant, in accordance with which desired
transmission characteristics of microphone arrangements
are obtained in principle from the phase shift of
acoustic signals arriving at microphone arrangements
and their specific processing.
It is an object of the present invention to propose a
further procedure in order to implement a desired
transmission characteristic in the above sense.
According to the present invention, there is provided a method for
establishing a
desired transfer characteristic which converts an acoustical input signal
impinging on a microphone arrangement into an electric output signal as a
function of the angle at which said acoustical input signals impinge on said
microphone arrangement, said method comprising the steps of:
providing at said microphone arrangement a first microphone sub-
arrangement and a second microphone sub-arrangement, each microphone
sub-arrangement having a transfer characteristic which converts said
acoustical
input signal impinging on said microphone sub-arrangements into an electric
output signal of the respective sub-arrangement, said transfer characteristics
of
said first microphone sub-arrangements being different from said transfer
characteristic of said second microphone sub-arrangement with respect to said
acoustical input signal;
forming a ratio of said output signals of said first and second microphone
sub-arrangements, thereby generating a ratio result;
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forming a saturated product with said ratio result as one factor, thereby
clipping said product at a predetermined or predeterminable value and
generating a saturated product result; and
generating said electric output signal as a function of said saturated
product result.
According to the present invention, there is also provided a method for
establishing a desired transfer characteristic which converts acoustical input
signals impinging on a microphone arrangement into an electric output signal
as
a function of the angle at which said acoustical input signals impinge on said
microphone arrangement, said method comprising the steps of:
providing at said microphone arrangement at least two microphone sub-
arrangements, each microphone sub-arrangement having a transfer
characteristic which converts said acoustical input signals impinging on said
microphone sub-arrangements into an electric output signal of a respective sub-
arrangement, said transfer characteristics of said at least two microphone sub-
arrangements being different;
forming a ratio of said output signals of said at least two sub-
arrangements, thereby generating a ratio result;
forming a saturated product with said ratio result as one factor, thereby
performing saturating said product at a predetermined or predeterminable value
and generating a saturated product result;
generating said electric output signal as a function of said saturated
product result.
According to the present invention, there is also provided a method for
establishing a desired transfer characteristic which converts an acoustical
input
signal impinging on a microphone arrangement into an electric output signal as
a function of the angle at which said acoustical input signals impinge on said
microphone arrangement, said method comprising the steps of:
at said microphone arrangement providing:
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a first microphone sub-arrangement having a transfer characteristic
which converts said acoustical input signal impinging on said first
microphone into an output signal represented by c,,; and
a second microphone sub-arrangement having a transfer characteristic
which converts said acoustical input signal impinging on said second
microphone into an output signal represented by cZ; and
generating said electric output signal according to the equation:
l
s -l (A -la f- ICz1
1 cJ n I satB
wherein:
S is said electric output signal,
A is a predetermined or adjusted value,
ICn I is the amplitude value of the output signal c,,,
ICZI is the amplitude value of the output signal cZ,
satB is the saturation of the product [] to a predetermined or adjusted
minimum or maximum value B, and
a is a predetermined or adjustable factor.
When "saturation" is spoken of within the scope of the
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present application, this means that the value of a
mathematical function under consideration is clipped
starting from when a prescribed value is reached, so
that, as against the course of a mathematical function,
it remains constant starting from when this value is
reached.
Although a saturation of the product mentioned, that is
to say the weighted quotient, to a minimum value can by
all means be sensible, it is preferably proposed that
the product in any event also be saturated to a maximum
value.
In what follows, the second factor of the saturated
product can assume an arbitrary non-vanishing value,
and thus certainly also the value 1.
In a further preferred embodiment, it is proposed that
the function mentioned comprises a difference between a
constant - settable, if appropriate - and the saturated
product, the value of the constant preferably being
selected to be at least approximately equal to the
saturation value.
Furthermore, the quotient mentioned is preferably
determined from the amplitude values of the output
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signals without taking account of their phase angle.
In a particularly preferred embodiment of the method
according to the invention, the quotient mentioned is
used within the scope of the following function:
S=cn, A- a =IcZI
I~ N ~ satB
in which
S signifies the output signal of the microphone
arrangement
A signifies a prescribed or prescribable signal
value
ICN I signifies the amplitude value of the output signal
of a first microphone sub-arrangement whose
transmission characteristic exhibits maximum gain
for one angle of incidence where the
characteristic to be formed is also to exhibit
maximum gain
IcZ1 signifies the amplitude value of the output signal
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of the second microphone sub-arrangement
satB signifies a saturation of the quotient to a
prescribed or prescribable maximum signal value B
a signifies a prescribable or prescribed factor.
In a particularly preferred embodiment, in particular
within the scope of the use of the method according to
the invention for hearing devices, the transmission
characteristics of the microphone sub-arrangements are
selected such that they respectively exhibit maximum
signal gains for acoustic signals incident from
substantiallv inverse directions...
According to the present invention, there is also provided a microphone
arrangement comprising:
two microphone sub-arrangements each having an output, each of said
microphone sub-arrangements also having a respective transfer characteristic
with which acoustical input signal impinging on said microphone sub-
arrangements are converted into respective electrical output signals at said
outputs as a function of the angle at which said acoustical input signals
impinge
on said microphone sub-arrangements, said transfer characteristics of said
microphone sub-arrangements being different with respect to said acoustical
input signal;
a computing unit having at least two inputs and an output, said outputs of
said microphone sub-arrangements being respectively operationally connected
to said inputs of said computing unit, said computing unit including:
a ratio forming and weighing unit having an output, a denominator
input, a numerator input and a weighing input, wherein
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one of said inputs of said computing unit is operationally connected
to said denominator input, and wherein
the other of said inputs of said computing unit is operationally
connected with said numerator input, and further wherein
said ratio forming and weighing unit generates at said output an
output signal saturated at a maximum or minimum value, the output of
said ratio forming and weighing unit being operationally connected to
the output of said microphone arrangement.
According to the present invention, there is also provided a microphone
arrangement comprising:
a first microphone sub-arrangement having a first output in the time
domain having a first transfer characteristic with respect to an impinging
acoustic signal;
a second microphone sub-arrangement having a second output in the
time domain having a second transfer characteristic with respect to an
impinging
acoustic signal, wherein
said first transfer characteristic and said second transfer characteristic are
different;
a first time to frequency converter unit for converting said first output into
a first frequency domain signal;
a second time to frequency converter unit for converting said second
output into a second frequency domain signal;
a computing unit having a first input, a second input, and an output,
wherein
said frequency domain signals of said time to frequency converter units
are connected to said inputs of said computing unit, respectively, wherein
said computing unit generates a ratio signal that is proportional to an
amplitude or an absolute value of one of said first and second frequency
domain
signals, and further wherein
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said ratio signal is inversely proportional to an amplitude or an absolute
value of the other of said first and second frequency domain signals, and
still
further wherein
said ratio forming and weighing unit multiplies said ratio signal by a non-
zero value to create a weighted ratio; and wherein
said ratio forming and weighing unit generates a saturated signal by
clipping said weighted ratio at a maximum or minimum value.
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Preferred design variants of the microphone arrangement
according to the invention are specified in claims 10
to 18.
The method according to the invention and the
microphone arrangement according to the invention are
particularly suitable for use on hearing devices.
Although it is certainly possible to implement the
method according to the invention and the microphone
arrangement according to the invention by means of
signal processing in the time domain, in a preferred
embodiment the signal processing is undertaken in the
the frequency domain with t.the use of time
domain/frequency domain converters and frequency
domain/time domain converters.
The invention is explained below by way of example with
the aid of figures, in which:
Figures la
and b show, by way of example, the transmission
characteristics of two (a and b) microphone
sub-arrangements used according to the
invention;
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Figure 2 shows, plotted against the angle axis cp in
accordance with figures la and lb, the
formation of a quotient function Q from the
characteristics in accordance with figures la
and lb, as well as the saturation of this
quotient function to the maximum value 0 dB;
Figure 3 shows, starting from the saturated quotient
function explained with the aid of figure 2,
the same saturated quotient function with
linear gain scaling, and the formation of a
function F from the difference between said
saturated quotient function and a fixed
value;
Figure 4 shows in a shaded fashion in a representation
similar to figures la and lb a transmission
characteristic implemented according to the
invention;
Figure 5 shows in a representation similar to figure 4
a further transmission characteristic
implemented according to the invention; and
Figure 6 shows the implementation of a microphone
arrangement according to the invention in the
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form of a simplified signal flow/function
block diagram.
The procedure according to the invention is to be
illustrated with the aid of figures 1 to 3 without
pretension to scientific exactitude with the aid of
simple transmission characteristics corresponding in
each case to first-order cardoids. This comprehensible
and simple procedure provides the person skilled in the
art with the instructions as to how a desired
transmission characteristic can be implemented
according to the invention even when starting from more
complex transmission functions.
Let a first microphone sub-arrangement have the three-
dimensional transmission characteristic, illustrated in
two dimensions in figure la, with reference to its
transmission or gain characteristic with reference to
acoustic signals incident on it from the direction T.
In a representation similar to figure la, there is
illustrated in figure lb the transmission
characteristic of a second microphone sub-arrangement
which may be a mirror image with reference to the axis
n/2; 3n/2 of the transmission characteristic of the
first microphone sub-arrangement. The transmission
characteristic in accordance with figure la may be
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denoted by CN, and that in accordance with lb by cZ.
The magnitude of the transmission characteristics CN and
cZ, respectively, is illustrated qualitatively and in dB
in figure 2 against the angle axis cp in accordance with
figures la and lb.
In the case of acoustic standard signals incident on
the two microphone sub-arrangements, the transmission
characteristics illustrated in figures la and lb
correspond at the same time to the respective signal
values on the output side of the microphone sub-
arrangements considered.
According to the invention, a quotient, for example,
Q ICZ1
ICN I
is now formed according to the invention from these two
output signal values, which are likewise denoted by CN
and cz, respectively. This quotient formation results in
the function Q, represented qualitatively with a dash-
dotted line in figure 2 and having a pole at cp =n. In
the case of real quotient formation, the pole resulting
for the zero of the denominator function IcN I is
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captured in any case, that is to say the quotient
function Q is saturated. The quotient function is
preferably saturated at a prescribed or prescribable
value B, in accordance with figure 1 preferably at the
value "one", in the case of a maximum value of the
transmission functions in accordance with figures la, b
of "one".
If it is now assumed that the denominator transmission
characteristic, in the present case cN, be that which is
to be the dominant one for the transmission
characteristic result to be achieved, that is to say be
a transmission characteristic that has a high signal
gain in an angular range iq which the desired
characteristic to be implemented is also to have a high
signal gain, then the advantage of the quotient
formation according to the invention is already to be
seen now. A pole of the quotient results in the zero
angular range from this transmission characteristic
dominant for the result to be targeted. The zero
angular range of the dominant transmission
characteristic or of those angular ranges with reduced
signal gain will, however, be those that are to be
changed, that is to say can be "improved" in order to
obtain the desired characteristic. It is precisely
there that the possibility now exists of intervening
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simply, specifically by saturation to a prescribable or
prescribed constant value of the quotient function.
For reasons of clarity, the quotient function Qsati
saturated to "1" is now introduced into figure 3 with
linear gain scaling. It may now be seen further from
this that the saturated quotient function Qsatl exhibits
the profile of a directed transmission characteristic
in the non-saturated angular ranges, in the present
case between 0 and n/2, as well as between 3n/2 and 2n.
If the aim is now the directional characteristic
expressed for the desired transmission characteristic
to be implemented, the region of the quotient function
set according to the invention to the prescribed
saturation value, "one" in the example described, is
utilized for the purpose of achieving a defined minimum
gain of the desired transmission characteristic there,
that is to say in this angular range. This is achieved
in the example presented by virtue of the fact that the
saturated quotient function is subtracted from a
prescribed or prescribable fixed value A, for example,
and preferably in the example presented, having the
value "one". The result is the function
F = A - QsatB,
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represented once again with a continuous line in figure
3, or, as a special and preferred case, the function
F = 1 - Qsati =
It may be seen from this that a transmission function,
F, must be achieved that exhibits a non-vanishing
signal gain exclusively in the angular range
05 tp < 2 and 32 < cp <_ 2~.
The following can now be set forth with reference to
the procedure according to the invention:
~ Fundamentally, the transmissioft characteristic to be
implemented is implemented on the output side of the
microphone arrangement according to the invention as
a function of the quotient, saturated to a
prescribed or prescribable maximum value, of the
output signals of two microphone sub-arrangements
having a different transmission characteristic.
It is preferred in this case, as is to be shown
later, to multiply the quotient function Q, as
factor, by a further permanently prescribed or
settable weighting factor before saturation takes
place at the resulting product. The weighting factor
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mentioned is 1 in the example presented with the aid
of figures 1 to 3.
Furthermore, it can well be advantageous to
undertake the saturation at the product of the
factor mentioned and the quotient at least also when
the prescribed minimum values are reached.
== The quotient formation can be performed in this case
directly by quotient formation of the signal
amplitude values, without taking account of phase.
== Although the saturated product can be used, if
appropriate, in the form of 4nother function, that
is to say as F = F[(a = Q) sats], in general, it is
much preferred to implement a directed charac-
teristic by subtracting the saturated product
mentioned from a prescribed or prescribable fixed
value.
As will be shown later the possibility of varying the
targeted directional characteristic results in a very
simple way from varying the fixed value mentioned
and/or the multiplicative factor a of the saturated
product.
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= In principle, it is possible to use as microphone
sub-arrangements all known microphones and their
combinations that have different transmission
characteristics as required in the position of use
and as required with reference to the direction of
incidence tp of acoustic signals that strike.
== In order, in particular, to implement directed
characteristics, it is preferred to use microphone
sub-arrangements whose transmission characteristics
are identical, but inversely directed with reference
to the direction of incidence of acoustic signals.
== The implementation of such macrophone arrangements
can be performed, in particular, by using the known
"delay and add" principle.
Particularly in the case of this form of
implementation, as well, the inversely acting
microphone arrangements just named can be
implemented with two microphones whose outputs, as
still to be shown, are time delayed and
appropriately added in each case in order to form
the two microphone sub-arrangements.
= It goes without saying that it is possible to
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implement very highly complex transmission functions
and transmission function combinations by developing
the procedure according to the invention with three
and more microphone sub-arrangements.
The transmission function preferably used according to
the invention may be reproduced once again in summary,
specifically:
S=cN A- a= IcZI
IcN) atB
Figure 4 illustrates the transmission function that was
formed according to the invention from inversely
directed, identical cardoid transmission charac-
teristics Ca, corresponding to the transmission
function
S'=cN I- 1. ZI
I ccNI atl
The resulting transmission characteristic is
illustrated in figure 5 when the following is true:
c
S"=cN I- 4= Z
cN
I I satl
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A microphone arrangement operating using the method
according to the invention is illustrated by way of
example, in particular for use in a hearing device, as
well, in figure 6 with the aid of a simplified signal
flow/functional block diagram.
In accordance with figure 6, an arrangement 1 having at
least two microphone sub-arrangements la and lb is
provided at the microphone arrangement according to the
invention. Output signals appear at their outputs Ala
and Alb as a function of the acoustic signals incident
at the microphones on the input side as a function of the direction
cp. As illustrated in figure 6, the two microphone sub-
arrangements can well be impleipented by means of a
single pair of microphones whose outputs are coupled to
one another using the "delay and add" technique. What
is essential is that basically signals having different
transmission characteristics with reference to the
direction cp of arriving acoustic signals are generated
at the outputs Ala and Alb.
The outputs Ala and Alb are preferably led to time
domain/frequency domain converter units FFT 3a and 3b,
respectively, if, as preferred, the subsequent signal
processing is to be performed in the frequency domain.
The outputs mentioned are operationally connected to
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inputs E5a and E5b, respectively, of magnitude forming
units 5a and 5b. The outputs of the magnitude forming
units mentioned are, as illustrated, led to the
denominator and numerator inputs N and Z of a division
unit 7. The output A7 is operationally connected to one
input Ella of a subtraction unit 11 in a fashion
multiplied by a weighting unit 9 by a weighting factor
a that can be prescribed at a control input S9.
As boxed in a dashed manner in figure 6, the division
unit 7 and weighting unit 9 form a weighted quotient
forming unit 10. The factor a illustrated by way of
example in figure 6 and capable of being set at the
weighting unit 9 can assume a3;bitrary non-vanishing
values.
As further illustrated schematically in figure 6, the
signal at the output Ay of the weighted quotient forming
unit 10 is fed to a saturation unit 12 whose output is
firstly fed to the input E11a= At the saturation unit
12, which can, of course, be integrally combined with
the weighted quotient forming unit 10, the output
signal of the weighted quotient forming unit 10 is
saturated downward (indicated by dashes in block 12 of
figure 6) and/or upward to a prescribed or prescribable
value B - as set in a way illustrated schematically at
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the input satB. This preferably at least also in this
case to a maximum value. At the subtraction unit 11,
the signal present there is subtracted from a fixed
value A set, or settable, at the second input Ellb. The
output A11 of the subtraction unit 11 is operationally
connected to one input E13a of a multiplication unit 13. The output signal Ala
of the
microphone sub-arrangement la is operationally connected to the second input
E13b of the multiplication unit 13 via the converter unit FFT 3a, and is also
operationally connected to the denominator input N of the division unit 7 via
the
magnitude forming unit 5a. If appropriate for changing the saturation angular
range
explained with the aid of figures 1 to 3, it is
possible, as illustrated by dashes at 15, for the
denominator signal, and if cippropriate also the
numerator signal, which is fed to the input N or the
input Z of the division input 7, to be weighted.
The output signal Sout of the microphone arrangement
according to the invention appears on the output side
of the multiplication unit 13. It has the desired
transmission characteristic as a function of the solid
angle tp with which acoustic signals are incident on the
microphone arrangement 1 on the input side.
As has already been mentioned, it is preferred to
select identical characteristics that act in inverse
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directions relative to one another for the transmission
characteristics of the microphone sub-arrangements la
and lb. The desired transmission characteristic is set
at the output signal Soõt by setting the weighting
factor a, the saturation value B, the fixed value A,
and further weighting factors such as (3, if
appropriate.
The method according to the invention and the
microphone arrangement according to the invention are
excellently suited to use with hearing devices,
particularly also because of the low outlay on signal
processing and, as was shown with the aid of figures 3
and 4, the pronounced possibil#y of suppressing the
signal transmission from undesired directions of
incidence such as from behind with reference to a
hearing device that is worn. Instead of microphone sub-
arrangements with cardoid characteristics Ca, it is
rather those with hypercardoid characteristics Hca
(figure 5) that are preferably used for hearing
devices.
