Note: Descriptions are shown in the official language in which they were submitted.
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Method for electronically selecting the dependency of an output
signal from the spatial angle of acoustic signal impingement
and hearing aid apparatus
The present invention is generically directed on a technique
according to which acoustical signals are received by at least
two acoustical/electrical converters as e.g. by multidirec-
tional microphones, respective output signals of such convert-
ers are electronically computed by an electronic transducer
unit so as to generate an output signal which represents the
acoustical signals weighted by a spatial characteristic of am-
plification. Thus, the output signal represents the received
acoustical signal weighted by the spatial amplification charac-
teristic as if reception of the acoustical signals had been
done by means of e.g. an antenna with an according reception
lobe or beam. Thus, the present invention is generically di-
rected on an electronically preset, possibly electronically ad-
justed and tailored reception "lobe".
Figure 1 most generically shows such known technique for such
"beam forming" on acoustical signals. Thereby, at least two
multidirectional acoustical/electrical converters 2a and 2b are
provided, which both - per se - convert acoustical signal irre-
spective of their impinging direction A and thus substantially
unweighted with respect to impinging direction 8 into first and
second electrical output signals Al and A2. The output signals
Ai and Az are fed to an electronic transducer unit 3 which gen-
erates from the input signals A1, AZ an output signal Ar. As
shown within the block of unit 3 the signals Al,Zare treated to
result in the result signal Ar which represents either of A1 or
A2, but additionally weighted by the spatial amplification
function F1(A). Thus, acoustic signals may selectively be am-
CONFIRMATION COPY
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plified dependent from.the fact under which spatial angle A
they impinge, i.e. under which spatial angle the transducer ar-
rangement 2a, 2b "sees" an acoustical source. Thereby, such
known approach is strictly bound to the physical location and
intrinsic "lobe" of the converters as provided.
One approach to perform signal processing within transducer
unit 3 shall be exemplified with the help of Fig. 2. Thereby,
all such approaches are based on the fact that due to a prede-
termined mutual physical distance pP of the two converters 2a
and 2b, there occurs a time-lag dt between reception of an
acoustical signal at the converters 2a, 2b.
Considering a single frequency - c,~ - acoustical signal, re-
ceived by the converter 2a, this converter will generate an
output signal
(1) Al - A ~ sinwt,
whereas the second transducer 2b will generate an output signal
according to
AZ - A ~ s inw ( t+dt ) ,
whereat dt is given by
2 0 ( 3 ) dt - Pp sin6
c
therein, c is the sound velocity.
By time-delaying e.g. A1 by an amount
(4) T - PP~c
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and forming the result signal A= from the difference of time-
delayed signal Al' - as a third signal - namely from
(5) Al' - A ~ sinw (t+i) , and
(2) AZ - A ~ sinw (t+dt) ,
there results, considered at the frequency w, a spatially car-
doid weighted output signal Ar as shown in the block of trans-
ducer unit 3:
(6) ~Ar~ - ~Al' - A~~ - 2A sin (w (i-dt) /2)
- 2A sin (w (T-pp*sin9/c) /2 ) .
At 8 - 90° Ar becomes zero and
at 8 - -90° Az becomes
( 7 ) Azm~ - 2A Sin w pp/c .
Such processing of the output signals of two omnidirectional
order converters leads to a first order cardoid weighing func-
tion F1(6) as shown in Fig. 3. By respectively selecting con-
verters with higher order acoustical to electrical conversion
characteristic i.e. "lobe" and/or by using more than two con-
verters, higher order - m - weighing functions Fm(8) may be re-
alised.
In Fig. 4 there is shown the amplitude Ar",,a,~-characteristic, re-
sulting from first order cardoid weighing as a function of fre-
quency f = w/2n. Additionally, the respective function for a
second order cardoid weighing function FZ(8) is shown. Thereby,
there is selected a physical distance pp of the two converters
28 and 2b of fig. 1 to be 12 mm.
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As may clearly be seen at a frequency fr which is
(8) fr = c/ (4pp)
maximum amplification occurs of +6 dB at the first order car-
doid and of +12 dB at a second order cardoid. For pP = 12 mm,
fr is about 7 kHz.
From fig. 4 a significant roll-off for low and high frequencies
with respect to fr is recognised, i.e. a significant decrease
of amplification.
Techniques for such or similar type of beam forming are e.g.
known from the US 4 333 170 - acoustical source detection -,
from the European patent application 0 381 498 directional mi-
crophone - or from Norio Koike et al., "Verification of the
Possibility of Separation of Sound Source Direction via a Pair
of Pressure Microphones", Electronics and Communications in Ja-
pan, Part 3, Vol. 77, No. 5, 1994, page 68 to 75.
Irrespective of the prior art techniques used for such beam
forming with at least two converters, the distance pP is an im-
portant entity as may be seen e.g. from formula (8) and di-
rectly determines the resulting amplification/angle dependency.
Formula (8) may be of no special handicap if such a technique
is used for narrow band signal detection or if no serious lim-
its are encountered for geometrically providing the at least
two converters at a large mutual physical distance pP.
Nevertheless, and especially for hearing aid applications, the
fact that fr is inversely proportional to the physical distance
pp of the transducers is a serious drawback, due to the fact
that for hearing aid applications the audio frequency band up
CA 02296414 2004-07-28
to about 4 kHz for speech recognition should be detectable by
the at least two transducers which further should be mounted
with the shortest possible mutual distance pp. These two re-
quirements are in contradiction: The lower fr shall be real-
ised, the larger will be the distance pp required.
It is thus a first object of the present invention to remedy
the drawbacks encountered with respect to pP-dependency of
known acoustical "beam forming".
The first object of the present invention is reached by providing a method for
electronically selecting the dependency of an electric. ouput signal of an
electronic transducer unit from spatial direction wherefrom acoustical signals
impinge on at least a first and a second acoustical/electrical converter
operationally connected to the input of said transducer unit and thereby
inputting
first and second electric signals thereto, comprising the steps of:
~ generating at least one third electrical signal in dependency
from mutual phasing of said first and second electric signals
multiplied by a constant larger than unity or a frequency-
dependent factor and further from a fourth electric signal
which is dependent from at least one of said first and second
electric signals, thereby generating by said multiplying a
mutual phasing of said first and second electric signals as
if said first and second converters were more distant from
each other than they actually are;
~ generating said output signal of said transducer unit in de
pendency of said third electric signal and a~fifth electric
signal being dependent from at least one of said first and
second electric signals.
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5a
Thereby, it becomes possible, irrespective of the actual physical mutual
distance
of the two converters. to select said de-
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pendency, thereby pre-selecting same and possibly tuning and
adjusting same, to result in a dependency as if the at least
two converters were physically arranged at completely different
physical positions than they really are.
In a first preferred manner of realising the inventive method
the fourth electric signal is selected to be linearly dependent
only from one of the first and second electric signals, thereby
being preferably directly formed by such first or second elec-
tric signal.
Nevertheless, in a today's more preferred manner of realising
the inventive method, the fourth electric signal is dependent
on both first and second electric signals. In a preferred form
the fourth electric signal has a predetermined or adjustable
"lobe" characteristic, i.e. dependency from spatial impinging
direction. Thereby in a preferred form of "lobe" realisation
the fourth electric signal is generated by delaying one of the
first and second signals and then summing the delayed signal
and the other, undelayed signal of said first and second sig-
nals. Thereby, the fourth electric signal per se has an ampli-
fication to impinging angle dependency and thus defines - as
was said - for a "lobe", as an example according to a depend-
ency as was discussed with the help of the figs. 1 to 4.
In a further preferred form of realising the inventive method,
either per se or combined with either method to generate the
fourth signal as just stated, and especially combined with gen-
erating the fourth signal with a "lobe"-characteristic, it is
proposed to generate the fifth electric signal in direct or
linear dependency of at least one of the first and second elec-
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tric signals, thereby preferably using one the said first and
second electric signals as the fifth electric signal.
Thereby, and again per se or combined with either method of
generating the fourth electric signal, especially combined with
generating the fourth electric signal with a "lobe"-dependency,
it is proposed to generate the fifth electric signal as well
with a "lobe" dependency from spatial impinging angle, which is
realised in a first form by delaying one of the first and sec-
ond signals and summing the delayed signal and the other of
said first and second signals. Thereby, it becomes clear that
the fourth electric signal, generated to define for a "lobe"
characteristic, may directly be used as the fifth electric sig-
nal, having then the same "lobe"-characteristic.
In a further, clearly preferred realisation form of the inven-
tive method and combined with any of the preferred realisation
forms stated up to now and throughout the further description,
it is proposed to generate the first and second electric sig-
nals in their respective spectral representation, thereby gen-
erating the at least one third electric signal in dependency of
mutual phasing of respective spectral components of the first
and second signals and multiplied by a constant frequency-inde-
pendent or by frequency-dependent factors.
In a further preferred mode of operation, the frequency-
dependent multiplication factors are selected to be inversely
~ 25 proportional to frequency, at least in a first approximation.
With an eye specifically on hearing aid applications, wherefore
the present method is most suited, but may be clearly applied
to others, it is proposed that the real physical distance of
the first and second converters to be at most 20 mm, whereby
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the virtual distance, which is at least dependent from the
phasing multiplication factor, is selected to be larger than
the mutual physical distance of the two converters, in other
words dependency of the transducer unit's output signal from
spatial angle becomes so as if, physically, converters were
provided at considerably larger mutual distances than they
really are. It goes without saying, that such technique is of
very high advantage in any space-restricted applications, as
especially in hearing aid applications.
To resolve the object mentioned above and to realise especially
a hearing aid, whereat, irrespective of the physical position
of at least two acoustical/electrical converters, a desired re-
ception lobe may be tailored and possibly adjusted according to
the needs, is realised inventively by an acoustical/electrical
transducer apparatus comprising at least two acoustical/elec-
trical converters spaced from each other by a predetermined
physical distance, whereby the at Least two converters gener-
ate, respectively, first and second electrical output signals
and wherein the outputs of said acoustical/electrical convert-
ers are operationally connected to an electronic transducer
unit, which generates an output signal dependent from said
first and second output signals of said converters by an ampli-
fication function which function is dependent from spatial an-
gle under which said converters receive acoustical signals,
comprising:
- a phase difference detection unit, the inputs thereof being
operationally connected to the outputs of said converters and
generating at its output a phase difference-dependent signal,
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9
- a phase processing unit, one input thereof being operationally connected to
the output of said phase difference-detection unit, at least one second input
of
said processing unit being operationally connected to a factor-value-selecting
source, a third input of said phase processing unit being operationally
connected
to at least one of the outputs of said at least two converters, said phase
processing unit generating an output signal at its output according to a
signal at
said third input with a phasing according to a signal at said one input
multiplied
by a signal at said at least one second input,
- a beam-former processing unit with at least two inputs, one input being
operationally connected to the output of said phase-processing unit, the
second
input being operationally connected to at least one output of said at least
two
converters.
Under all the aspects of the invention there is thus possible to realise
(9) Pv' Pp .
This especially for low-space applications, as especially for hearing aid
applications.
Thereby, there is introduced the virtual distance pv of transducers, i.e. the
distance of converters which would have to be physically realised to get an
angle dependency as realised inventively.
Thereby, according to formula (8), fr may be shifted to lower frequencies:
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It becomes possible to realise fr values well in the audio-
frequency band for speech recognition (< 4 kHz) with physical
distances of microphones, which are considerably smaller than
this was possible up to now.
Multiplying the phase difference by a constant factor does nev
ertheless not affect the roll-off according to fig. 4. This
roll-off is significantly improved, leading to an enlarged fre-
quency band Br according to fig. 4 a.f - as was said - the pre-
determined function of frequency is selected as a function
which is at least in a first approximation inversely propor-
tional to the frequency of the acoustic signal.
For instance for the first order cardoid according to fig. 3
and fig. 4, there may be reached a flat frequency characteris-
tic between 0,5 and 4 kHz and thus a significantly enlarged
frequency band Br with well-defined roll-offs of amplification
at lower and higher frequencies by accordingly selecting the
frequency dependent function to be multiplied with the phase
difference.
Other objects of this invention will become apparent as the de-
scription proceeds in connection with the accompanying draw-
ings, of which show:
Fig. 1: A functional block diagram of a two-transducer acous-
tic receiver with directional beam forming according
to prior art;
Fig. 2: one of prior art beam forming techniques as may be
incorporated in the apparatus of fig. 1, shown in
block diagram form;
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Fig. 3: a two-dimensional representation of a three-
dimensional cardoid beam, i.e. amplification charac-
teristic as a function of incident angle of acousti-
cal signals;
Fig. 4: the frequency dependency of the maximum amplification
value according to fig. 3 for first and second order
cardoid functions;
Fig. 5: a pointer diagram resulting from the technique ac-
cording to fig. 2, still prior art;
Fig. 6: a pointer diagram based on fig. 5 (prior arty, but
according to the inventive method, which is performed
by an inventive apparatus;
Fig. 7: a simplified block diagram of a first realisation
form of an inventive apparatus, especially of an in-
ventive hearing aid apparatus, wherein the inventive
method is implemented;
Fig. 8: a simplified block diagram of a today preferred re-
alisation form of the inventive method and apparatus;
Fig. 9: a simplified block diagram of an inventive apparatus,
operating according to the inventive method, in a
generalised form;
Fig. 10: a generic signal-flow/functional block diagram of an
inventive apparatus operating according to the inven-
tive method;
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Fig. 11: the measured directivity characteristics resulting
from the inventive method and inventive apparatus ac-
cording to fig. 8;
Fig. 12: a second directivity characteristics in a representa-
tion according to fig. 11, resulting from the inven-
tive method and apparatus according to fig. 8.
As was mentioned above, in the figs. 1 to 4 known beam forming
techniques were based on at least two acoustical/electrical
transducers spaced from each other and directly on their mutual
physical distance pP.
In fig. 5 there is shown a pointer diagram according to (6).
The basic idea of the present invention shall be explained now
with the help of the still simplified one - o~ - frequency ex-
ample. The inventively realised pointer diagram is shown in
fig. 6. The phase difference w ~ dt between signal A2 and A1 ac-
cording to fig. 6 is
pP sin6
( 10 ) cz~ dt - ~ ~ - ~cp .
c
This phase difference is determined and is multiplied by a
value dependent from frequency, thus with the respective value
of a function M(co) , which may be also a constant Mo ~ 1.
By phase shifting one of the two signals Al, AZ according to
the respective pointers in fig. 6, e.g. of A2 by
M,~ - Ocp or by Mo ~ Ocp,
there results the phase shifted pointer Az~. This pointer would
have also occurred if dt had been larger by an amount according
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to M~, or Mo, thus if a "virtual transducer" had been placed
distant from transducer la by the virtual distance p~, for
which:
( 11 ) p" - M~, ~ pp or
(12) p~ = Mo ~ PP.
As we consider one single frequency for simplicity we may write
Mo = M~,.
With virtual T~
( 13 ) z~ = M~, ~ i and
sin6
(3~) dt~ - M~, - pp
c
we get according to the present invention:
( 1~) A1 - A1V - Asinc~t
(2~) Az~ = Asinc~ (t+dt") - Asinc~ (t+M«dt)
{5~) Al~ = Asinw (t+MwT)
(6~) Ar" = 2Asin ( (M~, ' cu (T-dt) /2)
With (8) we further get:
c 1
f Y~ 4 M~pp - MW ' f r .
Therefrom, we may see that for a given pp, which would lead to
a too high fr, fz~ is reduced by the factor M,~, taken Mw > 1.
In fig. 7 there is schematically shown a first preferred reali-
sation form of an inventive apparatus in a simplified manner,
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especially for implementing the inventive method into an inven-
tive hearing aid apparatus. Thereby, the output signals of the
acoustical/electrical transducer 2a and 2b are fed to respec-
tive analogue to digital converters 20a, 20b, the outputs
thereof being input to time domain to frequency domain - TFC -
converter units as to Fast-Fourier Transform units 22a and 22b.
A spectral phase difference detecting unit 27 spectrally de-
tects phase difference Ocpn for all n spectral frequency compo-
nents which are then multiplied by a set of constants cn. If M
is a function of w, M,~, then the c" can be different for dif-
ferent frequencies, and represent a frequency dependent func-
tion or factor. If on the other hand the phase differences ~cp~
are multiplied by the same co = cn ~ 1 this accords with using
a constant Mp .
This multiplication according to {3~) is done at a spectral
multiplication unit 28. Signal A1 in its spectral representa-
tion is then spectrally phase shifted at a spectral phase
shifter unit 29 by the multiplied spectral phase difference
signals output by multiplier unit 28.
According to fig 7 the signal A1 in its spectral representation
and inventively, spectrally phase shifted - A1 (w, Ocp' n) - is
computed in a spectral computing unit 23 together with A2 in
its spectral representation, as if transducer 2a was distant
from transducer 2b by a distance p~ = M~,pP. The resulting spec-
trum is transformed back by a frequency to time domain con-
verter - FTC - as by an Inverse-Fast-Fourier-Transform unit 24
to result in A~" .
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Thereby, other beam forming techniques than that described with
the help of figs 1 to 4, i.e. using the time delaying technique
- transformed in the frequency domain - may be used in unit 23.
' Nevertheless the time delaying technique is preferred.
With an eye on fig. 4 it has been explained that by inventively
introducing "virtual" converters with a virtually enlarged mu-
tual distance, it becomes possible to shift the high gain fre-
quency fr towards lower frequencies, which is highly advanta-
geous especially for hearing aid applications. This is already
reached if instead of a frequency dependent function M~" a con-
stant Mo is multiplied with the phase difference as explained.
In a preferred mode of the invention the frequency dependent
function M~, is selected to be, at least in a first approxima-
tion,
1
(14) M« ~ -
c~
Thereby, it is reached that, different from fig. 4, there will
be no roll-off and the gain in target direction will be con-
stant over the desired frequency range. By appropriately se-
letting the function Mw it is e.g. possible to reach a flat
characteristic within a predetermined frequency range, e.g. be-
tween 0.5 and 4 kHz with defined roll-offs at lower and higher
frequencies. With appropriately selecting the function M~, prac-
tically any kind of beam forming can be made.
For generating higher order cardoid weighing functions it is
absolutely possible to additionally use the not phase-shifted
output signal Al - as shown in fig. 7 by dotted line - as com-
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puting input signal to unit 23 too, thus "simulating" three
converters.
Fig. 8 shows a today's preferred embodiment of an inventive ap-
paratus in a functional-block/signal-flow representation in
analogy to the representation of fig. 7. Blocks and signals
which were already explained with the help of fig. 7 are de-
fined in fig. 8 by the same reference numbers.
The phase spectrum at the outside of multiplication unit 28,
..n is added at a summing unit 29' to a signal Akr, ~...n (~ ~ e)
also in spectral representation, which signal has a preselected
dependency from impinging angle 8, as especially a first or
higher order cardoid dependency.
To realise that signal Akr,l...n (W...n.6) and following the expla-
nation with respect to figures 2 to 4, the output signal Al(w),
and Az(w) in their spectral representation, are led to a beam-
former unit 32, which may be integrated in beam-former unit 23'
and which e.g. is built up according to the beam-former of fig.
2. Thereby, it must be clearly stated that instead of the beam-
former 32 as shown in fig. 8 other kinds of beam-former result-
ing in different than first order cardoid characteristics may
be implemented there.
The spectrum Akr,l...n(~~..n.9) is then phase-shifted by the phase
adding unit 29' by Ocp' l, _.n, resulting in an output signal of
that unit 29' which is the spectrum A,~",l...n (c~l...n~ O~P ~ ~...n. 9) as
shown in fig. 8 . The signal Ak=,l...n (W...n~ e) as well as the out-
put signal of summing unit 29' are led to the beam-former unit
23', where they are preferably again summed as shown at 33.
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At the output of beam-former unit 32 a signal is generated with
a real cardoid dependency from impinging angle A, whereas at
the output of unit 29', and thus after phase shifting, a de-
pendency function with respect to impinging angle 6 is realised
according to virtually positioned converters. When summing, as
with the unit 33 within beam-former unit 23', there results a
dependency of the output signal Ar from impinging angle 8 ac-
cording to a second order cardoid if the real cardoid depend-
ency at the output of unit 32 is a first order cardoid.
Thus, in a more generic representation, as shown in fig. 9, the
phase difference spectrum at the output of unit 27 is subjected
to a phase shifter unit 35, where it is modified as per cl to
Cn.
The generalised phase shifter 3S may receive directly one of
the output signals of one of the two converters 2a, 2b and/or a
signal which results from beam forming from the said converter
output signals to be phase shitted. In fig. 9 this is repre-
rented by the signal path fed back from beam former 37 to the
phase shifter 35. This feedback accords, with an eye on fig. 8,
to the signal path between beam former 32 and summing unit 29'.
According to fig. 9 beam former unit 32 of fig. 8 is integrated
in the overall beam former unit 37.
The beam former 37 in its generalised form of fig. 9 receives
at least one of the output signals of the converters 2a, 2b and
the output signal of the generalised phase shifter 35.
It is evident for the skilled artisan that
~ more than two real converters may be used and/or
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~ more than one M~, function or of co or cl...n sets may be used
to produce more than one "virtual transducer" signal from one
or from more than one real converter signals respectively.
With selecting the number of physical and virtual converters,
their characteristics and virtual "relocation" of these con-
verters, the spatial weighing function may be selectively tai-
lored.
The present invention under its principal object makes it pos-
Bible to realise practically any desired beam forming with at
least two converters separated by only a predetermined small
distance, due to the fact that electronically there is provided
a virtual mutual converter location of the physically provided
converter.
Thereby, roll-off may be significantly reduced by such virtual
transducer, which is especially established with realising a
virtual distance of the converter which is dependent from fre-
quency, especially inversely dependent. By selecting a fre-
quency-M«-dependent virtual distance of the converters, virtu-
ally an array of frequency-selective converters is established.
For a hearing aid apparatus the real distance between the at
least two transducers, i.e. microphones, is selected to be 20
mm at most, preferably less.
Fig. 10 shows in most generic form the principle proceeding and
apparatus structure as according to the present invention and
common to all embodiments of the invention as described above.
First and second electric signals S1 and Sz, which are derived
from the output signals of the at least two acoustical/electri-
cal converters 2a, 2b, are input to the transducer unit 3.
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Within unit 3, there is provided a phase difference detection
unit according to unit 27 of figures 7, 8 or 9. The phase dif-
ference detection unit 27 has respective inputs which are op-
erationally connected to the inputs of unit 3 and-thus to the
outputs of the converters 2a, 2b. The output of the phase dif-
ference detection unit 27 is operationally connected to an in-
put of a phase processing unit 40 shown in dashed-dotted lines
in fig. 10. The phase processing unit has a second input, which
is connected to a factor value-selecting source 42, generating
a constant or frequency-dependent factor h. A third input of
the phase processing unit is operationally connected as sche-
matically shown by combining unit 44 in an "AND" or in an "EX-
OR" dependency to respective outputs of the at least two con-
verters 2a and 2b. The phase processing unit 40 generates an
output signal, S3, in accordance with a signal, S4, applied to
the third input of the processing unit 40 and in accordance
with the signals applied to the first - from 27 - and second -
from 42 - inputs to the phase processing unit.
The signal at the first input of the phase processing unit,
which is operationally connected to the output of the phase
difference detection unit, is multiplied - by unit 28 - by the
constant or frequency-dependent factor, and, at a signal com-
bining unit 46, the output signal of the processing unit, sig-
nal S" is thus generated in dependency from mutual phasing of
the output signals of the converters, multiplied by a constant
or frequency-dependent factor and from signal S4 as applied to
the third input of the processing unit 40, which latter signal
S4 is dependent from at least one of the output signals of the
converters 2a, 2b. In unit 46 the dependency Fl of signal Sj
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from both, signal S4 and multiplied phasing signal as at the
output of unit 28, is generated.
The signal S3, which accords to A1(w) of fig. 7 or-to
Akz,~...n(W...n.e) of figs. 8 and 9, is input to a beam former
processing unit 48 according to unit 23 or 23' or 37, as of the
figs. 7 to 9. The beam former processing unit comprises a sec-
ond input to which S5, dependent from at least one of the out-
put signals of the converters 2a, 2b is fed. Latter signals are
thus operationally connected as schematically shown by block SO
in an "EX-OR" or in an "AND" combination to the beam former
processing unit 48.
In fig. 11 there is shown the "lobe" or directivity character-
istic - in dB - which was measured at an inventive apparatus
according to fig. 8 at single frequency 1 kHz of acoustical
signals impinging on the two acoustical/electrical converters
2a, 2b. In this apparatus there was valid:
converters 2a, 2b: omnidirectional microphones,
KNOWLES EK 7263
Physical distance pP: 12 mm
z: 35 ~tsec.
c: 2 at 1 kHz and at 4 kHz
There resulted a directivity index as defined in SPEECH COMMU-
NICATION 20 (1996), 229 to 240, Microphone array systems for
hands-free telecommunication, Gary W. Elco of 8.83.
In fig. 12 the result is shown at an inventive apparatus which
was used for the measurement according to fig. 11, but at 4 kHz
CA 02296414 2000-O1-14
WO 99/04598 PCT/IB98/01069
- 21 -
single frequency acoustical impinging signals. The directivity
index became 7.98.
There results from proceeding according to fig. 8a directivity
characteristics according to a second order cardoid. This would
conventionally have to be realised by means of four acousti-
cal/electrical converters as of 2a and 2b, which four convert-
ers define for a spacing of 24 mm between respective two of the
four converters. Thus, a.t might be seen that with the inventive
method and apparatus with only two acoustical/electrical con-
l0 verters with a mutual spacing of 12 mm a directivity result is
reached as if four acoustical/electrical converters had been
used with mutual spacing of 24 mm.
