[0001] The present invention is generically directed on a technique according to which acoustical
signals are received by at least two acoustical/electrical transducers as e.g. by
multidirectional microphones, respective output signals of such transducers are electronically
computed so as to generate a result signal which represents said acoustical signals
weighted by a spatial characteristic of amplification as a function of spatial angle
under which the acoustical signal impinges on the two transducers provided. Thus,
the result signal represents the received acoustical signal weighted by a spatial
amplification characteristic as if reception of the acoustical signals had been done
by means of e.g. an antenna with an according reception lobe or beam.
[0002] Figure 1 most generically shows such known technique for such "beam forming" on acoustical
signals. Thereby, at least two multidirectional acoustical/electrical transducers
2
a and 2
b are provided, which both convert acoustical signal irrespective of their impinging
direction θ and thus substantially unweighted with respect to impinging direction
θ into electrical output signals A
1 and A
2. The output signals A
1 and A
2 are fed to an electronic signal processor unit 3 which generates from the input signals
A
1, A
2 a result signal A
r. As shown within the block of unit 3 in that unit 3 the signals A
1,
2 are treated to result in the result signal A
r which represents either of A
1 or A
2, but additionally weighted by the spatial amplification function F(θ). Thus, acoustic
signals may selectively be amplified dependent from the fact under which spatial angle
θ they impinge, i.e. under which spatial angle the transducer arrangement 2a, 2b "sees"
an acoustical source.
[0003] One approach to perform signal processing within processor unit 3 shall be exemplified
with the help of Fig. 2. Thereby, all such approaches are based on the fact that due
to a predetermined mutual distance p
P of the two transducers 2
a and 2
b, there occurs a time-lag dt between reception of an acoustical signal at the transducers
2
a, 2
b.
[0004] Considering a single frequency - ω - acoustical signal, received by the transducer
2
a, this transducer will generate an output signal

whereas the second transducer 2
b will generate an output signal according to

whereat dt is given by

therein, c is the sound velocity.
[0005] By time-delaying A
1 by an amount

and forming the result signal A
r from the difference of time-delayed signal A
1', namely from

there results, considered at the frequency ω, a spatially cardoid weighted signal
A
r as shown in the block of processing unit 3:

[0006] At θ = 90° A
r becomes zero and
at θ = -90° A
r becomes

[0007] Such processing of the output signals of two omnidirectional order transducers leads
to a first order cardoid weighing function F
1(θ) as shown in Fig. 3. By respectively selecting transducers with higher order acoustical
to electrical conversion characteristic and/or by using more than two transducers,
higher order - m - weighing functions F
m(θ) may be realised.
[0008] In Fig. 4 there is shown the amplitude A
rmax-characteristic, resulting from first order cardoid weighing as a function of frequency

. Additionally, the respective function for a second order cardoid weighing function
F
2(θ) is shown. Thereby, there is selected a distance p
P of the two transducers 2
a and 2
b of fig. 1 to be 12 mm.
[0009] As may clearly be seen at a frequency f
r which is

maximum amplification occurs of +6 dB at the first order cardoid and of +12 dB at
a second order cardoid. For p
P = 12 mm, f
r is about 7 kHz.
[0010] From fig. 4 a significant roll-off for low and high frequencies with respect to f
r is recognised, i.e. a significant decrease of amplification.
[0011] 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 microphone - 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 Japan, Part 3, Vol. 77, No. 5, 1994, page 68 to 75.
[0012] Irrespective of the prior art techniques used for such beam forming with at least
two transducers, the distance p
P is an important entity as may be seen e.g. from formula (8).
[0013] Formula (8) may be of no special handicap if such technique is used for narrow band
signal detection or if no serious limits are encountered for geometrically providing
the at least two transducers at a large mutual distance p
P.
[0014] Nevertheless, and especially for hearing aid applications, the fact that f
r is inversely proportional to the distance p
P of the transducers is a serious drawback, due to the fact that for hearing aid applications
the audio frequency band up 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 p
P.
[0015] It is thus a first object of the present invention to remedy the drawbacks encountered
with respect to p
P dependency of acoustical "beam forming".
[0016] The first object of the present invention is reached by providing a method for electronically
changing the distance between a first and a second acoustical/electrical transducer,
generating, respectively, first and second electrical output signals which represent
substantially simultaneously impinging acoustical signals, whereby the first and second
electrical output signals are electronically treated so as to generate a result signal
which result signal is a function of acoustical signals at least substantially simultaneously
received at the two transducers and amplified by a spatial reception characteristic
of amplification as a function of spatial angle under which the acoustical signals
impinge on the two transducers, which method further comprises the steps of generating
a third signal from at least one of the first and second output signals by shifting
the phase of at least one of said output signals by an amount according to phase difference
of the first and second output signals - being dependent from the physical distance
p
P of said two transducers - multiplied by a constant factor not equal to unity or by
a factor which is a function of frequency, and generating the result signal from at
least one of the first and second output signals and the third signal, which latter
represents an acoustical signal as it would be received if a transducer was placed
at a distance from one of the provided transducers, which is given by the predetermined
factor.
[0017] Thereby, there is introduced a virtual distance p
V of transducers which becomes clearly preferable, as for hearing aid applications,

[0018] Thereby, according to formula (8), f
r is shifted to lower frequencies.
[0019] Thereby, it becomes possible to realise f
r 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.
[0020] Multiplying the phase difference by a constant factor does nevertheless not affect
the roll-off according to fig. 4. This roll-off is significantly improved, leading
to an enlarged frequency band B
r according to fig. 4 if the predetermined function of frequency is selected as a function
which is at least in a first approximation inversely proportional to the frequency
of the acoustic signal.
[0021] For instance for the first order cardoid according to fig. 3 and fig. 4, there may
be reached a flat frequency characteristic between 0,5 and 4 kHz and thus a significantly
enlarged frequency band B
r 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.
[0022] To fulfil the above mentioned object there is further provided a hearing aid apparatus
which comprises at least two acoustical/electrical transducers spaced from each other
by a predetermined distance, whereby the at least two transducers generate, respectively,
first and second electrical output signals and wherein the outputs of the acoustical/electrical
transducers are operationally connected to a signal processing unit which generates
a result output signal from the first and second output signals of the transducers,
which result output signal being a function of acoustical signals received at least
substantially simultaneously at the two transducers and amplified by a spatial reception
characteristic of amplification as a function of spatial angle under which the acoustical
signals impinge on the two transducers and which further comprises
- a phase difference detection unit operationally connected to the outputs of the two
transducers and generating a phase difference indicative signal; and wherein
- the output of the phase difference detecting unit is operationally connected to the
first input of a multiplier unit, the second input thereof being connected to the
output of a function or constant generator unit, the output of the multiplication
unit being operationally connected to the control input of a phase shifter unit with
a signal input operationally connected to the output of one of the transducers, the
output of the phase shifter unit and at least one of the outputs of the at least two
transducers being operationally connected to the processing unit.
[0023] Other objects of this invention will become apparent as the description proceeds
in connection with the accompanying drawings, of which show:
- Fig. 1:
- A functional block diagram of a two-transducer acoustic 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;
- Fig. 3:
- a two-dimensional representation of a three-dimensional cardoid beam, i.e. amplification
characteristic as a function of incident angle of acoustical 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 according to fig. 2, still prior art;
- Fig. 6:
- a pointer diagram based on fig. 5 (prior art), but according to the inventive method,
which is performed by an inventive apparatus;
- Fig. 7:
- a simplified block diagram of an inventive apparatus, especially of an inventive hearing
aid apparatus, wherein the inventive method is implemented.
[0024] As was mentioned above, in the figs. 1 to 4 known beam forming techniques based on
at least two acoustical/electrical transducers spaced from each other by a predetermined
distance p
P have been explained.
[0025] In fig. 5 there is shown a pointer diagram according to (6).
[0026] The basic idea of the present invention shall be explained now with the help of the
stil simplified one - ω - frequency example. The inventively realised pointer diagram
is shown in fig. 6. The phase difference ω · dt between signal A
2 and A
1 according to fig. 6 is

[0027] This phase difference is determined and is multiplied by a value dependent from frequency,
thus with the respective value of a function M(ω), which may be also a constant M
o # 1.
[0028] By phase shifting one of the two signals A
1, A
2 according to the respective pointers in fig. 6, e.g. of A
2 by

there results the phase shifted pointer A
2V. This pointer would have also occurred if dt had been larger by an amount according
to M(ω) or M
0, thus if a "virtual transducer" had been placed distant from transducer 1
a by the virtual distance p
V, for which:

[0029] As we consider one single frequency for simplicity we may write

.
[0030] With virtual τ
V

we get according to the present invention:

[0031] With (8) we further get:

[0032] Therefrom, we may see that for a given p
P, which would lead to a too high f
r, f
rV is reduced by the factor

, taken M
ω > 1. In fig. 7 there is schematically shown a preferred realisation form of an inventive
apparatus in a simplified manner, especially for implementing the inventive method
into an inventive hearing aid apparatus. Thereby, the output signals of the acoustical/electrical
transducer 2
a and 2
b are fed to respective 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
detects phase difference Δϕ
n for all n spectral frequency components which are then multiplied by a set of constants
c
n. If M(ω) is valid, then the c
n can be different for different frequencies, and represent a frequency dependent function
or factor. If on the other hand the phase differences Δϕ
n are multiplied by the same

this accords with using a constant M
0.
[0033] This multiplication according to (3
V) is done at a spectral multiplication unit 28. Signal A
1 in its spectral representation is then spectrally phase shifted at a spectral phase
shifter unit 29 by the multiplied spectral phase difference signals output by multiplier
unit 28.
[0034] According to fig 7 the signal A
1 in its spectral representation and inventively, spectrally phase shifted is computed
in a spectral computing unit 23 together with A
2 in its spectral representation, as if transducer 2a was distant from transducer 2b
by a distance
. The resulting spectrum is transformed back by a frequency to time domain converter
- FTC - as by an Inverse-Fast-Fourier-Transform unit 24 to result in A
r#.
[0035] 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.
[0036] Nevertheless the time delaying technique is preferred.
[0037] With an eye on fig. 4 it has been explained that by inventively introducing a "virtual"
transducer with a virtually enlarged distance from one of the two physically provided
transducers, it becomes possible to shift the high gain frequency f
r towards lower frequencies, which is highly advantageous especially for hearing aid
applications. This is already reached if instead of a frequency dependent function
M(ω), a constant M
0 is multiplied with the phase difference as explained.
[0038] In a preferred mode of the invention the frequency dependent function M(ω) is selected
to be, at least in a first approximation,

[0039] Thereby, it is reached that different from fig. 4 there will be no roll-off and the
gain in target direction will be constant over the desired frequency range. By appropriately
selecting the function M(ω) it is e.g. possible to reach a flat characteristic within
a predetermined frequency range, e.g. between 0.5 and 4 kHz with defined roll-offs
at lower and higher frequencies. With appropriately selecting the function M(ω) practically
any kind of beam forming can be made.
[0040] For generating higher order cardoid weighing functions it is absolutely possible
to additionally use the not phase-shifted output signal A
1 - as shown in fig. 7 by dotted line - as computing input signal to unit 23 too, thus
"simulating" three transducers.
[0041] It is evident for the skilled artisan that
- more than two real transducers may be used and/or
- more than one M(ω) function may be used to produce more than one virtual transducer
signal from one or from more than one real transducer signals respectively.
[0042] With selecting the number of physical and virtual transducers the spatial weighing
function may be selectively tailored, thereby e.g. to completely suppress signals
from unwanted directions, e.g. only to pass signals from within a narrow range of
target direction and suppress all others. On the other hand, it is also possible to
just suppress unwanted sources, thus realising an in fact omnidirectional behaviour,
thereby just suppressing one or more distinct noise sources at predetermined spatial
angles.
[0043] The present invention under its principal object makes it possible to realise beam
forming with at least two transducers separated by only a predetermined small distance,
due to the fact that electronically there is provided a virtual transducer distance
from one of the two physically provided transducers.
[0044] Thereby, roll-off may be significantly reduced by such virtual transducer, which
is established with a distance dependent from frequency. 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.
1. A method for electronically and virtually changing the effective distance between
a first and second acoustical/electrical transducer, generating respectively first
and second electrical output signals, representing substantially simultaneously impinging
acoustical signals, said first and second electrical output signals being electronically
treated so as to generate a result signal, said result signal being a function of
acoustical signals at least substantially simultaneously received at said two transducers
and amplified by a spatial reception characteristic of amplification as a function
of spatial angle under which said acoustical signal impinge on said transducers and
further comprising the steps of
- generating a third signal from at least one of said first and second output signals
by phase-shifting said at least one of said first and second output signals by an
amount given by the phase difference between said two output signals, said phase difference
being dependent from the physical distance of said two transducers, multiplied by
a constant factor not equal to unity or by a factor which is a function of frequency,
- generating the result signal from at least one of said first and second output signals
and said third signal, which latter representing an acoustical signal as it would
be received if a transducer was placed at a distance from one of the provided transducers,
which distance being given by said constant or said predetermined factor.
2. The method of claim 1, wherein said first and second output signals are transformed
in their respective frequency spectra and wherein said third signal is formed as a
frequency spectrum, wherein each spectral component at one of said output signals
and at a frequency considered is shifted in phase according to phase difference of
the spectral components of said first and second output signals at said frequency
considered, said phase difference being multiplied by equal or frequency-specific
coefficients.
3. The method of claim 1, wherein said frequency dependent function is selected as inversely
proportional to frequency at least in a first approximation.
4. The method of claim 2, wherein said frequency-specific coefficients are selected inversely
proportional to the frequency considered.
5. The method of claim 1, further comprising the step of selecting the mutual physical
distance of said first and second transducers to be at most 20 mm and selecting said
distance given by said factor to be larger than said physical distance and incorporating
said first and second transducers into a hearing aid apparatus.
6. A hearing aid apparatus comprising at least two acoustical/electrical transducers
spaced from each other by a predetermined distance, whereby the at least two transducers
generate, respectively, first and second electrical output signals and wherein the
outputs of said acoustical/electrical transducers are operationally connected to a
signal processing unit which generates a result output signal from said first and
second output signals of said transducers, which result output signal being a function
of acoustical signals received at least substantially simultaneously at said at least
two transducers and amplified by a spatial reception characteristic of amplification
as a function of spatial angle under which the acoustical signals impinge on said
at least two transducers, and which further comprises:
- a phase difference detection unit operationally connected to the outputs of said
two transducers and generating a phase difference indicative signal and wherein
- the output of said phase difference detection unit is operationally connected to
the first input of a multiplier unit, the second input thereof being operationally
connected to the output of a function generator or constant-generator unit, the output
of said multiplication unit being operationally connected to the control input of
a phase shifter unit having a signal input operationally connected to the output of
one of said transducers, the output of the phase shifter unit and at least one of
the outputs of said at least two transducers being operationally connected to said
signal processing unit.
7. The apparatus of claim 6, wherein the outputs of said transducers are operationally
connected to respective analogue to digital converters, the outputs of said converters
being fed to respective transform units generating output signals representing the
input signals to said transform unit in the frequency domain, and further providing
said phase difference detection unit, said multiplier unit, phase shifter unit as
operating in the frequency domain as well as said signal processing unit and further
retransforming the output signal of said processing unit into time domain by means
of a frequency to time domain conversion unit.
8. The apparatus of claim 6, wherein said function generator generates said function
at least in a first approximation inversely proportional to frequency of said acoustic
signals impinging on said at least two transducers.