[0001] This application has an Attachment A
[0002] The present invention is directed on a method for matching at least two acoustical
to electrical converters which generate, respectively, electrical output signals.
Signals which depend on the electrical output signals of the converters are computed
to result in a result signal. The transfer characteristic between an acoustical signal
impinging upon the at least two converters and the result signal is dependent on direction
of arrival - DOA - of the acoustical signal upon the at least two converters.
[0003] Acoustical pickup arrangements which have a transfer characteristic between acoustical
input and electrical output, the amplification thereof being dependent on the DOA
of acoustical signals on the acoustical inputs of such devices are called "beamformers"
and are widely used as e.g. for hearing devices, be it outside-the-ear hearing devices
or in-the-ear hearing devices, be it for such hearing devices to improve and facilitate
normal hearing or be it for such hearing devices for therapeutic appliances, i.e.
to improve hearing capability of hearing impaired persons. Further, beamformers may
also be applied for hearing protection devices, whereat the main target is to protect
an individual from excessive acoustical loads.
[0004] The addressed transfer characteristic, called the "beam" characteristic when represented
in polar coordinates, is of one or more than one lobe and has accordingly one or more
minima, called "Nulls", at specific values of DOA.
[0005] Beamformers may be conceived just by acoustical to electrical converters which per
se have a beamforming characteristic.
[0006] The present invention deals with other cases where at least two spaced apart acoustical
to electrical converters are used, signals dependent on their electrical output signals
being computed to generate a result signal. It is by such computing that the desired
beam characteristic is generated, between the acoustical input signals and the result
signal. Often the at least two converters have omni-directional characteristics and
it is only by the addressed computing that beamforming is achieved. Nevertheless,
converters which have intrinsic beamforming ability may also be used but the desired
transfer characteristic is conceived finally by the addressed computing.
[0007] Whenever a beam characteristic is realized by computing the electrical output signals
of at least two acoustical to electrical converters or from more than two of such
converters, whether a desired beam characteristic is accurately achieved depends from
how accurately the involved converters provide for assumed predetermined transfer
characteristics between their acoustical inputs and their electrical outputs.
Definition:
[0008]
- Two or more than two acoustical to electrical converters as microphones are considered
to be matched if their real transfer characteristics between acoustical input signals
and their electrical output signals is equal to such transfer characteristics as assumed
when tailoring a desired beam characteristic.
- Two or more than two of such converters are considered to be substantially matched
if due to adjustment of at least one of their electrical output signals it is achieved
that their respective real transfer characteristics are less different from the assumed
transfer characteristic than they are without such adjustment, i.e. given just by
the intrinsic behavior of the converters.
- We understand under "matching" two or more than two acoustical to electrical converters,
the process of mutually adjusting at least one characteristic feature of the transfer
characteristic of at least one converter and so that the resulting real transfer characteristics
of the at least two converters with the mutually adjusted electric output signals
become less different from the assumed characteristics than they are without such
adjustment. Characteristic features to be adjusted may e.g. be frequency response,
thereby gain response and/or phase response. Thus, by the action of converter matching
the converters become substantially matched, and not necessarily matched
[0009] Often, the desired beam characteristic is designed based on the assumption of identical
transfer characteristics of the converters involved. Obviously, in such case the converters
are made to be matched if the real transfer characteristics between acoustical input
signals and respective possibly mutually adjusted electrical output signals are identical.
[0010] In this case too the process of matching the converters means mutually adjusting
their electrical output signals so that the respective real transfer characteristics
differ less than without such mutual adjusting and become, due to the mutual adjustment,
in the ideal case, identical.
[0011] As a most common example - known as "delay and subtract" technique - beamforming
is performed using at least two e.g. omni-directional converters which are mutually
spaced by a predetermined distance, mutually delaying the output signals of the converters
and subtracting the mutually delayed electrical signals which results in an overall
beam characteristic which, with omni-directional converters, is of cardoid, hypercardoid,
bidirectional or some other shape. Directivity of the resulting beam characteristic
depends on one hand from the mutual distance of the converters, on the other hand
from the possibly adjustable, thereby often automatically adjustable mutual delay,
and from the accuracy with which the converters are matched.
[0012] If the two addressed converters are not matched the desired transfer characteristic
will only be reached approximately.
[0013] Attempts have been made to match the at least two converters by mutual converter
specimen selection or by mutually adjusting their electrical output signals, be it
statically or dynamically, i.e. during operation of the beamformer.
[0014] Recently, dynamic matching is the preferred approach which allows accounting for
time-varying transfer characteristics.
[0015] According to the DE-OS-19 822 021, which accords with the US-A 6 385 323, the electrical
output signals of two microphone converters are fed via controlled matching amplifier
units to a computing unit. The output signal of the computing unit has, with respect
to acoustical input signals, a beam characteristic. The output signal powers resp.
magnitudes of the matching amplifier units are averaged and the averaged signals compared
by difference forming. The comparing result signal is fed to an analyzing and controller
unit which controls the matching amplifier units. Thereby, the matching is performed
in a negative feedback structure up to the comparing result of the two averaged signals
vanishes. If this occurs the two input converters are considered to have been matched.
[0016] From the DE-OS-19 849 739 a similar approach as was discussed in context with the
DE-OS 19 822 021 is known but in a feed-forwards structure. Significant characteristics
as e.g. amplitude response or phase response of the analogue to digital converted
output signals of two input converter microphones are compared and the output signal
of one of the microphones is adjusted with respect to said characteristics as a function
of the comparing result. It is further taught that whenever the two microphones have
intrinsic beam characteristics directed in opposite directions, acoustical signals
impinging laterally should lead to identical microphone output signals. Any deviation
is then attributed to microphone mismatch and an appropriate adjustment is performed
on the electric output signal of one of the microphones. Such ideal acoustical situation
as only exploitable in free-field acoustical surrounding is apparently exploited for
finding an appropriate optimum of pre-matching.
[0017] According to the WO 01/69968 the output signals of two microphones are computed.
A result signal establishes with respect to the acoustical input signals a beamforming
transfer characteristic. Each of the electrical output signals of the microphones
is fed to a respective minimum estimation unit, the outputs thereof to a division
unit. The result of the division controls a matching unit, namely a multiplying unit.
It is recognized that because the microphones are often matched in free-field acoustical
surrounding and not in-situ, the microphones can be mismatched when used in real life
which degrades directionality. Matching is performed when the output signals of the
microphones are minimal which is assigned to a "only noise" acoustical situation.
This reference addresses multi-frequency band adaptive matching scheme.
[0018] From the EP 1 191 817 it is known to maintain a prevailing optimum directional transfer
characteristic over time by forming a difference of averaged signals of the analog
to digital converted microphone output signals and by feedback adjusting one of the
digitalized microphone output signals to reduce the difference of the averaged signals.
[0019] In the US 6 272 229 mismatch of the microphone converters with respect to phase is
also discussed. It is taught to provide acoustical delay compensation at two microphone
output signals, thereby trying to compensate for time delays between acoustical signals
impinging on the two microphones. A remaining time delay - after acoustical delay
compensation - between the two output signals is assigned to microphone phase mismatch.
[0020] The US 2001/0038699 teaches to disable the directivity of the transfer characteristic,
i.e. the beam characteristic of a two-microphone-based beamformer whenever "only noise"
situation is recognized, thereby disabling one of the two microphones to reduce overall
noise and maintaining only one microphone operative.
[0021] According to the DE-PS 19 918 883 which accords with the US 6 421 448 matching of
two microphones is established with respect to frequency response by adjusting a filter
arrangement between one of the microphone electrical outputs and a computing unit.
[0022] The present invention departs from the following recognitions:
[0023] Whenever a beamforming device or beamformer, which is based on at least two acoustical
to electrical input converters, signals dependent on the output signals of these converters
being computed, e.g. by delay-and-subtract operation, is applied in non-free field
acoustical surrounding, such non-free field surrounding presents per se acoustical
signal attenuation which varies as a function of spatial angle at which the acoustical
source is seen from the acoustical input of the device. Such non-free field acoustical
transfer characteristic, called "in-situ" characteristic, which varies with DOA is
often important to be maintained as an informative entity. Generically, whenever according
to known microphone matching approaches e.g. as described in the documents cited above,
adjustment of the output signals of the converters is performed, this would lead -
in the in-situ situation - to compensation of the in-situ transfer characteristic
if fast time constants for the matching procedure were employed. Prior art literature
like e.g. also US 6 385 323 or US 5 515 445 consider only aging, temperature, influence
of dirt etc. as influencing factors for microphone matching though, i.e. they apply
matching time constants in the range of minutes to days.
Definition
[0024] By matching time constant we understand the adaptation time constant to adapt the
converters involved from one matching situation to another matching situation.
[0025] In hearing device appliances the head-related transfer function HRTF provides for
an acoustic in-situ transfer characteristic between an acoustical source and the at
least two converters, which differs from individual to individual and which varies
significantly with varying DOA. If a sound source is thought to travel on a circular
locus around an individual's head, the in-situ transfer characteristic between the
acoustical source and individual's ear may vary by more than 10 dB as a function of
DOA. The individual exploits such DOA dependency for localizing acoustical sources.
Thus, such characteristic should not be spoiled by converter matching.
[0026] Prior art microphone matching algorithms employing long matching time constants to
guard against aging, dirt influences etc. will not be able to provide sufficient dynamic
matching in dependency of DOA without negatively influencing also HRTF related localization
by the user of the hearing device.
[0027] It is one object of the present invention to provide for a matching technique for
the at least two acoustical to electrical converters which maintains the effect of
acoustical, surrounding-based transfer characteristics - in-situ transfer characteristics
- to the converters.
[0028] This is achieved by the method for matching at least two acoustical to electrical
converters, wherein signals respectively dependent on electrical output signals of
the converters are computed to result in a result signal, the transfer characteristic
between an acoustical signal impinging upon said at least two converters and said
result signal being dependent on DOA of said acoustical signal upon the at least two
converters. The method comprises matching the at least two converters for acoustical
signals in dependency of an impinging direction of arrival within a range of direction
of arrival upon said converters, said range being determined before performing said
matching.
[0029] Thereby, the range of DOA of acoustical signals for which matching is performed is
selected so that the in-situ transfer characteristic is known and in advance, as an
example, is known to be neglectable. Techniques to evaluate the DOA of acoustical
signals impinging on at least two acoustical to electrical converters of a beamforming
device are known.
[0030] With respect to evaluation of the DOA we refer as an example to the WO 00/33634 which
accords with the US patent application no. 10/180 585 of the same applicant as the
present application. With respect to one possibility to monitor DOA the said WO 00/33634
as well as its US counterpart shall form by reference an integral part of the present
application.
[0031] DOA evaluation is also strongly linked to time delay estimation for which numerous
methods like cross-correlation, MUSIC, etc. are well known in the art. M. Brandstein
"Microphone arrays", Springer, ISBN 3-540-41953-5 gives a nice overview over such
methods. US 20010031053 shows another method for DOA estimation which is leaned on
processes found in nature.
[0032] It has further been recognized that a range of DOA which is most suited to be exploited
according to the present invention is where the desired transfer characteristic has
minimum gain, i.e. around a "Null". This because signals impinging from the respective
direction shall - according to the desired "Null" - be cancelled. Therefore, a realization
form of the method according to the present invention, whereat the transfer characteristic
has a minimum for a value of DOA, comprises matching the at least two converters for
acoustical signals which impinge within the range determined before matching which
includes such value of DOA.
[0033] Beamformers are further known which make use of at least two acoustical/electrical
converters, signals dependent from their output signals being computed by a first
computing and at least a second computing. The at least two computings result in respective
first and second result signals. Thereby, a first transfer characteristic between
an acoustical signal impinging on the at least two converters and the first result
signal and which is dependent on DOA is differently dependent on DOA than a second
transfer characteristic between the acoustical input signal and the second result
signal. Such beamforming devices are e.g. realized by the so-called Griffith Jim-based
beamformers as exemplified e.g. in the US 5 473 701 to AT&T.
[0034] According to an embodiment of the present invention in such a case matching is performed
independently for the addressed first and at least one second computing, for acoustical
signals which respectively impinge from ranges of DOA determined before matching upon
the at least two converters. These ranges may be selected to be equal or to be different.
[0035] In an embodiment of the present invention matching is performed selectively in frequency
bands determined before matching, whereby in a further embodiment of the invention
analog to digital and time-domain to frequency-domain conversion is performed between
the electrical output of the at least two converters and computing.
[0036] Attention is drawn to the enclosed
Attachment A which is a yet unpublished European patent application with application No. 04 006
073.3 filed March 15, 2004 and which accords to a US application filed same date with
a yet unknown Serial Number. This unpublished and therefore annexed patent application
is to be considered as a part of the present description by reference with respect
to the following subject matter:
[0037] In the Attachment A a method for suppressing feedback between an acoustical output
of an electrical/acoustical output converter arrangement and an acoustical input of
an acoustical/electrical input converter arrangement of a hearing device is addressed.
Thereby, acoustical signals impinging on an input converter arrangement are converted
into a first electrical signal by a controllably variable transfer characteristic
which is dependent on the angle (DOA) at which the acoustical signals impinge on the
input converter arrangement. The first electrical signal is then processed and a signal
resulting from such processing is applied to the output converter.
[0038] Thus, and with an eye on the present description the following may be established:
[0039] The acoustical/electrical input converter arrangement as addressed in the
Attachment A, wherein acoustical signals impinging on the input converter arrangement are converted
into a first electrical signal by a controllably variable transfer characteristic
which is dependent on the angle at which the acoustical signals impinge on the input
converter arrangement, accords in the present description to the at least two acoustical
to electrical converters, computing and generating the result signal.
[0040] When applying the device according to the present invention to hearing devices as
addressed above, the result signal is operationally connected via a processing unit
to an electrical/acoustical output converter arrangement. Further, the teaching according
to the
Attachment A addresses a method for suppressing feedback between the output of such electrical/acoustical
output converter arrangement and the input of the at least two converters as addressed
in the present description.
[0041] According to the present application as was already addressed the at least two input
converters are to be matched during operation, i.e. automatically, whereby in fact
the real transfer characteristic is adjusted. This accords with the definition in
Attachment A of an adaptive beamformer unit.
[0042] If according to one embodiment of the present invention the result signal is operationally
connected to an output electrical/acoustical converter as of a hearing device and
there is provided, as described in the
Attachment A in details, a feedback compensator, the input of which being operationally connected
to the input of the output converter arrangement, the output of which being fed back,
the complex task of estimating the feedback signal to be suppressed by the feedback
compensator e.g. by correlation leads to the fact that the feedback compensation process
has a relatively long adaptation time constant to adapt from one feedback situation
to be suppressed to another by appropriately varying the loop gain of the feedback
loop. As described in the
Attachment A such an adaptation time constant is customarily in the range of hundreds of msec.
[0043] The matching process which is addressed in the present application defines as well
for an adaptation time constant of the adaptive beamformer. The adaptation time constant
for "matching adaptation" is significantly shorter than the adaptation time constant
as realized by the feedback compensator. Therefore, and if according to one aspect
of the present invention a feedback compensator is provided as explained in detail
in the addressed
Attachment A, the same problems arise as also explained in the addressed
Attachment A, namely the problem that the feedback compensator may not follow quick changes of
feedback situations which are caused by the short adaptation time constants of matching
adaptation. Thus, and according to one aspect of the present invention, this is resolved
by that embodiment of the present invention, wherein the addressed result signal is
operationally connected to an electric input of an electrical to acoustical converter
and which comprises feeding back an electric feedback compensating signal which is
dependent on an input signal to the electrical to acoustical converter and superimposing
the fed-back signal to the result signal, wherein further the adaptation rate of matching
according to the present invention is controlled in dependency of the loop gain along
the feedback signal path.
[0044] The skilled artisan will recognize also from the
Attachment A or the respective applications once published, how to realize the just addressed
embodiment of the invention.
[0045] As was addressed above prior art matching is accomplished with matching time constants
τ which are very long, namely in the range of minutes up to days. Thereby, such matching
may not cope with converter matching needs which arise at short term.
[0046] This is remedied by the present invention under a second aspect by providing for
a method for matching at least two acoustical to electrical converters, signals dependent
on the electrical output signals of the converters being computed to result in a result
signal and wherein the transfer characteristic between an acoustical signal impinging
upon the at least two converters and the result signal is dependent on direction of
arrival of the acoustical signal on the at least two converters, wherein matching
of the converters is performed with a matching time constant τ, for which there is
valid:

[0047] Thereby, in a further embodiment there is established

[0048] And in a still further embodiment

[0049] A beamforming device according to the present invention comprises at least two acoustical
to electrical converters and at least one computing unit, the electrical output of
the converters being operationally connected via a matching unit to inputs of the
at least one computing unit. Thereby, the output of the beamforming device is operationally
connected to the output of the at least one computing unit. The computing unit further
generates a signal which is indicative of DOA of an acoustical signal which impinges
on the at least two converters. The device further comprises a matching control unit
which generates a matching control signal which is operationally connected to a control
input of the matching unit. The signal which is indicative of DOA is further operationally
connected to a control input of the matching control unit, which further has at least
two inputs which are operationally connected to respective outputs of the at least
two converters, in feedback structure downstream the matching unit, in feed-forwards
structure upstream the matching unit.
[0050] Under a second aspect of the present invention there is provided a beamforming device
comprising at least two acoustical to electrical converters and at least one computing
unit, the electrical output of said converter being operationally connected via a
matching unit to inputs of said at least one computing unit, the output of said beamforming
device being operationally connected to the output of said at least one computing
unit, a matching control unit generating a matching control signal operationally connected
to a control input of the matching unit, said matching unit comprising at least two
inputs operationally connected to the outputs of said at least two converters upstream
or downstream said matching unit and wherein said matching control unit generates
the matching control signal so as to match the at least two converters with a matching
time constant τ for which there is valid:

[0051] In a further embodiment under this second aspect the matching time constant τ is:

[0052] In a still further embodiment there is valid:

[0053] It is further to be noted that when we speak of a value or of a frequency band which
is determined before matching is performed, the meaning of "before" encompasses a
long time span before, e.g. when a respective device is fitted or even is manufactured
up to a very short time span when such a value or frequency band is determined dynamically
in situ just before the respective matching is performed.
[0054] Preferred embodiments of the present invention shall now be exemplified with the
help of figures. These as well as the appending claims will also reveal to the skilled
artisan additional embodiments of the device according to the invention.
[0055] The figures show:
- Fig. 1
- schematically and simplified, by means of a signal-flow/functional block diagram,
an embodiment of the device according to the present invention performing the method
according to the invention;
- Fig. 2
- a schematic representation of steps as performed by the method and device according
to fig. 1;
- Fig. 3
- in a representation in analogy to that of fig. 1, the implementation of the device
of fig. 1, e.g. in a hearing device as an embodiment of the invention with feedback
compensation, and
- Fig. 4
- a further embodiment of a device according to the present invention operating according
to the method of the present invention, again in a representation in analogy to that
of fig. 1.
[0056] According to fig. 1 a number of acoustical to electrical converters, as shown two
such converters 1a and 1b, have electrical outputs A
1a, A
1b which are operationally connected to inputs E
3a and E
3b of a matching unit 3. As shown in dashed lines within matching unit 3 signals which
are applied to the inputs E
3a and E
3b are adjusted with respect to at least one of their characteristics, e.g. with respect
to frequency response, amplitude and/or phase response or other characteristic features.
[0057] Respective adjusting members are provided in unit 3, e.g. as shown in channel a or
b or in both channels a and b. The outputs A
3a and A
3b are operationally connected to inputs E
7a and E
7b of a computing unit 7 which has an output A
7 and an output A
DOA.
[0058] Within computing unit 7 on one hand and as schematically shown by unit 7
BF beamforming is computed from the signals applied to the inputs E
7a, E
7b e.g. by delay-and-subtract computing. The result of beamforming is fed to output
A
7 as a result signal of the beamforming operation.
[0059] Additionally, in computing unit 7 the direction of arrival DOA of acoustical signals
impinging upon the converters 1a and 1b is computed from the signals applied to E
7a, E
7b resulting in an output signal fed to output A
DOA of computing unit 7 which is indicative of DOA of the addressed acoustical signals.
In unit 7
DOA performing monitoring of the DOA is e.g. realized as described in the WO 00/33634
which was already mentioned above or as taught by the following publications:
M. Brandstein "Microphone arrays", Springer, ISBN 3-540-41953 or US 2001003053.
[0060] At the output A
DOA of computing unit 7 there is generated a signal which is indicative of the direction
of arrival DOA. This signal is operationally connected to a comparator unit 9, where
it is checked, whether the instantaneously evaluated DOA signal is within a range
±ΔDOA around a value DOA
S. Determination, whether the actual DOA signal is within this range DOA
S±ΔDOA is performed by comparing the DOA indicative signal from the output A
DOA with a signal range which is preset at input E
9C of unit 9. Whenever it is detected in unit 9 that the prevailing DOA signal is within
the predetermined range, unit 9 generates at an output A
9 a control signal which is operationally connected to a control input E
11C of a matching control unit 11. The matching control unit 11 has two further inputs
E
11a and E
11b which are operationally connected to the electric output A
1a and A
1b of the respective converters. The signals applied to the input E
11a and E
11b are compared as shown in block 11 e.g. by difference forming and an output signal
is generated at output A
11 of matching control unit 11, which is dependent on the result of such comparison.
As further schematically shown within unit 11 the signal applied to control input
E
11c enables the comparison result dependent signal to become effective via output A
11 on adjustment control input E
3c of matching unit 3, controlling the adjustant members provided in matching unit 3.
Thereby, as a function of the comparing result in matching control unit 11, the at
least two signals which are fed to the computing unit 7 at E
7a and E
7b are adjusted to become less different.
[0061] Whereas fig. 1 shows a feed forwards structure the same technique may be realized
in a feed-back structure (not shown) by connecting the inputs E
11a and E
11b not to the outputs of the converters 1a and 1b upstream unit 3, but instead to the
outputs A
3a and A
3b downstream matching unit 3.
[0062] In fig. 2 processing as performed with the device and method exemplified with the
help of fig. 1 shall further be explained. Representation (a) shows as an example
the transfer characteristic in polar representation of an omnidirectional converter
as of converter 1a of fig. 1. Representation (b) shows such transfer characteristic
again as an example of the second converter as of 1b of fig. 1. Based on these two
converter-intrinsic omnidirectional transfer characteristics, beamforming within computing
unit 7 leads e.g. to the cardoid transfer characteristic as shown in representation
(c) which is e.g. realized by the delay-and-subtract method.
[0063] Within computing unit 7 and as shown in fig. 1 by block 7
DOA, the instantaneously prevailing DOA is estimated as shown in representation (d) to
be α. The range, which is determined before performing matching, DOA
S±ΔDOA as also shown in representation (d) is exemplified with DOA
S = 0, at which a "Null" of the desired transfer characteristic as of representation
(c) is expected. Only then when the estimated DOA according to α is within the range
DOA
S±ΔDOA, with an eye on fig. 1, matching of the converters is initiated by means of
the signal generated at the input E
11C. Techniques which are applicable for mutually adjusting the signals in matching unit
3 are well-known as has been shown by the referenced publications in the introductory
part of the present description. Accordingly, the matching control unit 11 is realized
to provide for the desired dependency between the comparison result of comparing the
signals applied to the inputs E
11a and E
11b and adjustment of the respective adjusting members in unit 3.
[0064] Instead of enabling/disabling, practically in a hard switching manner, matching of
the converters via matching control unit 11 it is possible to softly weigh the effect
of the comparing result computed in matching control unit 11 upon the adjusting members
in matching 3 e.g. as a function of deviation between estimated DOA and DOA
S as determined before performing matching. Such weighing may e.g. be realized so that
such effect becomes the weaker resp. the matching frozen the more that the estimated
DOA deviates from DOA
S.
[0065] In fig. 3 a further embodiment of a device according to the present invention operating
according to the method of the invention is shown. The same reference numbers are
used in fig. 3 as in fig. 1 for elements which have already been described in context
with fig. 1.
[0066] The unit comprising the converters 1a, 1b, matching unit 3, computing unit 7, matching
control unit 11, provides for an adaptive beamformer unit 20
A, whereby being adapted by adjusting the overall transfer function by converter matching.
[0067] The output A
7 of the adaptive beamformer 20
A is operationally connected to a superimposing unit 20
AP.
[0068] Attention is drawn to the convention with respect to the reference numbers applied
in fig. 3. The same reference numbers are used as used in the
Attachment A, fig. 3, which latter teaches in more details the technique as also applied in the
embodiment of fig. 3 of the present invention. Nevertheless, these linking reference
numbers are indexed with "AP" (for Appendix).
[0069] The output of the superimposing unit 20
AP is input to processing unit 14
AP, the output thereof being operationally connected to the input of an electrical to
acoustical converter arrangement 16
AP. Thereby, the combined structure of beamformer 20A, processing unit 14
AP and electrical to acoustical converter arrangement 16
AP is a structure typical e.g. in hearing device applications.
[0070] A compensator unit 18
AP has an input operationally connected to the input of the converter arrangement 16
AP and an output operationally connected to one input of the superimposing unit 20
AP. The negative feedback loop with compensator unit 18
AP provides for compensation of acoustical feedback from the acoustical output of converter
arrangement 16
AP to the acoustical input of the converters 1a, 1b.
[0071] As schematically shown in fig. 3 the compensator unit 18
AP has an output A
GAP, whereat a signal is generated which is indicative of the loop gain of the negative
feedback loop. This loop gain may e.g. be estimated by multiplying the linear gains
along the loop which primarily consists of the compensator unit 18 and of processing
unit 14
AP or by adding these gains in dB.
[0072] The loop gain indicative signal at output A
GAP is fed to a control input C
12RAP of the adaptive beamformer 20
A and therein to a control input of matching control unit 11. By means of the loop
gain indicative signal applied to this control input, the matching adaptation rate
at matching unit 3 and via matching control unit 11 is slowed down at least down to
the adaptation rate of compensator unit 18
AP in dependency of the prevailing feedback effect and thus of the loop gain of compensator
unit 18
AP. Thereby, combination of the beamformer unit 20A with automatically matched converters
1a and 1b according to the present invention and of feedback compensation becomes
feasible.
[0073] In fig. 4 a further embodiment of the present invention is shown. Again, reference
numbers which were already used in context with fig. 1 or 3 are used for elements
which have already been described. According to the embodiment of fig. 4 the outputs
A
1a and A
1b of the at least two converters 1a and 1b are operationally connected to a first matching
unit 3
I and to a second matching unit 3
II.
[0074] The outputs of the two matching units 3
I and 3
II are operationally connected to respective computing units 7
I and 7
II. At the output A
7I there appears a first result signal. Between an acoustical input signal impinging
on the converters 1a and 1b and the first result signal at output A
7I there prevails a first transfer characteristic which is differently dependent on
DOA than a second transfer characteristic which prevails between the acoustical input
signal upon converters 1a and 1b and a signal generated at output A
7II of the second computing unit 7
II.
[0075] Thus, in fact based on the converters 1a and 1b two beamformers are realized with
different beam characteristics. Matching is performed independently at both beamformers
as follows:
[0076] Matching of the converters with respect to first beamformer I is performed via unit
9
I, matching control unit 11
I in analogy to the one beamformer technique of fig. 1. Further in complete analogy
matching of the converters 1a and 1b with respect to the second beamformer II is performed
via unit 9
II, matching control unit 11
II. As may be seen in fig. 4 in opposition to the representation in fig. 1 a feedback
structure is shown in that the outputs of the respective matching units 3
I and 3
II are fed for comparison purposes to the matching control units 11
I and 11
II.
[0077] In all the embodiments of the invention signal processing may be performed in analog
or digital or hybrid technique. Converter matching selectively in frequency bands
which are determined before performing matching is simplified by signal processing
in the frequency domain.
[0078] Due to the fact that according to the one aspect of the present invention converter
matching is only then performed when an acoustical signal impinges on the input converters
within a range of DOA and this range may be selected in an optimum direction with
an eye on in-situ situation, it is achieved that automatic in-situ converter matching
is feasible without affecting the effects of the in-situ acoustic situation.
[0079] As was already addressed above generically matching time constants for direction
of arrival controlled matching as was described with the help of figures 1 to 4 may
be performed with a matching time constant τ for which there is valid:

[0080] Thereby, such time constant τ may be even selected to be:

or even to be

[0081] Nevertheless and irrespectively of controlling converter matching in dependency of
direction of arrival, more generically, a beamformer technique is addressed under
a second aspect which makes use of at least two acoustical to electrical converters
and where converter matching is performed with matching time constants τ for which
the addressed ranges are valid.
Attachment A
Feedback suppression
[0082] The present invention deals with a method for suppressing feedback between an acoustical
output of an electrical/acoustical output converter arrangement and an acoustical
input of a acoustical/electrical input converter arrangement of a hearing device,
wherein acoustical signals impinging on the input converter arrangement are converted
into a first electrical signal, by a controllably variable transfer characteristic
and which is dependent on the angle at which said acoustical signals impinge on the
input converter arrangement. The first electrical signal is processed and a resulting
signal is applied to the output converter. There is further provided an electrical
feedback-compensating signal, generated in dependency of the result signal which is
applied via a feedback signal path upstream the processing.
Definition
[0083] A unit to which the output of the input converter arrangement is input and which
provides a signal transfer characteristic to its output which has an amplification
dependent on spatial angle at which acoustical signals impinge on the acoustic input
of the input converter arrangement is called a beamformer unit. The transfer characteristic
in polar representation is called the beam.
[0084] An adaptive beamformer unit is a beamformer unit, the beam generated therefrom being
controllably variable.
[0085] From the EP 0 656 737 there is known such a method which nevertheless does not apply
beamforming. The input of a feedback-compensator is operationally connected to the
input of the output converter arrangement of the device, the output of the compensator
is operationally connected to the output of the input converter arrangement, thereby
forming a feedback signal path.
[0086] Due to the complex task of estimating the feedback-signal to be suppressed e.g. by
correlation at the feedback-compensator, the feedback-compensation process has a relatively
long adaptation time constant to adapt from one feedback situation to be suppressed
to another by appropriately varying its gain. Such an adaptation time constant is
customarily in the range of hundreds of milliseconds.
[0087] Feedback signals to be suppressed impinge upon the input acoustical/electrical converter
arrangement substantially from distinct spatial angles. As schematically shown in
Fig. 1, a behind-the-ear hearing device 3 with an input converter arrangement 5 applied
at the pinna 1 of an individual, experiences feedback to be suppressed from a distinct
direction as shown at d1. An in-the-ear hearing device 7 according to Fig. 2 which
has, as an example, a vent 9 and two acoustical ports 11 to the input converter arrangement,
experiences feedback signals to be suppressed from the distinct directions d2.
[0088] Therefore, a further approach for suppressing feedback is to install high signal
attenuation between the input and the output converter of the device for signals which
impinge on the input converter under such distinct spatial angles. This accords with
applying a beamformer technique generating a beam having zero or minimum amplification
at such angles.
[0089] Hearing devices which have adaptive beamformer ability are known e.g. from the WO
00/33634. For feedback suppression at a hearing device with adaptive beamforming ability,
it seems, at first, quite straight forward to combine on the one hand feedback compensation
techniques as e.g. known from the EP 0 656 737 with adaptive beamformer technique
as e.g. known from the WO 00/33634 and thereby to place minimum amplification of the
beam at those angles which are specific for feedback signals to be suppressed impinging
on the input converter. This especially because these angles are clearly different
from the target direction range within which maximum amplification of the beam is
to be variably set.
[0090] Thereby, it has to be noted that the adaptation time constant of an adaptive beamformer
unit is considerably smaller, in the range of single to few dozen milliseconds, than
the adaption time constant of a feedback-compensator which is, as mentioned above,
in the range of hundreds of milliseconds.
[0091] One approach is known where a beamformer unit is provided, the input thereof being
operationally connected to two mutually distant microphones of an input converter
arrangement. As both spaced apart microphones experience the feedback signal to be
suppressed differently, two feedback compensators are provided with inputs operationally
connected to the input of the output converter arrangement. The respective output
signals are superimposed to the respective output signals of the two microphones.
[0092] The fact that the adaptation time constant of the beamformer unit is much shorter
than the adaptation time constant of the compensators does not pose a problem in this
configuration, because the fast adapting beamformer unit is placed within the closed
feedback loop formed by the feedback-compensation feedback paths.
[0093] Nevertheless, this known approach has the serious drawback that for each of the microphones
one compensator feedback path must be provided which unacceptably raises computational
load.
[0094] A further approach for beamformer/feedback-compensation combination is known from
M. Brandenstein et al. "Microphone arrays", Springer Verlag 2001. Here the feedback
compensation path is fed back to the output of the beamformer unit. By this approach
only one compensation path is necessary and thus computational load is reduced. Nevertheless,
here the fast adapting beamformer is outside the negative feedback loop. Thus, whenever
the adaptive beamformer is controlled to rapidly change its beam pattern, the compensator
will not be able to adequately rapidly deal with the new situation of feedback to
be suppressed.
[0095] Therefore, M. Brandenstein et al. "Microphone arrays" considers this approach as,
at least, very difficult to realise.
[0096] A third approach is proposed in M. Brandenstein et al. as mentioned and in W. Herbold
et al. "Computationally efficient frequency domain combination of acoustic echo cancellation
and robust adaptive beamforming". A generalised side lobe cancelling technique for
the beamformer is used whereat only a not-adaptive beamformer is placed upstream the
compensation feedback path, thus eliminating the adaptation time problem as well as
double computational load. Nevertheless, by this approach placing minimum amplification
of the beam in the direction of feedback signal arrival may not be realised.
[0097] It is an object of the present invention to provide a method for suppressing feedback
as addressed above at a hearing device which has an adaptive beamformer on the one
hand, and a feedback compensator on the other hand, thereby avoiding the drawbacks
as addressed above.
[0098] This is achieved on the one hand by superimposing the fed back feedback compensating
signal to the signal downstream the beamforming, and, on the other hand, by controlling
the adaptation rate of beamforming in dependency of the gain along feedback signal
path with the compensator.
[0099] Thus, there is proposed a method for suppressing feedback between an acoustical output
of an electrical/acoustical output converter arrangement and an acoustical input of
an acoustical/electrical input converter arrangement of a hearing device, wherein
acoustical signals impinging on the input converter arrangement are converted into
a first electric signal by a controllably variable transfer characteristic which is
dependent on the angle at which said acoustical signals impinge on said input converter
arrangement. The first electric signal is processed and a resulting signal is applied
to the output converter arrangement. The feedback to be suppressed is compensated
by a feedback compensating signal which is generated in dependency of the resulting
signal and is fed back by a feedback signal path to a location along the signal path
upstream the processing. Thereby, the feedback-compensating signal is fed back to
the first electric signal - thus downstream the beamformer - and the adaptation rate
of converting to variations of the transfer characteristic - and thus of beamforming
- is controlled in dependency of gain along the compensator feedback signal path.
Definition
[0100] We understand by the adaptation rate of the adaptive beamformer unit the speed with
which the beamformer unit reacts on an adaptation command to change beamforming operation
as e.g. changing target enhancement or noise suppression direction. The adaptation
rate accords with an adaptation time constant to change from one beamforming polar
pattern to another.
[0101] We understand by the adaptation rate of feedback-compensating the rate with which
the respective compensator reacts on a detected change of feedback situation until
the compensator has settled to a new setting. The compensator thereby estimates the
prevailing situation of feedback to be suppressed e.g. by a correlation technique
between the signal applied to the output converter arrangement and the signal received
from the input converter arrangement as e.g. described in the EP 0 656 737. The adaptation
rate of the compensator accords with an adaptation time constant too. Whenever the
loop gain along the compensating feedback signal path increases, this is caused by
an increasing amount of feedback to be suppressed and thus to be compensated. This
means that the adaptation rate of the beamformer unit is to be slowed down so that
the compensator feedback signal may model the response of the beamformer unit too.
Thus, in a preferred embodiment, the adaptation rate of converting i.e. of beamforming
is slowed down with increasing loop gain along the feedback signal path.
[0102] As was addressed above, feedback signals, which are acoustical and which have to
be suppressed, impinge on the acoustical input of the input converter arrangement
substantially and dependent on the specific device at specific angles. Thus, in a
most preferred embodiment of the method according to the present invention, amplification
of the transfer characteristic representing beamforming is minimized at one or more
than one specific angles which accord to angles at which the feedback to be suppressed
predominantly impinges on the input converter arrangement.
[0103] Thus, and considered in combination with slowing down the adaptation rate of beamforming
with increasing gain along feedback compensation fed back signal path, it becomes
apparent that the compensator may still model the beamformer without losing the established
minimum or minima in the direction of the said specific angles.
[0104] Further, it has to be noted that the feedback to be suppressed is a narrow band acoustical
signal, thus in a further improvement of the method according to the present invention,
it is not necessary - so as to deal with a feedback to be suppressed - to control
and especially to slow down the adaptation rate of beamforming conversion in the entire
frequency range beamforming is effective at, but it suffices to controllably adapt
the adaptation rate of the beamforming conversion at frequencies which are significant
for the feedback signal to be suppressed. Therefore, in a further preferred embodiment
of the present invention, controlling of the adaptation rate of the beamforming conversion
is performed frequency selectively.
[0105] In spite of the fact that the principal according to the present invention may be
applied at hearing devices where signal processing is performed in analog technique,
it is preferred to perform the method in devices where signal processing is performed
digitally. Thereby, and in view of the addressed preferred frequency selective control,
in a most preferred embodiment, at least signal processing in the beamforming conversion
as well as along the feedback compensation path, is performed in frequency domain,
whereby time domain to frequency domain conversion may be realised in a known manner,
be it by FFT, DCT, wavelet transform or other suitable transforms. The respective
reconversion for the signal applied to the output converter arrangement is performed
with the respective inverse processes. The adaptation rate is controlled at selected
frequencies in dependency of the compensator gain at these selected frequencies. Thereby
the following approach is achieved:
[0106] As beamforming is only effective with respect to the feedback to be suppressed at
specific frequencies or at a specific frequency band on the one hand the control of
the adaptation rate of beamforming is in fact only to be performed at these specific
frequencies or for the addressed frequency band. Further, selecting minimum amplification
at the specific feedback impingement angles must be provided at the beamformer only
for the specific frequencies or for the frequency band of the feedback to be suppressed
too. Thus, this leads to the recognition that in fact beamforming may be subdivided
in beamforming for frequencies which are not significant for the feedback to be suppressed
and beamforming for frequencies or the frequency band which is specific for the feedback
signal to be suppressed. Thus, beamforming in the addressed specific frequencies may
be performed and its adaptation rate controlled independently from tailoring beamforming
at frequencies which are not specific for the feedback signal to be suppressed. This
beamforming may be performed at adaption rates which are independent from feedback
compensation and thus faster and which generates a beam which is not dealing with
the specific impinging angles of the feedback signal to be suppressed.
[0107] Therefore, in a further preferred embodiment of the method according to the present
invention, performing controlling of beamforming is done selectively at frequencies
which are significant for the feedback to be suppressed. Further preferred minimalising
the amplification of the beamforming transfer characteristic is only done at specific
angles in a frequency selection manner. In fact two independent beamforming actions
are superimposed, a first dealing with the generically desired beamforming behaviour,
a second dealing with feedback suppression as concerns frequencies and as concerns
beamshaping. It becomes possible e.g. to switch off first beamforming, thereby maintaining
the second and thereby preventing acoustical feedback to become effective. The method
according to the present invention may be applied to behind-the-ear hearing devices
or to in-the-ear hearing devices, monaural or binaural systems, and further may be
applied to such devices which are conceived as ear protection devices i.e. protecting
the human ear from excess acoustical load, or to hearing improvement devices be it
just to improve or facilitate hearing by an individual, or in the sense of a hearing
aid, to improve hearing of a hearing impaired individual.
[0108] It is to be noted that feedback caused not by acoustical but by electrical or mechanical
reasons is often fed into the microphones of the input converter arrangement with
equal gains and phases, thus appearing to originate from a direction perpendicular
to the port axis of the input converter arrangement. In an endfire array, as typically
used in hearing instruments, this conforms to a 90° direction or arrival, and may
be suppressed by a beamformer arrangement according to the present invention as well.
[0109] To resolve the object as mentioned above, there is further, and according to the
present invention, provided a hearing device which comprises:
- an acoustical/electrical input converter arrangement and a adaptive beamformer unit
generating at an output an electric output signal dependent on acoustical signals
impinging on said input converter arrangement and in dependency of angle at which
said acoustical signals impinge, said beamformer unit having a first control input
for varying beamforming characteristics and a second control input for controllably
adjusting adaptation rate;
- a processing unit with an input operationally connected to the output of said beamformer
unit with an output operationally connected to an input of an electrical/acoustical
output converter arrangement;
- a feedback compensator unit, the input thereof being operationally connected to said
input of said electrical/acoustical output converter arrangement, the output thereof
being operationally connected to the input of said processing unit and having a loop
gain output, said loop gain output being operationally connected to said second control
input of said beamformer unit.
[0110] Preferred embodiments of the method according to the present invention, as well as
of a hearing device according to the present invention, shall additionally become
apparent from the following detailed description of preferred embodiments with the
help of further figures and from the claims. The figures show:
- Figs. 1 & 2:
- as discussed above, schematically specific angles at which feedback signals impinge
on the acoustical input port of outside-the-ear (Fig. 1) and in-the-ear (Fig. 2) hearing
devices.
- Fig. 3:
- by means of a simplified functional block/signal flow-diagram, a device according
to the present invention operated according to the method of the present invention.
- Fig. 4:
- in polar diagram representation preferred beamforming at the device according to Fig.
3 taking into account specific angles with which the feedback to be suppressed impinges
on the acoustic input as exemplified in the Figs. 1 or 2.
- Fig. 5a:
- as an example and quantitatively, beamforming by the device of Fig. 3 at specific
frequencies which are significantly present in the feedback signal to be suppressed.
- Fig. 5b:
- beamforming at the device of Fig. 3 for frequencies which are not significantly present
in the feedback signal to be suppressed.
[0111] In Fig. 3 there is schematically shown, by means of a signal flow-/functional block-diagram
a device according to the present invention, whereat the method according to the invention
is realised. The device comprises an input acoustical/electrical converter arrangement
10, which cooperate with a beamformer unit 12. The conversion characteristics of the
input converter arrangement 10 together with signal processing in beamformer unit
12 provides a beamformer characteristic between acoustical input E
10 to input converter arrangement 10 and electrical output A
12 of the beamformer unit 12. The beamformer unit 12 has an adaptation control input
C
12A and α adaptation rate control input C
12R.
[0112] The transfer characteristic between E
10 and A
12 has an amplification which is dependent on the angle α at which acoustical signals
impinge on the acoustical port of input converter 10. Thus, there is generated by
the combined units 10 and 12 a beam characteristic as exemplified with B in unit 12.
[0113] As further schematically shown by the variation arrow V within block 12, the transfer
characteristic, in polar representation the beam B, may be varied with respect to
its characteristics as e.g. with respect to target direction, maximum amplification
etc. as shown in dotted line within block 12. Variation of the beam characteristic
B is controlled by control input C
12A which latter is, as shown in dotted line, normally connected to a processing unit
14 for adapting the beam characteristic B e.g. to prevailing acoustical situations
automatically or program controlled or by an individual wearing the hearing device.
[0114] Beamforming units which may be adapted are known. One example thereof is described
in the WO 00/33634.
[0115] Variation of the beam characteristic B may also be caused at the beamformer itself,
i.e. by beamformer internal reasons.
[0116] Therefore, it must be emphasised that the input C
12A and control signals applied thereto are merely a schematic representation of beam
characteristic variation ability or occurrence.
[0117] The electrical output of beamforming unit 12, A
12, is operationally connected to an input E
14, of the signal processing 14 unit whereat input signals are processed and output
at an output A
14 operationally connected to an electric input E
16 of an output electrical to acoustical converter arrangement 16 so as to provide desired
ear protections or hearing improvement to the individual carrying such device. We
understand under ear protecting ability the ability of reducing or even cancelling
acoustical signals which impinge on the input converter arrangement 10, so as to protect
individual's hearing or even provide the individual with silent perception in non-vanishing
acoustical surroundings. Under hearing improvement, we understand the improvement
of individual's hearing in an acoustical surrounding, be it for customary applications
of normal hearing individual or be it in the sense of hearing aid to improve individual's
impaired hearing.
[0118] As perfectly known to the skilled artisan, one ongoing problem in context with such
hearing devices is the acoustical feedback AFB between the acoustical output of the
output converter 16 and acoustical input E
10 of the input converter arrangement 10. As principally known e.g. from the EP 0 656
737, there is provided a feedback compensator 18 whereat the prevailed acoustical
feedback AFB, which is to be suppressed, is estimated e.g. with a correlation technique,
correlating the signal applied to output converter 16 with a signal dependent on the
output of input converter 10 as shown in dashed line at A. Thereby the gain G of compensator
18 is estimated so a to compensate for the AFB by negative feedback.
[0119] By means of compensator unit 18, a signal as predicted is fed back to the input of
processor unit 14 downstream the output of beamformer unit 12 so as to compensate
for the feedback AFB. As shown in Fig. 3, the compensator unit 18 has an input E18
which is operationally connected to the output A
14 of the processing unit 14 and has an output A
18 which is superimposed to the output E
12 of beamformer unit 12, the result of such superimposing being input to input E
14 of processing unit 14.
[0120] Customarily, the compensator unit 18, which computes estimation of the acoustical
feedback to be suppressed, has an adaptation rate in the range of several hundred
ms and is thus considerably slower than the adaptation rate of beamforer unit 12.
Thus without additional measures according to the present invention, whenever the
beamformer unit 12 is controlled or caused to vary its beamforming characteristic
B as schematically represented by a control at input C
12A, the compensator 18 will not be able to accurately rapidly deal with the varied situation
with respect to acoustical feedback AFB.
[0121] Therefore, there is provided a control of the adaptation rate of beamformer unit
12 which control is performed by the compensator unit 18, according to Fig. 3 at control
input C
12R. Whenever the feedback signal loop gain via compensator 18 rises, indicating the
increase in acoustical feedback AFB to be suppressed, the adaptation rate or time
constant of beamformer unit 12 is lowered to or below the adaptation rate of compensator
unit 18.
[0122] The loop gain may at be least estimated e.g. by multiplying the linear gains along
the loop, primarily consisting of the compensator 18 and the processing unit 14 in
Fig. 3 or by adding these gains in dB.
[0123] Thereby, it is prevented that an adjustment of the beamformer unit 12 with respect
to its beamforming characteristic B may not be dealt with by compensator unit 18.
[0124] Thus, in fact, adaptation rate control of beamformer unit 12 is performed in dependency
of the loop gain along the feedback loop with compensator unit 18. The rate control
input C
12R to beamforming unit 12 is operationally connected to a loop gain output A
G of unit 18. With the embodiment according to the present invention as shown in Fig.
3, it becomes possible to slow down the adaptation rate of the beamformer unit 12
at least down to the adaptation rate of the feedback compensator unit 18 in dependency
of prevailing feedback of compensator 18.
[0125] Thereby, combination of adaptive beamforming and feedback compensating becomes feasible.
[0126] As has already been mentioned, the direction with which acoustical feedback signals
AFB to be suppressed impinge on the acoustical port of the input converter 10 is specific.
Therefore, at the beamformer unit 12, there is generated a beam characteristic B
AFB, as shown in Fig. 4, which has minimum amplification for these specific angle or,
as shown e.g. for an in-the-ear hearing device, at two specific angles α
AFB. Thus and in addition to compensation of AFB by compensator unit 18, beamforming
is realised with minimum amplification for those spatial angles α
AFB with which the acoustical feedback AFB to be suppressed impinges on the input converter
10.
[0127] Further, it has to be noticed that acoustical feedback AFB to be suppressed occurs
substantially within a specific frequency band. This frequency band is dependent,
among others, on the specific output converter 16 used, the type of device e.g. in-the-ear
or outside-the-ear device. Therefore, in a further improved embodiment, overall feedback
suppression may be performed within that specific frequency band, thereby leaving
beamforming in frequencies not within this specific frequency band unaffected and
tailored according to needs different from acoustic feedback suppression. According
to Fig. 5 (a), beamforming B

for minimum amplification of acoustical feedback AFB to be suppressed, is performed
frequency selectively for frequencies f

of the acoustical feedback signal AFB.
[0128] Beamforming for frequencies f
AFB which are not significantly present in the acoustical feedback AFB is performed by
a second beamforming B

which may be selected independently from B
AFB.
[0129] In fact, two independent beam forms are superimposed each operating in respective,
distinct frequency-bands. Frequency selective feedback compensation and adaptation
beamforming may easily be realised, if at least beamforming in unit 12 as well as
compensation in unit 18 are performed in frequency domain respectively in sub-bands.
Beamforming is then realised at the frequencies f
AFB with minimum amplification at the specific angles α
AFB, whereas beamforming at other frequencies f

is performed according to other needs. Consequently the adaptation rate of beamforming
in unit 12 is only controlled by the gain of compensator unit 18 at the frequencies
f
AFB.
[0130] Thus, even when beamforming B

is switched off to minimum overall amplification, beamforming B
AFB may be maintained active to suppress feedback also in such "quiet" mode. Thereby,
and with an eye on processing in frequency domain, in each sub-band, which is significant
for AFB, the loop gain, as estimated in compensator unit 18, may be compared with
a threshold value and adaptation rate control at C
12R is only established, if the instantaneous loop gain at least reaches such threshold.
The control of the adaptation rate may then be lowered to practically zero, which
means that beamforming is switched off for frequencies F
AFB. This establishes a hard on/off-switching of beamforming in the F
AFB frequency-range. In a further approach, such switching may be performed steadily
which may be realised on the one hand by lowering the adaptation rate of B
AFB steadily and/or by reducing beamforming amplification of B
AFB steadily.
[0131] Due to the inventively improved suppression of acoustical feedback from the output
of the output converter to the input of the input converter, there is reached additional
stability of the device. The inter dependencies of vent tailoring at in-the-ear hearing
devices and acoustical feedback problems is resolved to a significantly higher degree
than was possible up to now when the device had the ability of adaptive beamforming.
1. A method for suppressing feedback between an acoustical output of an electrical/acoustical
output converter arrangement and an acoustical input of an acoustical/electrical input
converter arrangement of a hearing device, wherein
- acoustical signals impinging on the input converter arrangement are converted into
a first electric signal by a controllably variable transfer characteristic which is
dependent on the angle at which said at acoustical signals impinge on said input converter
arrangement;
- said first electric signal is processed and a resulting signal is applied to the output
converter arrangement;
- said feedback to be suppressed is compensated by a feedback compensating signal which
is generated in dependency of the resulting signal and is fed back by a feedback signal
path upstream said processing;
wherein further
- said electric feedback compensating signal is fed back to and superimposed upon the
first electric signal and
- adaptation rate of said converting to variations of said transfer characteristic is
controlled in dependency of the loop gain along said feedback signal path.
2. The method of claim 1, further comprising slowing down the adaptation rate of said
converting with increasing loop gain along said feedback signal path.
3. The method of claims 1 or 2, further comprising minimising amplification of said
transfer characteristic at one or more specific angles which accord to angles at which
said feedback to be suppressed predominantly impinges on said input converter arrangement.
4. The method of one of claims 1 to 3, further comprising frequency selectively controlling
said adaptation rate.
5. The method of one of claims 1 to 4, further comprising performing said converting
in said first electric signal, and said processing along said feedback signal path
in frequency domain and controlling said adaptation rate at selected frequencies in
dependency of said loop gain at said selected frequencies.
6. The method of one of claims 1 to 5, further comprising minimizing amplification
of said transfer characteristic at specific angles frequency selectively.
7. The method of one of claims 1 to 6, further comprising performing said converting
into said first electric signal independently for frequencies present in said feedback
to be suppressed and for frequencies substantially not present in said feedback to
be suppressed.
8. The method of one of claims 1 to 7, further comprising performing said control
of said adaptation rate selectively for frequencies present in said feedback to be
suppressed, said control comprising switching said converting on and off for said
frequencies present.
9. The method of claim 8, further comprising performing switching from on to off and/or
vice versa steadily during a predetermined timespan.
10. The method of one of claims 1 to 9, said hearing device being a behind-the-ear
or a in-the-ear hearing device.
11. The method of one of claims 1 to 10, said hearing device being a ear protection
or a hearing improvement device.
12. A hearing device, comprising:
- an acoustical/electrical input converter arrangement and an adaptive beamformer unit,
generating at an output an electric output signal dependent on acoustical signals
impinging on said input converter arrangement and in dependency of angle at which
said acoustical signals impinge, said beamformer unit having a first control input
for varying beamforming characteristics
- a processing unit with an input operationally connected to the output of said beamformer
unit and with an output operationally connected to an input of an electrical/acoustical
output converter arrangement
- a feedback compensator unit, the input thereof being operationally connected to said
input of said electrical/acoustical output converter arrangement, an output thereof
being operationally connected to the input of said processing unit
and wherein further
- said beamformer unit has a second control input for adjusting adaptation rate,
- said output of said feedback compensator unit is operationally superimposed with the
output of said beamformer unit,
- said feedback compensator unit has an output for a loop gain indicative signal, being
operationally connected to said second control input of said beamformer unit.
13. The device of claim 12 being a behind-the-ear hearing device or an in-the-ear
hearing device.
14. The device of one of claims 12 or 13, being a hearing protection device or a hearing
improvement device.

