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Claims  |
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What is claimed is:
1. A method to control acoustic feedback in a communications system
operating in an acoustic environment and having a microphone component for
inputting audio information into the system, an amplifier component for
amplifying audio frequency signals inputted to the microphone, and a
speaker component for outputting audio frequency signals into said
environment, said method comprising:
(a) identifying those parameters associated with acoustic feedback,
including identifying time-variations in these parameters;
(b) adjusting the transfer function of said amplifier component in
accordance with the identified parameter for cancelling the effects of
said acoustic feedback in response to identification of the parameters
associated therewith without attenuating any audio frequencies.
2. A method according to claim 1 including the step of configuring the
communication system in an identification mode before parameter
identification is carried out, and configuring the system to operate in an
operational mode after parameter identification is carried out, the
cancelling of the effects of said acoustic feedback being done while the
system is configured in its operational mode.
3. A method according to claim 2 including the step of monitoring the
condition of the communication system for effecting a change in
configuration of the system from its operational to its identification
mode when a predetermined condition exists.
4. A method according to claim 2 wherein the step of cancelling the effects
of said acoustic feedback includes establishing the transfer function of a
correction circuit in accordance with the identified parameters, and
coupling said correction circuit to said communication system when it is
configured in its operational mode.
5. A method to control acoustic feedback in a communication system
operating in an acoustic environment and having a microphone component for
inputting audio information to the system, an amplifier component for
amplifying audio frequency signals inputted to the microphone, and a
speaker component for outputting said audio frequency signals into said
environment, said method comprising:
(a) identifying those parameters associated with acoustic feedback,
including identifying time-variations in these parameters;
(b) adjusting the transfer function of said amplifier component in
accordance with the identified parameters for cancelling the affects of
said acoustic feedback in response to identification of parameters
associated therewith;
(c) configuring the communication system to operate in an identification
mode before parameter identification is carried out;
(d) configuring the communication system to operate in an operational mode
after parameter identification is carried out, the cancelling of the
effects of said acoustic feedback being done when the system is configured
in its operational mode; and
(e) wherein the communication system is configured in its identification
mode by disconnecting said speaker from said amplifier means, and
parameter identification is achieved by injecting a noise signal into said
speaker, and cross-correlating the output of said microphone with said
noise signal while said speaker is disconnected from said amplifier means.
6. A method according to claim 5 including the step of maintaining the
configuration of the system in its identification mode for a predetermined
period of time after the configuration is changed from its operational to
its identification mode.
7. A method according to claim 6 wherein said predetermined condition is
system turn-on, or a threshold change in the gain of the amplifier
component.
8. A method according to claim 7 wherein the identification of said
parameters is carried out independently of audio information inputted into
the microphone.
9. A method according to claim 7 wherein the communication system is
configured in its identification mode by disconnecting said speaker from
said amplifier means, and parameter identification is achieved by
injecting a noise signal into said speaker, and cross-correlating the
output of said microphone with said noise signal while said speaker is
disconnected from said amplifier means.
10. A method according to claim 5 wherein said noise signal is
pseudo-random.
11. Apparatus according to claim 5 wherein the communication system is
reconfigured by terminating injection of noise into said speaker, and
reconnecting said speaker to said amplifier means.
12. A method according to claim 5 wherein said noise signal is provided
from a pre-recording stored in memory.
13. A method according to claim 5 wherein the step of configuring the
communication system in an identification mode before parameter
identification is carried out, and after parameter identification is
carried out, the cancelling of the effects of said acoustic feedback being
done while the system is configured in its operational mode, and including
the step of cancelling the effects of said acoustic feedback and
establishing the transfer function of a correction circuit in accordance
with the identified parameters, and coupling said correction circuit to
said communication system when it is configured in its operational mode.
14. A method according to claim 13 wherein said noise is pseudo-random.
15. In an audio system operating in an acoustic environment and having a
microphone for inputting audio information into the system, amplifier
means for amplifying audio frequency signals inputted to the microphone,
and a speaker for outputting amplified audio frequency signals into said
environment which provides acoustic feedback between said speaker and said
microphone, the improvement comprising:
(a) an identification circuit for dynamically identifying those parameters
associated with said acoustic feedback;
(b) correction circuit means having a transfer function set in response to
parameters identified by said identification means for cancelling the
effect of said acoustic feedback; and
(c) wherein said identification circuit includes an identifier circuit
having two inputs for cross-correlating the signals at said inputs, a
noise generator for generating a noise signal that is applied to one of
said inputs, switch means having a first state that connects said speaker
to said amplifier means and disconnects said noise generator from said
speaker, and having a second state that disconnects said speaker from said
amplifier means and connects said noise generator to said speaker, and
means for applying the output of said microphone to the other of said
inputs of said identification circuit while said switch means is in its
second state.
16. The invention of claim 15 including switch control means for
controlling the state of said switch means.
17. The invention of claim 16 including gain sensor means operatively
associated with said amplifier means for sensing changes in the gain of
said amplifier means; and wherein said switch control means is responsive
to said gain sensor means for maintaining said switch means in its first
state until the occurrence of either turn-on of the system, or a
predetermined change in the gain of said amplifier means, and then
changing the switch means to its second state.
18. The invention of claim 17 wherein said switch control means is
constructed and arranged to maintain the switch means in its second state
for predetermined period of time before effecting a change of the switch
means to its first state.
19. The invention of claim 17 wherein said amplifier means includes
manually adjustable means for controlling the gain of said amplifier
means, and said gain sensor means is responsive to the adjustment of said
manually adjustable means for controlling the state of said switch means.
20. The invention of claim 17 wherein said amplifier means is a
preamplifier stage, and the gain sensor means is responsive to the voltage
across the preamplifier stage.
21. The invention of claim 20 wherein said gain sensor means includes a
divider-limiter circuit.
22. The invention of claim 21 wherein said divider limiter includes circuit
for dividing the voltage appearing across the input and output of said
preamplifier stage.
23. The invention of claim 15 wherein said identification circuit is
constructed and arranged to produce discrete time parameters associated
with said accoustic feedback.
24. The invention of claim 15 wherein said identification circuit is
constructed and arranged to produce continuous time parameters associated
with said accoustic feedback.
25. The invention of either of claims 23 or 24 wherein said identification
circuit is constructed and arranged to produce only pole parameters.
26. The invention of claim 25 wherein said identification circuit is
constructed and arranged to produce autoregressive parameters.
27. The invention of claim 16 including an amplitude sensor responsive to
the output of said amplifier means for sensing a rise in amplitude of such
output above a predetermined value, and wherein said switch-control means
is constructed and arranged to maintain said switch means in its first
state until occurrence of either a condition of turn-on of the system, or
a condition of a rise in amplitude of the output of said amplifier above
said predetermined value, and to change said switch means to its second
state in response to occurrence of either condition, said switch-control
means being further constructed and arranged to maintain said said switch
means in its second state for a predetermined period of time and to
thereafter change the state of said switch means to its first state.
28. The invention of claim 27 wherein said sensor is responsive to a rise
in amplitude of only those frequency components in the output of said
amplifier which have a frequency exceeding a predetermined value.
29. In an accoustic environment, apparatus comprising:
(a) a microphone component for responding to audio signals in said
accoustic environment which are received by the microphone to produce an
audio output signal;
(b) an amplifier component for producing an amplified output signal in
response to an audio input signal;
(c) a speaker component for producing an audio signal broadcast into the
acoustic environment in response to an input signal;
(d) acoustic feedback identification means for selectively identifying
those parameters associated with accoustic feedback between the speaker
and the microphone;
(e) an adaptive correction circuit component coupled to said amplifier
component and having a transfer function established by the parameters
identified by said accoustic feedback identification means; and
(f) configuration means for interconnecting the components such that said
adaptive correction circuit component is effective to cancel said acoustic
feedback without attenuating audio frequencies feedback.
30. Apparatus according to claim 29 wherein said configuration means is
selectively operable for alternately connecting the components into an
identification configuration that includes said acoustic feedback
identification means, or into an operation configuration that excludes
said acoustic feedback identification means.
31. Apparatus according to claim 30 wherein said configuration means
includes two-state switch means, and switch control means for establishing
and maintaining the state of said switch means, one state of said switch
means interconnecting said microphone component to the speaker and
amplifier components and disconnecting the acoustic feedback
identification means from the components thereby establishing the
operation configuration of the system.
32. The apparatus of claim 31 wherein said apparatus is a part of a public
address system.
33. In an acoustic environment, apparatus comprising:
(a) a microphone component for producing an audio-output signal and
responsive to audio signals in said acoustic environment which are
received by the microphone;
(b) an amplifier component for producing an amplified output signal in
response to an audio input signal;
(c) a speaker component for producing an audio signal broadcast into the
acoustic environment in response to an input signal;
(d) acoustic feedback identification means for selectively identifying
those parameters associated with acoustic feedback between the speaker and
the microphone;
(e) an adaptive correction circuit component coupled to said amplifier
component and having a transfer function established by the parameters
identified by said acoustic feedback identification means;
(f) configuration means for interconnecting the components such that said
adaptive correction circuit component is effective to cancel said acoustic
feedback;
(g) said configuration means being selectively operable for alternately
connecting the components into an identification configuration that
includes said acoustic feedback identification means, or into an operation
configuration that excludes said acoustic feedback identification means;
(h) said configuration means including two-state switch means, and switch
control means for establishing and maintaining the state of said switch
means, one state of said switch means interconnecting said microphone
component to the speaker and amplifier components and disconnecting the
acoustic feedback identification means from the components thereby
establishing the operation configuration of the system;
(i) wherein said acoustic feedback identification means includes: (1) an
identifier circuit having first and second inputs for cross-correlating
signals at said inputs and producing an output signal; and (2) a noise
generator connected to said first of the inputs of said identifier
circuit; (3) the other state of said switch means connecting the speaker
component to said first of the inputs to said identifier circuit for
effecting injection of noise from said noise generator into said
identifier circuit and into said speaker, and connecting the microphone
component to said second of the inputs to said identifier circuit for
applying to the latter signals received by the microphone component due,
in part, to the acoustic coupling between said speaker and said
microphone.
34. Apparatus according to claim 33 including a hearing aid housing
containing said apparatus.
35. The invention of claim 33 wherein said switch control means is
responsive to a threshold change in the gain of said amplifier component
for switching said switch means from its first to its second state.
36. The apparatus of claim 35 wherein said switch control means is
responsive to a turn-on condition of said amplifier component for
switching said switch means from its first to its second state for a
predetermined period of time.
37. Apparatus according to either of claims 35 or 36 wherein said noise
generator generates pseudo-random noise.
38. Apparatus according to claim 37 wherein said switch control means is
constructed and arranged to maintain said electronic switch in its second
state for a preselected period of time.
39. Apparatus according to claim 38 wherein said identifier circuit
produces discrete time parameters.
40. The invention of claim 38 wherein said identifier circuit produces
continuous time parameters.
41. Apparatus according to claim 40 wherein said parameters are pole
parameters.
42. Apparatus according to claim 41 wherein said parameters are
autoregressive parameters.
43. In an audio system operating in an acoustic environment and having a
microphone for inputting audio information into the system, amplifier
means for amplifying audio frequency signals inputted to the microphone,
and a speaker for outputting amplified audio frequency signals into said
environment which provides acoustic feedback between said speaker and said
microphone, the improvement comprising;
(a) an identification circuit for dynamically identifying those parameters
associated with said acoustic feedback; and
(b) correction circuit means having a transfer function set in response to
parameters identified by said identification means for cancelling the
effect of said acoustic feedback without attenuating any audio
frequencies.
44. The invention of claim 43 wherein said identification circuit is
constructed and arranged to identify said parameters in response to a
condition selected from the group of conditions: turn-on of the system,
and gain a change in said amplifier means.
45. The invention of claim 44 wherein said identification circuit includes
means responsive to threshold changes in said acoustic feedback for
effecting identification of the parameters thereof.
46. A method to control acoustic feedback in a communications system
operating in an acoustic environment and having a microphone component for
inputting audio information into the system, an amplifier component for
amplifying audio frequency signals inputted to the microphone, and a
speaker component for outputting audio frequency signals into said
environment, said method comprising;
(a) identifying those parameters associated with acoustic feedback,
including identifying time-variations in these parameters;
(b) adjusting the transfer function of said amplifier component in
accordance with the identified parameters for cancelling the effects of
said acoustic feedback in response to idenfification of the parameters
associated therewith;
(c) configuring the communication system to operate in an identification
mode before parameter identification is carried out;
(d) configuring the communication system to operate in an operational mode
after parameter identification is carried out, the cancelling of the
effects of said acoustic feedback being done while the system is
configured in its operational mode;
(e) cancelling the effects of said acoustic feedback by establishing the
transfer function of a correction circuit in accordance with the
identified parameters;
(f) coupling said correction circuit to said communication system when it
is configured in its operational mode; and
(g) wherein the step of configuring the communication system in its
identification mode is carried out by disconnecting said amplifier
component from said microphone and speaker components.
47. A method to control acoustic feedback in a communications system
operating in an acoustic environment and having a microphone component for
inputting audio information into the system, an amplifier component for
amplifying audio frequency signals inputted to the microphone, and a
speaker component for outputting audio frequency signals into said
environment, said method comprising:
(a) identifying those parameters associated with acoustic feedback,
including identifying time-variations in these parameters;
(b) cancelling the effects of said acoustic feedback in response to
identification of the parameters associated therewith; and
(c) wherein the step of identifying those parameters associated with
acoustic feedback utilizes a pseudo-random noise source and is carried out
at least during a short interval immediately following system turn-on, or
threshold gain change, and the step of cancelling the effects of said
acoustic feedback is accomplished by a correction circuit whose parameters
are set in accordance with the identified parameters and which feeds back
to said amplifier component, a feedback signal whose phase is
approximately opposite to the phase of the acoustic feedback and whose
amplitude is approximately the same as the amplitude of the acoustic
feedback.
48. A method according to claim 47 wherein the identification of said
parameters is carried out periodically after system turn-on.
49. A method according to claim 47 wherein the identification of said
parameters is carried out at fixed time intervals, and the parameters of
the correction circuit are reset at fixed time intervals.
50. A method according to claim 47 wherein identification is based on
minimizing the squared integrated error between the output of the
microphone and the output of the correction circuit.
51. A method according to claim 49 wherein identification is based on
minimizing the squared integrated error between the output of the
microphone and the output of the correction circuit.
52. A method according to claim 50 wherein identification is based on
minimizing the gradient of the squared integrated error between the output
of the microphone and the output of the correction circuit.
53. A method according to claim 52 wherein minimization of the gradient of
the squared integrated error is done sequentially in time by an iterative
process.
54. A method according to claim 51 wherein identification is based on
minimizing the gradient of the squared integrated error between the output
of the microphone and the output of the correction circuit.
55. A method according to claim 54 wherein minimization of the gradient of
the squared integrated error is done sequentially in time by an iterative
process.
56. Apparatus comprising:
(a) a microphone component for producing an output audio signal in response
to an applied input audio signals;
(b) amplifier means including a correction circuit having an adjustable
transfer function for producing an amplified output in response to an
applied input audio signal;
(c) a speaker component for broadcasting audio signals into an accoustic
environment in response to an input audio signal;
(d) an identifier circuit means;
(e) switch means for selectively connecting either said identifier circuit
means, or said amplifier means between the microphone and speaker
components;
(f) said identifier means being constructed and arranged to identify those
parameters associated with acoustic feedback when said switch means
connects said identifier circuit means between the microphone and speaker
components; and
(g) means responsive to parameters identified by said identifier means for
adjusting the transfer function of said correction circuit such that
acoustic feedback between the speaker and the microphone component is
substantially cancelled without attenuating audio frequencies when the
switch means interconnects said amplifier means between the microphone and
speaker components.
57. Apparatus according to claim 56 wherein said identifier circuit means
includes a noise generator for generating noise, and cross-correlation
means, said switch means being constructed and arranged such that when
said identifier circuit means is interconnected between said microphone
and said speaker components, noise from said generator is applied to both
the cross-correlator and said speaker component, and said crosscorrelator
cross-correlates the output of said microphone component with said noise
to produce an output that identifies the parameters associated with
acoustic feedback.
58. Apparatus according to claim 57 including configuration means
responsive to the application of power to the amplifier means for
interconnecting said identifier circuit means between the microphone and
speaker components for a predetermined period of time.
59. Apparatus according to claim 57 including configuration means
responsive to a threshold change in gain of the amplifier means for
interconnecting said identifier circuit means between the microphone and
speaker components for a predetermined period of time.
60. Apparatus according to claim 57 including configuration means
responsive to either the application of power to the amplifier means, or a
threshold change in gain of the amplifier means for interconnecting said
identifier circuit means between the microphone and speaker components for
a predetermined period of time. |
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Claims |
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Description |
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CROSS REFERENCE TO RELATED MATTERS
The following patents, helpful to an understanding of the present
invention, are hereby incorporated by reference:
[1] U.S. Pat. No. 4,025,721, issued May 24, 1977; and
[2] U.S. Pat. No. 4,185,168, issued Jan. 22, 1980.
TECHNICAL FIELD
This invention relates to a method of and means for adaptively filtering
screeching noise caused by acoustic feedback in communication systems such
as hearing aids or public address systems.
BACKGROUND OF THE INVENTION
Hearing impaired persons fitted with hearing aids, as well as persons
around them, are familiar with loud, unpleasant, and often uncomfortable,
screeching noises that often eminate from a hearing aid when it is turned
on, and at other times as well. Persons with normal hearing have
experienced similar problems with public address systems. In both hearing
aids and public address systems, hereinafter referred to as communication
systems of the type described, which are used in acoustic environments,
acoustic feedback is the culprit. That is to say, some of the acoustic
energy radiated from the speaker of a communication system into the
acoustic environment is picked up by the microphone of that same system,
is amplified by the system's electronics, and then rebroadcast into the
environment. Under some conditions, signal reinforcement, or
bootstrapping, occurs; and the result is a screeching noise that is both
loud, physically uncomfortable, and annoying to all those in the vicinity
of the speaker.
Screeching noise caused by acoustic feedback is a major irritation to
hearing aid users as well as to persons with unimpaired hearing in their
vicinity, and to persons in the vicinity of a malfunctioning public
address system. Conventionally, the user of a hearing aid controls screech
caused by acoustic feedback by reducing the gain on the amplifier in the
hearing aid, but this expedient solves the problem at the expense of a
reduction in the level of amplification of information, which is the basis
for wearing a hearing aid in the first place. In addition, manual
adjustment of the volume of a relatively small hearing aid is usually
difficult, or at least inconvenient. In public address systems, on the
other hand, resort to rearranging the microphone is often the only
practical way to alleviate screeching noise. Thus, the elimination of
screeching noise in a communication system caused by acoustic feedback
often requires manual intercession into the operation of such system which
may be inconvenient or inappropriate. Apparatus that automatically, and
adaptively, overcomes the problem of screeching noise caused by acoustic
feedback would therefore be very desirable.
It is an object of the present invention to provide both a method of and
apparatus for automatically and adaptively overcoming the problem of
screeching noise caused by acoustic feedback in a communication system of
the type described.
BRIEF DESCRIPTION OF INVENTION
The present invention is incorporated into a communication system of the
type described which operates in an acoustic environment. The system
includes a microphone component for inputting audio information, an
amplifier component for amplifying audio frequency signals inputted into
the microphone, and a speaker component for outputting amplified audio
frequency signals into the environment which provides an acoustic feedback
path between the speaker component and the microphone. According to the
present invention, an identification circuit dynamically identifies those
parameters associated with acoustic feedback, and a correction circuit,
whose transfer function is established by the parameters identified by the
identification circuit, and which is coupled to the amplifier component,
cancels the effect of the acoustic feedback.
In order for the acoustic feedback parameters to be identified, the
configuration of the communication system is altered from a conventional
operational mode to a parameter identification mode. In its operational
mode, the system is configured with the microphone coupled to the speaker
through the amplifier of the system; and the speaker is coupled to the
microphone through the acoustic medium of the environment. In its
parameter identification mode, the amplifier is decoupled from the
microphone and speaker and is replaced by an identifier circuit. By
decoupling the amplifier according to the present invention,
identification is performed in a manner that virtually ignores the
amplifier and concentrates essentially only on the acoustics of the
system. Identifying the parameters of the acoustics of the system is the
goal of the present invention because it is these parameters that must be
cancelled in order to eliminate acoustic feedback.
The identifier circuit, which is part of an identification circuit that
also includes a pseudo-random noise generator, is a cross-correlation
circuit that may be of type disclosed in Chapter 4 of the textbook
Identification of Systems, D. Graupe, Krieger Publishing Company,
Huntington, NY (1976) listed as reference [1] of U.S. Pat. No. 4,025,721
identified above. Such a cross-correlation circuit cross-correlates the
signals that appear at each of two inputs to the circuit and produces
either discrete or continuous time parameters as output. Instead of a
cross-correlation circuit, the identifier circuit may utilize a least
square minimization circuit (that minimizes a function of the squared
integrated identification error), or a gradient, or a sequential gradient,
minimization circuit (that minimizes the gradient of the squared
integration identification error, e.g., using an iterative process in
which minimization is done sequentially in time).
These last mentioned mimimization circuits are described in Chapters 5 and
7 of Identification of Systems. The parameters so obtained establish the
transfer function of the correction circuit enabling the effect of
acoustic feedback to be significantly reduced or eliminated.
In order to establish the configuration of the communication system, a
two-state switch means, operated by switch control means, is interposed
between the microphone and the amplifier, and between the speaker and the
amplifier. In its first state, the switch means is effective to configure
the system in its operational mode, i.e., the amplifier is interposed
between the microphone and the speaker, and the identification circuit is
not in the loop. When the switch means is in its second state, the system
is configured in its parameter identification mode wherein the identifier
circuit of the identification circuit replaces the amplifier, i.e., one
input to the identifier circuit is connected to the microphone and the
other input is connected to both the speaker and the noise generator. In
this configuration, the output of the noise generator is injected both
into the speaker and into one input of the identifier circuit. Some of the
noise broadcast by the speaker is received by the microphone because of
the acoustic coupling therebetween; and the received noise is applied to
the other input of the identifier circuit.
The cross correlation between the noise directly injected into the
identifier circuit, and the noise (and perhaps information that is
uncorrelated with the noise) received by the microphone via the acoustic
link between the speaker and the microphone, is a representation of a
transfer function related to the individual transfer functions of the
microphone, the speaker, and the acoustic link. In this way, the
parameters associated with the acoustic feedback are dynamically
identified by the identification circuit in prepration for establishing
the transfer function of the correction circuit.
The identification process requires a finite time; and the switch control
means maintains the switch means in its second state of a predetermined
period interval of time. At the end of such time interval, the switch
control means changes the state of the switch means from its second to its
first state, and the configuration of the system is converted from its
identification mode to its operational mode. At the same time, the
identified parameters are applied to the correction circuit to establish a
transfer function that is directly associated with the acoustic feedback
as it existed during the time the system was operated in its
identification mode. Both the correction circuit and the amplifier of the
communication system are coupled between the microphone and speaker of the
system in its operational mode, preferably in a simple feedback
connection. Because the transfer function of the correction circuit is in
accordance with the output of the identifier circuit during the
identification mode, the presence of the correction circuit is effective
to alter the system transfer function in a way that cancels the effect of
acoustic feedback as long as it remains substantially the same as it was
during the identification time interval.
As indicated above, the switch control means establishes the states of the
switch means which, in turn, determine the configuration of the
communication system. The switch control means senses turn-on of the
system and monitors the gain of the amplifier or threshold increases in
amplitude, or in the root-means square (RMS) amplitude of the output of
the amplifier, or in the high frequency portion of such output. Such
threshold increases are assumed to be caused by acoustic feedback. In
response to system turn-on, or gain change, or a threshold change in the
output of the amplifier, or in the high-frequency (e.g., above say 1200
Hz.) portion of the output of the amplifier, or at periodic intervals
(e.g., every 60 seconds), the switch-control means forces the switch means
into their second state and maintains them in such state for a
predetermined period of time during which the system is configured in its
identification mode. During this period of time, parameter identification
takes place. At the end of the predetermined period of time, the switch
control means is effective to return the switch means to their first state
at which the communication system reverts to its operational mode. In this
way, the present invention provides a dynamic and adaptive way to filter
screeching noise caused by acoustic feedback from communication system
without manual intercession.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention are illustrated in the accompanying
drawings wherein:
FIG. 1 is a schematic representation of a prior art hearing aid that is
representative of a communications system into which the present invention
can be incorporated;
FIG. 2 is a schematic block diagram of the hearing aid shown in FIG. 1
illustrating the transfer functions of its components;
FIG. 3 is a block diagram of the present invention incorporated into a
communication system such as a hearing aid housing or a public address
system;
FIG. 4 is a block diagram of the device shown in FIG. 3 configured to
operate in a parameter identification mode;
FIG. 5 is a schematic block diagram of the correction circuit shown in FIG.
3;
FIG. 6 is block diagram of an embodiment of logic circuit which provides
for the sensing of gain changes in the amplifier means of the apparatus
shown in FIG. 3;
FIG. 7 is a block diagram showing one type of noise generator shown in FIG.
3;
FIG. 8 is a block diagram showing electronic switch means by which the
microphone and speaker are connected to the amplifier means of the system
and the switch control means for controlling the switch means.
DETAILED DESCRIPTION
Referring now to FIG. 1 of the drawings, reference numeral 10 designates a
prior art communication system such as a hearing aid or public address
system. Communication system 10 comprises microphone component 12 for
producing an audio output signal on line 13 in response to audio signals,
figuratively identified by lines 14 and 15, in an acoustic environment
within which the communication system is located. System 10 further
includes amplifier component 16 for producing an amplified output on line
17 in response to an audio input signal on line 13. Finally, system 10
includes speaker component 18 for producing audio signals, designated
figuratively by reference numeral 19, that are broadcast into the acoustic
environment in response to an input signal on line 17. For the purpose of
illustrating the prior art, the acoustical environment within which the
communication system is located provides an acoustic link designated
figuratively by reference numeral 20 between speaker 18 and microphone 12.
As a consequence of the arrangement shown in FIG. 1, audio information
broadcast by speaker 18 may be received by microphone 12, amplified in
amplifier 16, and supplied to speaker 18 such that reinforcement occurs.
The result is the familiar screeching noise associated with acoustic
feedback.
Reference is now made to FIG. 2 for the purpose of illustrating the problem
of acoustic feedback in terms of the transfer function associated with
each of the components shown in FIG. 1. Each of the components shown in
FIG. 1, namely microphone 12, amplifier 16, speaker 18, and acoustic link
20 has associated with it a transfer function characteristic of the
component. The transfer function of a component, as is well known, is the
ratio of the Laplace transform of the output of the component to the
Laplace transform of the input of the component. Expressed in this manner,
the transfer function of a component is characteristic of the component
regardless of the form of the input thereto.
In FIG. 2, the transfer function of the components shown in FIG. 1 are
shown in corresponding blocks. That is to say, the transfer function of
microphone 12 is G.sub.1 (s) as indicated by block 12A, etc. The present
invention contemplates the addition to the conventional circuitry in a
communication system of a correction circuit, whose transfer function C(s)
is such that the presence of the correction circuit in the communication
system is effective to cancel the acoustic feedback link. Such a circuit
is shown in FIG. 2 and is designated by reference numeral 21A. While
correction circuit 21A is shown connected in feedback relationship to the
amplifier component, it is also possible for the correction circuit to be
connected serially, or in parallel, to the amplifier component but the
resulting corrections and analyses are more complex.
By conventional circuit analysis, the transfer function of the entire
communication system T(s) is defined as the Laplace transform of the
output at point 22, in the circuit shown in FIG. 2, to the Laplace
transform of the input at point 23 in the circuit. The circuit transform
T(s) can be expressed as follows:
##EQU1##
where G.sub.l (s) is the transfer function of microphone 12, G.sub.2 (s)
is the transfer function of amplifier 16, G.sub.3 (s) is the transfer
function of speaker 18, and H(s) is the transfer function of acoustic link
20.
From inspection of Eq. (1), the effect of acoustic link 20 can be
eliminated form the transfer function of the system provided that the
transfer function of correction circuit 21A is adjusted, or is
established, such that:
C(s)+H(s)G.sub.1 (s)G.sub.3 (s)=0 (2)
In the event that Eq. (2) is satisfied, Eq. (1) reduces to:
T(s)=G.sub.1 (s)G.sub.2 (s)G.sub.3 (s) (3)
Eq. (3) represents the transfer function of the communication system when
no acoustic link exists between the output and the input of the system.
From inspection of Eq. (2), one can see that the product G.sub.l
(s)G.sub.3 (s)H(s) must be identified rather than the quantity H(s), and
the transfer function of the correction circuit must be set in accordance
with Eq. (2).
Basically, the present invention provides an acoustic feedback
identification circuit for selectively identifying those parameters
associated with acoustic feedback between the speaker and microphone
components of a communication system, and an adaptive correction circuit
component which is coupled to the amplifier component, and which has a
transfer function established by the parameters identified by the acoustic
feedback identification means. In order to achieve this end, the present
invention also includes configuration means for interconnecting the
components such that the acoustic feedback identification circuit first
identifies the parameters associated with acoustic feedback between the
speaker and the microphone, and then, automatically, and self adaptively,
establishes the transfer function of the correction circuit such that the
latter is effective to cancel acoustic feedback.
Apparatus according to the present invention is designated by reference
numeral 30 in FIG. 3 to which reference is now made. This figure is
similar to FIG. 2 in that the transfer functions of the components of the
communication system are illustrated in FIG. 3. That is to say, the
microphone compo | | |
