Null adaptation in multi-microphone directional system

Improved approaches to adaptively suppress interfering noise in a multi-microphone directional system are disclosed. These approaches operate to adapt the direction null for the multi-microphone directional system in accordance with a dominant noise source.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to noise suppression and, more particularly, to noise suppression for multi-microphone sound pick-up systems.

2. Description of the Related Art

Suppressing interfere noise is still a major challenge for most communication devices involving a sound pick up system such as a microphone or a multi-microphone array. The multi-microphone array can selectively enhance sounds coming from certain directions while suppressing interferes coming from other directions. The pattern of the direction selection can be fixed or adaptive. Adaptive selection is more attractive because it intends to maximize SNR depending on the sound environment. However, because the relative low frequency range of audio applications, existing adaptation techniques are effective only for microphone array with large physical dimension. For applications where physical dimension is limited, such as the case in hearing aid applications, traditional adaptation using Finite-Impulse-Response (FIR) adaptive filtering techniques is not effective. As a result, most hearing aids that have directional processing can only give a fixed directional pattern which is effective in improving Signal-to-Noise Ratio (SNR) in some conditions but less effective in other conditions.

FIG. 1shows a typical directional processing system in a two-microphone hearing aid. The two microphones pick-up sounds and convert them into electronic or digital signals. The output signal from the second microphone is delayed and subtracted from the output signal of the first microphone. The result is a signal with interference from certain directions being suppressed. In other words, the output signal is dependent on which directions the input signals come from. Therefore, the system is directional. The physical distance between the two microphones and the delay are two variables that control the characteristics of the directionality. For hearing aid applications, the physical distance is limited by the physical dimension of the hearing aid. The delay can be set in a delta-sigma analog-to-digital converter (A/D) or by use of an all-pass filter.

FIGS. 2(a)-2(c) illustrate polar patterns of a directional processing system corresponding to three different delay values. The term “polar pattern” has often been used to describe the characteristics of a directional processing system. The physical distance between the two microphones of the directional processing system is fixed. When a sound source is at 0 degrees, which is the direction along the axis of the two microphones and on the side of the front microphone, the directional processing system has a maximum output. When the sound source is away from 0 degrees, the output is reduced. The direction at which the output of the directional processing system has a maximum reduction is called directional null. Ideally, the directional null occurs at the direction of an unwanted noise source. The location of the directional null is related to the value of the delay. If the noise source is in the direction of 180 degrees, the delay should be set to a value so that the polar pattern is a cardioids with the directional null at 180 degrees (seeFIG. 2(a)). If the noise source is in the direction of 115 degrees, the delay should be set to a value so that the polar pattern is a hyper-cardioid with the directional null at 115 degrees (seeFIG. 2(b)). If the noise source is in the direction of 90 degrees, the delay should be set to a value so that the polar pattern is a bi-directional with the directional null at 90 degrees (seeFIG. 2(c)). Ideally, the delay should be set in such a way that the null is placed in the direction of the dominant noise source so that the noise can be highly suppressed. If the direction of the noise source is known, the optima delay can be calculated as:
delay=d/c*cos(180°−q),
where d is distance of the two microphones, c is sound propagation speed, and q is direction angle in degree of the noise source.

One problem with conventional noise suppression approaches is that the direction of a noise source to be suppressed by the directional processing is often unknown. Conventionally, the estimating of the direction of a noise source is difficult because the frequency of audio sounds is relative low. The direction of the noise source is often merely a rough estimate from which a delay is then fixed to provide directional processing. In fact, most hearing aids currently available in the market merely set the delay to a fixed value so that directional processing has a fixed polar pattern for all conditions. Unfortunately, the noise suppression of such devices is often inadequate because the noise source is often at a direction other than that corresponding to the fixed delay.

Thus, there is a need for improved approaches to directional processing by adapting a directional null according to the direction of interfering noise source.

SUMMARY OF THE INVENTION

Broadly speaking, the invention relates to improved approaches adaptively suppress interfering noise in a multi-microphone directional system. These approaches operate to adapt the direction null for the multi-microphone directional system. These approaches are particularly useful for hearing aid applications in which directional noise suppression is important.

One aspect of the invention pertains to techniques for adjusting a delay adaptively so that a directional null is placed in the direction of a dominant noise source. This would produce maximum Signal-to-Noise Ratio (SNR) improvement across all conditions. In other words, the dominant noise source is attenuated (e.g., suppressed) but the desired sound from a particular direction is not attenuated

The invention can be implemented in numerous ways including as a method, system, apparatus, device, and computer readable medium. Several embodiments of the invention are discussed below.

As an adaptive directional sound processing system, one embodiment of the invention includes at least: a least two microphones spaced apart by a predetermined distance, each of said microphones producing an electronic sound signal; a delay circuit that delays the electronic sound signal from at least one of said microphones by an adaptive delay amount; a subtraction circuit operatively connected to said microphones and said delay circuit, said subtraction circuit producing an output difference signal from the electronic sound signals following said delay circuit; and a delay amount determination circuit operatively coupled to receive the output difference signal, said delay amount determination circuit produces a delay control signal that is supplied to said delay circuit so as to control the adaptive delay amount.

As an adaptive directional sound processing system, another embodiment of the invention includes at least: a least two microphones spaced apart by a predetermined distance, each of said microphones producing an electronic sound signal; a delay circuit that delays the electronic sound signal from at least one of said microphones by an adaptive delay amount; a logic circuit operatively connected to said microphones and said delay circuit, said logic circuit producing an output signal from the electronic sound signals following said delay circuit; and a delay amount determination circuit operatively coupled to receive the output signal, said delay amount determination circuit produces a delay control signal based on the output signal, the delay control signal being is supplied to said delay circuit so as to control the adaptive delay amount.

As an adaptive directional sound processing system, another embodiment of the invention includes at least: at least two microphones spaced apart by a predetermined distance, each of said microphones producing an electronic sound signal; a delay circuit that delays the electronic sound signal from at least one of said microphones by an adaptive delay amount; logic means for producing an output signal from the electronic sound signals following said delay circuit; and delay determination means for producing a delay control signal based on the output signal, the delay control signal being is supplied to said delay circuit so as to control the adaptive delay amount.

As a method for adaptively controlling delay induced on a sound signal so that unwanted noise is directionally suppressed, one embodiment of the invention includes at least the acts of: producing a difference signal from at least first and second sound signals respectively obtained by first and second microphones; estimating an energy amount of the difference signal; and producing a delay signal to control a delay amount induced on at least one of the first and second sound signals based on the energy amount of the difference signal.

As an adaptive delay method for directional noise suppression in a hearing aid device, the hearing aid device having at least first and second microphones, one embodiment of the invention includes at least the acts of: receiving first and second microphone outputs; delaying at least the second microphone output by an adaptive delay amount; combining the first microphone output and the delayed second microphone output to produce an output signal; estimating an energy amount associated with the output signal; and adapting the adaptive delay amount based on the energy amount.

As a method for adaptively controlling delay induced on a sound signal in a multi-microphone directional processing system so that unwanted noise is directionally suppressed, another embodiment of the invention includes at least the acts of: receiving at least first and second sound signals respectively obtained by first and second microphones; delaying at least one of the first and second sound signals by a plurality of different delay amounts; producing, following the delaying act, a plurality difference signals from at least first and second sound signals respectively obtained by first and second microphones; estimating energy amounts for each of the difference signals; and choosing the one of the difference signals as an output of the directional processing system based on the energy amounts of the difference signals.

As an adaptive directional sound processing system, another embodiment of the invention includes at least: at least two microphones spaced apart by a predetermined distance, each of the microphones producing an electronic sound signal; a plurality of delay circuits that each delay the electronic sound signal from at least one of the microphones by a different delay amount; logic means for producing candidate output signals from the electronic sound signals following the delay circuits; and output selection means for selecting one of the candidate output signals as an output based on energy levels of the candidate output signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:

FIG. 1shows a typical directional processing system in a two-microphone hearing aid;

FIGS. 2(a)-2(c) illustrate polar patterns of a directional processing system corresponding to three different delay values;

FIG. 3is a block diagram of a two-microphone directional processing system according to one embodiment of the invention;

FIG. 4shows a block diagram of an optimal delay determination unit according to one embodiment of the invention;

FIG. 5Ais a block diagram of a delay generator according to one embodiment of the invention;

FIG. 5Bis a schematic diagram of a circuit suitable for use as a delay increment calculation circuit according to one embodiment of the invention;

FIG. 5Cis a schematic diagram of a circuit suitable for use as a delay increment calculation circuit according to another embodiment of the invention;

FIG. 5Dis a schematic diagram of a circuit suitable for use as the delay increment calculation circuit according to still another embodiment of the invention;

FIG. 6shows an alternative method for adapting the direction null to maximize SNR in a two-microphone directional processing system;

FIG. 7is a graph illustrating a spectrum of a 1 kHz pure tone in white noise without any directional processing for noise reduction;

FIG. 8is a graph illustrating a spectrum of a 1 kHz pure tone in white noise with fixed-pattern (hypercaidiod) directional processing for noise reduction; and

FIG. 9is a graph illustrating a spectrum of a 1 kHz pure tone in white noise with adaptive directional processing according to one embodiment of the invention to provide enhanced noise reduction.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to improved approaches adaptively suppress interfering noise in a multi-microphone directional system. These approaches operate to adjust a direction null for the multi-microphone directional system.

One aspect of the invention pertains to techniques for adjusting a delay adaptively so that a directional null is placed in the direction of a dominant noise source. This would produce maximum Signal-to-Noise Ratio (SNR) improvement across all conditions. In other words, a dominant noise source is attenuated (e.g., suppressed) but the desired sound from a particular direction is not attenuated

Consequently, the invention enables multi-microphone directional processing systems to adaptively suppress a noise source. The invention is described below with respect to embodiments particularly well suited for use with hearing aid applications. However, it should be recognized that the invention is not limited to hearing aid applications, but is applicable to other sound pick-up systems.

FIG. 3is a block diagram of a two-microphone directional processing system300according to one embodiment of the invention. The two-microphone directional processing system300includes a first microphone302and a second microphone304. The first microphone302produces a first electronic sound signal, and the second microphone304produces a second electronic sound signal. The first and second electronic sound signals can be either analog or digital signals. In one implementation, the first and second microphones302and304are physically spaced by a distance of at least 3 mm. A delay unit306delays the second electronic sound signal by a delay amount. A subtraction unit308then subtracts the delayed second electronic sound signal from the first electronic sound signal to produce an output signal. At this point, the output signal has undergone directional processing by the two-microphone directional processing system300. Such directional processing enables unwanted interference from certain directions to be suppressed.

The two-microphone directional processing system300also includes an optimal delay determination unit310. The output signal produced by the substation unit308is supplied to the optimal delay determination unit310. The optimal delay determination unit310determines a delay amount (e.g., optimal delay) that the delay unit306should induce on the second electronic sound signal so that the directional null associated with the directional processing occurs at the direction of a noise source. The delay amount, or a corresponding control signal, is supplied to the delay unit306where the delay is imposed. Hence, the optimal delay determination unit310causes the delay amount for the delay unit306to self-adjust based on the output energy (e.g., output signal) of the two-microphone directional processing system300. In other words, the delay induced by the delay unit306automatically adjusts based on the output energy.

When interfering noise is present, the total energy of the signals picked up by the microphones302and304are greater than the output energy would be if the interfering noise were not present. According to one embodiment, the delay amount for the delay unit306can be adjusted so that the output of the two-microphone directional processing system300has minimum energy. Because change in the delay amount does not change the system response to desired sound coming from 0 degrees, minimizing the output energy by adjusting the delay is equivalent to achieving a maximum attenuation of noise (assuming the desired sound is coming from 0 degrees).

The output signal of the directional processing system300can be further processed by other processing functions. In the case of hearing aid applications, the output of the directional processing is further processed by other hearing aid functions such as amplification and noise suppression.

FIG. 4shows a block diagram of an optimal delay determination unit400according to one embodiment of the invention. The optimal delay determination unit400is, for example, suitable for use as the optimal delay determination unit400. The optimal delay determination unit400includes an energy estimator402and a delay generator404. The energy estimator402receives a feedback signal406. The feedback signal406is the output signal produced by the directional processing system300. The energy estimator402receives the feedback signal406and creates an energy signal408. The delay generator404receives the energy signal408and generates a delay signal410(delay amount; control signal) based on the energy signal408. More particularly, the delay generator404controls the delay amount induced by the delay unit306in such a way that the output energy is statistically minimized and, therefore, the Signal-to-Noise Ratio (SNR) is maximized.

The energy estimator402can create the energy signal408by any one of the following: (1) forcing its input into positive signal; (2) squaring the input; (3) calculating a Root-Mean-Square (RMS) signal for the input; or (4) estimating a minimum signal from the input. The energy signal408can be down-sampled first before being used to generate the delay signal410.

The delay generator404produces the delay signal410based on the energy signal408. In one embodiment, the delay signal410is a delay amount obtained by determining a change in the energy signal, creating a delay increment signal in accordance with the change, and adding the delay increment signal to a current delay amount to produce a next delay amount.

It should be noted that the optimal delay determination unit400can also utilize a down sampling process between the energy estimator402and the delay generator404as typically the energy estimate from the energy estimator402will have a higher sampling rate than that of the delay generator404. The result is that the output of the energy estimator402changes fast while the output from the delay generator404changes slowly. For example, in one embodiment, the input to and output from the energy estimator402can be digital signals at a higher sampling rate, e.g., 16 kHz, while the input to and output from the delay generator404can be digital signals at a lower sampling rate (e.g., 1 kHz). A down sampling process can thus be provided between the energy estimator402and the delay generator404to accommodate the difference in sampling rates. Down sampling is a term frequently used in digital signal processing to describe a process that reduces the sampling frequency from high to low.

Alternatively, instead of reducing the sampling frequency through down sampling, a similar effect can be achieved by using a slower time constant in the delay generator404than that of the energy estimator402. Time constant describes how fast the output of a processing block changes with its input. Here, the time constant for the delay generator404can be slower than the time constant of the energy estimator402so that the output of the energy estimator402changes fast while the output from the delay generator404changes slowly.

FIG. 5Ais a block diagram of a delay generator500according to one embodiment of the invention. The delay generator500is, for example, suitable for use as the delay generator404illustrated inFIG. 4. The delay generator500includes a subtraction circuit502. The subtraction circuit502receives the energy signal408from the energy estimator402. A sample delay circuit504delays the energy signal408by a specified amount (e.g., 1/z) before supplying the delayed energy signal to the subtraction circuit502. The subtraction circuit502subtracts the energy signal408from the delayed energy signal to produce an energy change signal. The energy change signal is supplied to a delay increment calculation circuit506.

The delay increment calculation circuit506calculates a current delay increment based on the energy change signal. The current delay increment is then supplied to an add circuit508. The add circuit508adds the current delay increment to a previous delay increment509to output an unrestricted optimal delay. The unrestricted optimal delay is then supplied to a maximum delay circuit510and a minimum delay circuit512. The unrestricted optimal delay, after passing through the maximum delay circuit510and the minimum delay circuit512, outputs an optimal delay516. The maximum delay circuit510limits the upper range for the optimal delay to a maximum value, and the minimum delay circuit512limits the minimum delay to a minimum value. Although the limits will vary widely with application, in one embodiment, the maximum value can be36and the minimum value can be zero. The optimal delay516is also fed back through a sample delay circuit518which produces the previous delay increment509that is supplied to the add circuit508. The optimal delay516is, for example, the delay signal410illustrated inFIG. 4.

The circuitry for the delay increment calculation circuit506can take many forms.FIGS. 5A,5B and5C illustrate three of many different approaches to calculate or determine the current delay increment.

FIG. 5Bis a schematic diagram of a circuit520suitable for use as the delay increment calculation circuit506according to one embodiment of the invention. The circuit520calculates the current delay increment from the energy change signal. The circuit520includes a switch circuit522, a negate circuit524, and a sample delay circuit526. The energy change signal is supplied to a control terminal of the switch circuit522to control its switching. The switch circuit522outputs the delay increment signal. The delay increment signal is also fed back to the sample delay circuit526which produces a previous delay increment signal. The previous delay increment signal is supplied to the negate circuit514as well as to a first switch terminal of the switch circuit522. The negate circuit524inverts the previous delay increment signal and supplies the inverted previous delay increment signal to a second switch terminal of the switch circuit522.

The switch circuit522is controlled in accordance with the energy difference signal. When the switch circuit522determines that the energy difference signal is greater than zero (0), then the delay increment signal being output by the circuit520corresponds to the previous delay increment signal. Alternatively, when the switch circuit522determines that the energy difference signal is less than zero (0), then the delay increment signal being output by the circuit520corresponds to the inverted previous delay increment. Hence, when the energy difference signal is greater than zero (0), the delay increment signal remains the same as it previously was. On the other hand, when the energy difference signal is less than zero (0), then the delay increment signal is negated from its previous value. As an example, the energy difference signal and the delay increment being output can be represented in multiple bits, such as 16 bits, of either integer or floating point numerical storage.

FIG. 5Cis a schematic diagram of a circuit540suitable for use as the delay increment calculation circuit506according to another embodiment of the invention. The circuit540calculates the current delay increment from the energy change signal. The circuit540includes a multiply circuit542and a sample delay circuit544. The energy difference signal is received at the multiply circuit542. In addition, the multiply circuit542receives a previous delay increment signal from the sample delay circuit544. Here, the multiply circuit542multiplies the energy difference signal with the previous delay increment signal to produce the delay increment signal. The delay increment signal is also supplied to the sample delay circuit544which delays the signal by a specified amount (1/z) to produce the previous delay increment signal.

FIG. 5Dis a schematic diagram of a circuit560suitable for use as the delay increment calculation circuit506according to still another embodiment of the invention. The circuit560calculates the current delay increment from the energy change signal. The circuit560includes a scaling circuit562, a multiply circuit564, and a sample delay circuit566. The energy difference signal is supplied to the scaling circuit562that scales the energy difference signal in accordance with a parameter K. Here, in one embodiment, the scaling parameter K is negative (−K). The scaled energy difference signal is then supplied to the multiply circuit564. The multiply circuit564also receives a previous delay increment signal produced by the sample delay circuit566. The multiply circuit564multiplies the previous delay increment signal by the scaled energy difference signal to produce the delay increment signal. The delay increment signal is also supplied to the sample delay circuit5664which delays the signal by a specified amount (1/z) to produce the previous delay increment signal.

FIG. 6is a block diagram of a two-microphone directional processing system600according to another embodiment of the invention. The two-microphone directional processing system600includes a first microphone602and a second microphone604. The first microphone602produces a first electronic sound signal, and the second microphone604produces a second electronic sound signal. The first and second electronic sound signals can be either analog or digital signals.

The directional processing system600also includes a series of different delay units606,608and610. Each of these delay units606,608and610operate to induce different delays to the second electronic sound signal. In addition, the directional processing system600also includes subtract circuits612,614and616. Each of the subtract circuits612,614and616receives the first electronic sound signal from the first microphone604. In addition, the subtract circuit614receives the delayed second electronic sound signal from the delay unit606. The subtract circuit614receives the delayed second electronic sound signal from the delay unit608. The subtract circuit616receives the delayed second electronic sound signal from the delay unit610. Each of the subtract circuits612,614and616produce a difference signal. The difference signals produced by the subtract circuits612,614and616are each supplied to a signal selection circuit618. Under the control of a control signal, the signal selection circuit618outputs one of the difference signals as the output signal. At this point, the output signal has undergone directional processing by the directional processing system600. Such directional processing enables unwanted interference from certain directions to be suppressed.

The control signal to the signal selection circuit618is provided by a selector620together with energy estimators622,624and626. The energy estimator622estimates the energy on the difference signal produced by the subtract circuit612, and supplies the energy estimate to a first input to the selector620. The energy estimator624estimates the energy on the difference signal produced by the subtract circuit614, and supplies the energy estimate as a second input to the selector620. The energy estimator626estimates the energy of the difference signal produced by the subtract circuit616and supplies the energy estimate to a third input to the selector620. The selector620then selects one of the estimated energy values supplied by the energy estimators622,624and626as the selected output which forms the control signal that controls the signal selection circuit618.

The directional processing system600selects the difference signal that has the lowest energy as the system output (output signal). The lowest energy means that the channel or path undergoing the most noise suppression is selected. The different delay units606,608and610together with the subtract units612,614and616for the channels or paths. In this embodiment, the delays for the delay elements are fixed and thus do not adapt. Instead, the various delay units offer different delays and the channel or path providing the best noise suppression is chosen. Although the directional processing system600provided only three channels or paths, it should be recognized that additional paths can be provided. In general, the directional processing system600operates with two or some channels or paths.

The signal energy can be estimated in a variety of ways. For example, the energy signal can be estimated using one of the followings: (1) forcing its input into positive signal; (2) squaring the input; (3) calculating a Root-Mean-Square (RMS) signal for the input; or (4) estimating a minimum signal from the input. Also, it should be noted that the rate at which the energy signal is estimated need not be the same as the rate in which the delay signal is updated. In other words, the energy signal can be updated with a different time constant that a time constant used in updating the delay signal. For example, for a fixed sampling rate, the energy signal can be updated for every sample, while the delay signal can be updated every 100 samples.

The adaptive directional processing system includes at least two microphones, typically physically spaced by a distance of at least three (3) mm. The microphones are used to convert sound into electronic signals. The electronic signals can be either analog or digital. The system further includes delay means to delay the electronic signals from one or both microphones. The system further includes addition or subtraction means to generate a differential signal of the microphone outputs as delayed by the delay means. The system also includes means for estimating the energy of the differential signal. The delay from the delay means is used to adapt the directional null to suppress a dominant noise source. The delay means, the addition/subtraction means, and the energy estimate means can be used more than once in parallel so that multiple delayed signals, multiple differential signals, and multiple energy signals are created.

Although the above-described embodiments of the directional processing systems have utilized two microphones, it should be understood that the directional processing systems can also use more than two microphones. Furthermore, following directional processing, the output of the directional processing system can be further processed by other processing functions. In the case of hearing aid applications, the output of the directional processing is further processed by other hearing aid functions such as amplification and noise suppression.

FIG. 7is a graph illustrating a spectrum of a 1 kHz pure tone in white noise without any directional processing for noise reduction. The SNR of the spectrum is about 6 dB.

FIG. 8is a graph illustrating a spectrum of a 1 kHz pure tone in white noise with fixed-pattern (hypercaidiod) directional processing for noise reduction. The SNR of the spectrum is about 14 dB.

FIG. 9is a graph illustrating a spectrum of a 1 kHz pure tone in white noise with adaptive directional processing according to one embodiment of the invention to provide enhanced noise reduction. The SNR of the spectrum is about 30 dB, which is a dramatic improvement over the conventionally available SNRs associated withFIGS. 7 and 8.

The invention is preferably implemented in hardware, but can be implemented in software or a combination of hardware and software. The invention can also be embodied as computer readable code on a computer readable medium. The computer readable medium is any data storage device that can store data which can be thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, magnetic tape, optical data storage devices, carrier waves. The computer readable medium can also be distributed over a network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion.

The advantages of the invention are numerous. Different embodiments or implementations may yield one or more of the following advantages. One advantage of the invention is that a dominant noise source can be directionally suppressed. Another advantage of the invention is that the directional suppression is adaptive and thus changes as the directional of the dominant noise source changes. Still another advantage of the invention is that desired sound from a particular direction is not interfered with even though a dominant noise source is able to be directionally suppressed.