WO2003034784A1

WO2003034784A1 - Improved hearing aid

Info

Publication number: WO2003034784A1
Application number: PCT/DK2002/000675
Authority: WO
Inventors: Mie Ø. JØRGENSEN; Lars Bramsløw
Original assignee: Oticon A/S
Priority date: 2001-10-17
Filing date: 2002-10-08
Publication date: 2003-04-24
Also published as: US7245732B2; EP1438873A1; US20040258262A1

Abstract

The invention relates to a digital hearing aid system comprising a signal path with an input transducer, a signal processor and an output transducer, where a part of the system is intended for delivering sound into an ear canal of a hearing aid user, where this part leaves the ear canal with an non obstructed cross sectional area corresponding to a vent channel with a diameter of at least 3 mm, and where the signal path is designed to have a signal delay less than 8 ms. Preferably the hearing aid signal path furthermore comprises means for providing an adaptive feedback compensation. Furthermore the signal processor is adjusted to provide increased gain in low frequency areas.

Description

Improved hearing aid

AREA OF THE INNENTION

The invention relates to hearing aids, which are intended to be placed in or on the ear. More particularly the invention relates to the function of such hearing aids where a remedy for an occlusion problem is provided.

BACKGROUND OF THE INNENTION

i connection with hearing aids the occlusion problem is normally experienced by the user of the hearing aid when the hearing aid or the earmould of a hearing aid is introduced into the ear canal. The hearing aid user often experiences the occlusion effect as very uncomfortable.

In order to provide remedy for the occlusion effect a ventilation channel of a significant size may be provided in the hearing aid or in the earmould. However providing an increased size vent often will have the effect of creating an acoustic feedback path. The size of the vent that may be created is therefore limited.

hi the recent years feedback cancellation systems have been introduced for the purpose of eliminating or reducing acoustic feedback in normal hearing aid systems, i.e. with normal vent sizes, where the occlusion problem is present.

One objective of the present invention is to provide a digital hearing aid where the occlusion problem is widely reduced.

A second objective is to provide a hearing aid where the occlusion problem is widely reduced and where at the same time a sufficient gain for the compensation of a hearing loss may be provided with reduced occurrence of acoustic feedback. A further objective of the present invention is to provide a hearing aid system where the occlusion problem has been widely reduced, where at the same time a sufficient gain for the compensation of a hearing loss may be provided with reduced occurrence of acoustic feedback .

SUMMARY OF THE INNENTION

According to the invention the first objective is achieved by means of a hearing aid as defined in claim 1.

By introducing the size of the vent of the size indicated the occurrence of the occlusion effect is significantly reduced if not totally absent. Having the delay as defined means that any undesired effect of the wearer's voice, in the form of an echo, is avoided.

Preferably where the delay is less than 5 ms.

According to the invention the second objective is achieved by means of a hearing aid as defined in claim 2.

The presence adaptive feedback cancellation system will at the same time ensure the reduction of the possible acoustic feedback occurring due to a significant amplification of the input.

According to the invention the third objective is achieved by means of a hearing aid as defined in claim 3.

In this advantageous embodiment the hearing aid according to the invention provides an increased gain in the lower frequency areas in order to compensate for the now almost open or totally open ear canal.

Further embodiments are depicted in the dependent claims. As the vent is increased in size a loss of low frequency sound pressure level will occur and therefore the gain compensation for the sound pressure lost through the vent is carried out in the frequency area below 1000Hz, primarily in the frequency area below 500 Hz.

The gain compensation in at least one frequency band corresponds to at least 25 % of the actual loss of sound pressure level lost due to ventilation, preferably at least 35 %, more preferably at least 45 %.

The invention will be described more detailed in connection with the following preferred embodiment with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the hearing aid according to the invention. FIG. 2 is a schematic diagram showing more detailed a feedback compensation path.

DESCRIPTION OF A PREFERRED EMBODIMENT

A well-known principle for feedback cancellation in hearing aids is shown in fig. 1. All the components described below, except blocks (1), (5) and (50), operate in the discrete time domain.

The components are as follows: (1) is a microphone which picks up the sound from the environment (51) ("External input") and the feedback signal (52) ("FBSignal"); (2) is a microphone amplifier and an analog-to-digital converter (A/D); (3) is the hearing aid amplifier with filters, compressors, etc.; (4) is a digital-to-analog converter and a power amplifier; (5) is the hearing aid receiver; (50) is the acoustic feedback path (outside the hearing aid); (6) is a delay unit whose delay matches the delay through the components (4), (5), (50), (1) and (2). (7) is an N-tap finite impulse response (FIR) filter which is intended to simulate the combined impulse response of components (4), (5), (1), (2) and (50). (8) is an adaptive algorithm which will adjust the coefficients (9) of the filter (7) so as to minimize the power of the error signal (10).

The algorithm (8) is well known as the Least Mean Square (LMS) algorithm. The algorithm requires a reference signal (11), which is used to excite the path consisting of the components (4), (5), (1), (2) and (50). The correlation between the reference signal (11) and the error signal (10) is used to compute the adjustment of the coefficients (9).

No noise generator is included in the system shown in fig. 1. The system utilizes the output signal (11) from the hearing aid amplifier block (3) as a driving signal for the LMS algorithm, thereby eliminating the need for a disturbing noise in the receiver (5).

For some external input signals, the LMS based algorithm used in the application shown in fig. 1 is known to have difficulty adjusting the coefficients (9) as desired, i.e. to match the path consisting of components (4), (5), (1), (2) and (50). The difficulties are greatest for signals with long autocorrelation functions. Mismatched coefficients may lead to audible side effects, which can be very disturbing to a hearing aid user. One general remedy against this problem is to use a low adaptation speed, but this leads to poorer performance of the system because the coefficients cannot track changes in the acoustic feedback path (50) quickly, resulting in a long feedback cancellation time.

The basic system shown in fig. 1 may be improved in various ways to minimize the side effects associated with certain input signals. Many authors have proposed additional system blocks, which will improve the sound quality while maintaining an acceptable adaptation speed.

The present invention is based on the system diagram shown in fig. 1, and the invention consists of additional features, which will improve the sound quality and maintain an acceptable adaptation speed.

FIG.2 shows the block diagram of the general system and the components of the invention. The embodiment shown includes three features: Adaptation rate control a frequency- selective adaptation procedure, and a feedback oscillation detector.

Adaptation rate control

Two well known operation modes for the LMS algorithm are the "standard" mode and the "normalized" mode. In the "standard" mode, the coefficients are updated by an amount that depends on the short-term power of the error signal and the reference signal. This means that the update rate is faster when more powerful signals are processed by the hearing aid. In the "normalized" mode, the update rate is made nearly independent of the signal power, due to a normalization of the update equation.

As described earlier, a low adaptation speed generally improves the sound quality for signals with long autocorrelation functions. In contrast, a high adaptation speed is desirable to reduce feedback oscillations quickly.

Other authors have previously proposed changing the adaptation rate factor (often known as "μ") when feedback oscillations are detected. Although this does increase the adaptation speed, it also allows coefficients to deteriorate proportionally faster, in those situations where signals with long autocorrelation functions are present at the hearing aid input.

In the present invention, we utilize the fact that feedback oscillations often have a high power. In many hearing aids, the output level is limited by compressor circuits, and in many cases the maximum output level is well above the normally used output level, for example when speech and other environmental signal are present. We will therefore assume that the feedback oscillations have a higher power than the environmental signal, in most cases where feedback problems exist.

Additionally, the feedback oscillation has the desirable property that its frequency is generally equal to the frequency where the loop gain currently is highest, i.e. where the fastest adaptation is needed. For the reasons mentioned above, it is very effective to utilize the feedback oscillation signal itself as a driving signal for the adaptation.

When the "normalized" adaptation approach is used, the high-power feature of the feedback oscillation is not utilized. If, instead, the "standard" update approach were used, the high power feature of the feedback oscillation would be utilized. At the same time, however, stronger signals in general would cause a higher adaptation speed, which could lead to more autocorrelation problems.

The present invention introduces a new normalization scheme which will generally maintain the low adaptation speed and the normalized operation mode, except when a feedback oscillation is detected. When a feedback oscillation is detected, the system is switched from normalized operation to standard operation by the switch (13), and the full power of the feedback oscillation signal is therefore allowed to adapt the coefficients. During "standard" operation, the update parameter (14) is chosen to such a value (53) that the external input (51) produces approximately the same update rate as it would in "normalized" operation. Assuming that the external input signal (51) maintains nearly constant properties before and during the feedback oscillation, the switch of normalization procedure will be nearly transparent to the external signal (51). This ensures that the sound quality remains high, even though the adaptation speed has been increased due to the higher power in the feedback oscillation. The update parameter (53) to be used during standard mode is estimated in component (12) before the feedback oscillation is detected. During intervals of feedback oscillations, controls signal (15) prevents (12) from updating the parameter (53).

The switch from normalized mode to standard mode may be controlled by a feedback oscillation detector (49) through its output signal (15). The switch (13) may also be controlled by other conditions, which could result in feedback oscillations, for example when the acoustic feedback is rapidly decreased. Such devices are not included in the invention.

The adaptive LMS algorithm (8) may be implemented as the following set of equations: Normalized operation:

a • r(n - k) • e(ή) hk(n + 1) = hk(ή) + , p = l..N

(El)

Standard operation:

(E2)

In these equations, h_k(n) is the k'th coefficient in the FIR filter at sample time n; a is a constant wliich determines the general adaptation speed of the algorithm (this constant is sometimes referred to as "μ"); b is a small constant which prevents division by 0 for very small values of the reference signal; N is the number of coefficients in the filter (7); r(n) is the reference signal (30) sample value at time n; e(n) is the error signal (28) sample value at time n; and LTs_u is a value computed as described below.

The sum term of the denominator of El is equal to the signal (54). LT_sum is equal to the signal (53).

LT_sum (equal to (53)), which is computed by component (12), may be updated according to eq. (E3):

LTsum(n + 1) = LTsum(n) • βxr + SumSq(n) • an

(E3)

SumSq(ή) = ∑r(n -p)² , p = l..N In equation (E3) SumSq(n) is defined as follows (E4):

(E4) CC _T and P_LT are time constants which control the length of the exponential window over which the value of LT_SUm is computed.

Eq. (E3) should not be updated while feedback oscillation is present, since LT_sum should reflect the long-term value of SumSq for segments without oscillation. Once the feedback oscillation has disappeared, eq. (E3) may be updated again.

In El and E4, the reference signal r(n) is used for normalizing the update equation. However, other signals in the system shown in fig. 2 may also be used instead of r(n). In the literature, the error signal e(n) has been used instead of r(n) for normalization; and even combinations of r(n) and e(n) have been used. The present invention will work for any type of normalization, in which the denominator in El and E2 is increased when the power level in the feedback loop consisting of (1), (2), (3), (4), (5) and (50) is increased.

Frequency-selective adaptation

Many feedback cancellation systems proposed earlier contain some form of frequency weighting of the signals which enter the LMS algorithm (8). The purpose of such weighting is to attenuate frequency ranges in which the autocorrelation of the external input signal (51) is long, and thereby reduce the possibility of poorly adjusted coefficients and poor sound quality. Several possibilities exist for frequency weighting. Various combinations of fixed and adaptive filters have been suggested in the past.

In the present invention, we include steep highpass filters with high attenuation (20) in the inputs to the LMS algorithm. The purpose of these filters is to prevent low frequency contents from the reference signal (11) from entering the LMS algorithm. The cutoff frequency for the highpass filters (20) must be lower than the lowest frequency for which feedback cancellation should take place, and otherwise as high as possible.

With the highpass filters (20) in place, the LMS algorithm (8) would not experience an increased level of the error signal (10) when the coefficients (9) are poorly adjusted in the low frequency range. Filter (7) with poorly adjusted coefficients, combined with components (3) and (6), may lead to a system with a high loop gain, and instabilities may result.

In order to avoid this problem, a parallel feedback cancellation filter (21) is added. This filter is intended to provide low frequency information to the LMS algorithm. The two filters (7) and (21) use identical coefficients (9). While filter (7) is designed to simulate the path consisting of components (4), (5), (1), (2) and (50), filter (21) is designed to simulate the artificial path (25) with an impulse response of constant '0'. The adder (33) computes an error signal as the difference between the desired '0' output and the actual output (34) from the filter (21). The error output (10) from the high frequency range and the error output (27) from the low frequency range are combined into a single error signal (28) which is fed to the error input of the LMS algorithm (8). In order to generate a low frequency signal as input to the filter (21) and to the reference input to the LMS algorithm, a noise generator (22) is included. The noise generator output (29) is lowpass filtered by a fixed filter (23). The cutoff frequency for the lowpass filter (23) is selected approximately equal to the cutoff frequency of the highpass filters (20), to obtain a reasonably flat input spectrum to the LMS algorithm. The low frequency signal (32) and the high frequency signal (31) are combined by the adder (24) to form the complete reference signal (30) for the LMS algorithm. Clearly, the components (25) and (33) may be removed immediately, and the signal (34) can be comiected to the signal (27).

The noise generator (22) may be implemented by randomly swapping the numerical sign of each sample of the signal (35). In other words, for each sample instant it is randomly decided whether the sample value should be multiplied by 1 or by (-1). The advantage of using this type of noise generator is that noise samples at (35) and at (29) always have the same amplitude. The power spectrum of the reference signal (30) is therefore reasonably balanced at all times. In the literature, the noise generated as described above is sometimes referred to as 'Schroeder' noise.

Feedback oscillation detector

Feedback oscillations may be produced by a system which contains an amplifier and a feedback loop, under some circumstances. A hearing aid with acoustic amplification, combined with an acoustic path from the hearing aid telephone through a ventilation channel ("vent") and possibly other leaks, form a loop which may have a gain higher than 0 dB, at least for some frequencies. With more than 0 dB loop gain, the system may become unstable and produce feedback oscillations.

The present invention is designed to detect a feedback oscillation in the input signal (55), and set a flag (15) which indicates Oscillation' or 'no oscillation'.

Some assumptions about the feedback oscillations in hearing aids are included in the design of the detector. The signal produced as a feedback oscillation typically consists of a single frequency, namely the frequency at which the loop gain is highest, taking into account both the linear and non-linear components of the hearing aid. The level of the feedback oscillation is relatively stable, after a certain settling time. The feedback oscillation often dominates the signal in the feedback loop, since its level is often determined by the hearing aid compressors.

The feedback detection process is complicated by the presence of other signals in the feedback loop. Many environmental signals, including music, may contain segments of periodic nature which may resemble a feedback oscillation. However, in the frequency range where oscillations may occur, relatively few environmental signals consist of a single frequency only, at least when considered over a period of a few hundred milliseconds or more.

The feedback oscillation detector in the present invention is based on measuring the overall 'bandwidth' of the signal in the feedback loop consisting of components (1), (2), (3), (4), (5) and (50). In the preferred embodiment, the signal (55) is used as input to the detector, but with slight modifications the detector may obtain its input anywhere in the loop. When the bandwidth of the signal (55) has been small for a certain minimum period of time, the detector will flag a 'feedback oscillation' condition.

FIG. 3 describes the detector (49). The input signal (55) is highpass filtered by an 8-tap FIR filter (36). The filter helps prevent false feedback oscillation detection for low frequency input signals since it suppresses the fundamental frequencies for a wide range of signals. The 3 dB roll-off frequency for the filter should be higher than the lowest expected feedback oscillation frequency. The 8-tap FIR filter is just one example of a usable filter, but many other types maybe used. The highpass filtered signal (37) is fed to a modeling device (38), which attempts to model the spectrum of the signal (37), using a second-order auto-regressive model as shown in E4:

y(n) = x(n) ■ K - a\y(n - 1) - aιy(n - 2)

(E4)

where x(n) represents the excitation signal, which drives the model input, while y(n) is the output from the model.

The signal model E4 represents a second-order HR filter with a single complex- conjugated pole-pair. Based on the model coefficients z._\ and a₂, the filters center frequency and bandwidth may be computed. This computation is performed by the unit (40), which produces a bandwidth (41) and a center frequency (48). These two values are compared by (47) to preset thresholds (43) and (46). The comparator sets flag (44) TRUE if the bandwidth (41) is lower than the preset threshold (43) AND the center frequency (48) is higher than the acceptable mimmum feedback oscillation frequency (46). Otherwise the flag (44) is set FALSE.

All components (38), (40), (47) and (45) are working on a frame based schedule. A frame length of 40 ms may be used, but other values of the length would also work. For each frame, a new value of the flag (44) is computed. Since many environmental input signals contain short segments of narrow bandwidth, the flag (44) may occasionally be set TRUE while no feedback oscillations are present. To avoid this, the flag (44) is fed to a stability estimator (45). hi here, the flag (44) is placed in a delay line which, at any point in time, holds the values of the flag from the last N_se frames. N_se maybe selected as 10, but other values would also work. The stability estimator (45) sets the detector flag (15) TRUE when and only when at least N_min out of the N_se past values of the flag (44) were TRUE. For example, N_miπ maybe set to 4. The coefficients Ά_\ and a₂ in E4 are computed from the autocorrelation coefficients R(0), R(l) and R(2), by solving the equations:

R(0) -αι + R(ι) -α₂ = -R(l)

(E5a)

(E5b)

The autocorrelation coefficients may be computed using the following equations:

R(1) =~ Nf -∑ ^ )- + ^ , n = l..Nf -\

^Ra⁾ = Nτf -∑^x(n>^{x(n + 2}ϊ > n = ^\..N_f -2

(E6a) (E6b)

(E6c)

where Nf corresponds to the frame length, and x(i) is the i'th sample of signal (37) from the current frame.

The 3-dB bandwidth of the filter represented by the auto-regressive model E4 may be computed as

Bandwith = 2 • (1 - *κjaι )

(E7)

and the center frequency may be computed as fcenter =

(E8)

In both equations (E7) and (E8) the result is given in radians. Simple calculations, in which the system sample rate is included, may be used to convert the values of Bandwidth and the fcenter into Hz.

Example of compensation:

Audiogram

Frequency, Hz 125 250 500 750 1000 1500 2000

Air conduction hearing loss 35 35 30 30 30 35 35

Fitted with BTE and Adapto non-linear fitting rule 'Slow'

Frequency 250 750 1 k 2 k 3 k 4 k 5 k

No vent

0.8 mm vent

1.4 mm vent

2.4 mm vent

4 mm vent

Open vent

Claims

1. A digital hearing aid system comprising a signal path with an input transducer, a signal processor and an output transducer, where a part of the system is intended for delivering sound into an ear canal of a hearing aid user, where this part leaves the ear canal with an non obstructed cross sectional area corresponding to a vent channel with a diameter of at least 3 mm or an equivalent area; and where the signal path is designed to have a signal delay less than 15 ms.

2. A hearing aid according to claim 1, where the hearing aid signal path furthermore comprises means for providing an adaptive feedback compensation.

3. A hearing aid according to claim 1 or 2, where the signal processor is adjusted to provide increased gain in low frequency areas.

4. A hearing aid according to claim 3, where the gain compensation for the sound pressure lost through the vent is carried out in the frequency area below 1000Hz, primarily in the frequency area below 500 Hz.

5. A hearing aid according to claim 1,2, 3 or 4, where gain compensation in at least one frequency band corresponds to at least 25 % of the actual loss of sound pressure level lost due to ventilation, preferably at least 35 %, more preferably at least 45 %.

6. A hearing aid according to claim 1, where the delay is less than 10 ms than 5 ms.