EP2648424B1 - Method for limiting the output level in hearing aids - Google Patents

Description

The present invention relates to a method for amplifying an input signal in a hearing device by predetermining a respective channel-specific compression characteristic in a plurality of spectrally separated processing channels of the hearing device, which defines a relationship between an input level and an output level in the respective processing channel of the hearing device, and amplifying a respective input signal portion of the hearing device in each processing channel as a function of a channel-specific operating compression characteristic. A hearing device here means any device which can be worn in or on the ear and causes a hearing, in particular a hearing aid, a headset, headphones and the like.

Hearing aids are portable hearing aids that are used to care for the hearing impaired. In order to meet the numerous individual needs, different types of hearing aids such as behind-the-ear hearing aids (BTE), hearing aid with external receiver (RIC: receiver in the canal) and in-the-ear hearing aids (IDO), e.g. Concha hearing aids or canal hearing aids (ITE, CIC). The hearing aids listed by way of example are worn on the outer ear or in the ear canal. In addition, bone conduction hearing aids, implantable or vibrotactile hearing aids are also available on the market. The stimulation of the damaged hearing takes place either mechanically or electrically.

Hearing aids have in principle as essential components an input transducer, an amplifier and an output transducer. The input transducer is usually a sound receiver, z. As a microphone, and / or an electromagnetic receiver, for. B. an induction coil. The output transducer is usually used as an electroacoustic transducer, z. As miniature speaker, or as an electromechanical transducer, z. B. bone conduction, realized. The amplifier is usually integrated in a signal processing unit. This basic structure is in FIG. 1 illustrated by the example of a behind-the-ear hearing aid. In a hearing aid housing 1 for carrying behind the ear, one or more microphones 2 for receiving the sound from the environment are installed. A signal processing unit 3, which is also integrated in the hearing aid housing 1, processes the microphone signals and amplifies them. The output signal of the signal processing unit 3 is transmitted to a loudspeaker or earpiece 4, which outputs an acoustic signal. The sound is optionally transmitted via a sound tube, which is fixed with an earmold in the ear canal, to the eardrum of the device carrier. The power supply of the hearing device and in particular the signal processing unit 3 is effected by a likewise integrated into the hearing aid housing 1 battery. 5

The performance of a hearing aid is determined in accordance with the standard (see IEC 60118-7: 2005) by the achievable output sound pressure level at an input level of 90 dB SPL (Sound Pressure Level). The resulting so-called OSPL 90 playback curve must be optimally adjusted in order to avoid too loud output levels and excessive distortion of the output signal on the one hand and to preclude operation in the saturation region of the listener if the speech intelligibility is insufficient.

In general, there are two methods for limiting the maximum output sound pressure level, as shown in FIG Dillon H .: "Hearing aids" Turramurra, Australia, Boomerang Press (2001 ) is described. On the one hand, so-called "peak clipping" signal peaks can be cut off. Alternatively, high compression rate output level controlled compression compression may be performed, representing the ratio of input level range to output level range. In most cases the output level controlled dynamic compression is used. Only in hearing aids to supply very severe hearing loss, only the signal peaks are cut off.

Common to all methods is the comparison of the output sound pressure level with a certain level threshold. The respective limiting algorithms then become effective when this level threshold is exceeded. For the limitation by output level controlled dynamic compression both a frequency-dependent and frequency-independent threshold level can be specified. With the appropriate choice of frequency-dependent level thresholds, the achievable maximum output sound pressure level in individual frequency ranges is optimized.

Signal processing in digital hearing aids usually takes place in several (eg 48 or 64) channels. Each of these channels is assigned a specific frequency band. In each of the channels, an input signal component is then processed frequency-dependent or channel-specific.

The level thresholds which are frequency-dependent or channel-specific (converted to the frequency bands of the respective channel signal processing) are generally below a frequency-independent, d. H. broadband level threshold to which the broadband overall level of the output signal is related. In addition to the channel-specific output level limitation, the broadband output level limitation, which is applied in the signal flow after the frequency-dependent or channel-specific level limitation, can also act. As a result, however, narrowband signals (eg, sinusoids) at a lower broadband level are limited than broadband (eg, noise-like) signals. As a result, only the frequency-independent level threshold becomes effective for very loud broadband input signals. The associated distortions are very annoying with loud signals.

Furthermore, it is known that the level of a narrowband signal can be higher given the same loudness must be considered the level of a broadband signal. So z. For example, a sinewave signal at the frequency of 1 kHz with the level of 78 dB SPL is equally loud to a uniformly stimulating noise with the level of 60 dB SPL felt like this E. Zwicker, H. Fastl: "Psychoacoustics, Facts and Models", Springer (1999 ). This is in contrast to the above behavior for output level limiting by dynamic compression with fixed frequency dependent thresholds for the output level. Due to this limitation, the loudness of a broadband signal is then much higher than that of a narrowband signal.

The US 5 553 151 A relates to a method and a device for hearing aids for improved processing of acoustic speech signals. For this purpose, an input signal is amplified or attenuated based on output signals of one or more level detectors.

From the US 2007/263891 A1 a hearing aid is known in which a sound pressure of an input signal is detected, and in which a sound pressure of an output signal of the hearing aid is reduced when the sound pressure of the input signal reaches a predetermined threshold value.

The object of the present invention is therefore to provide a method for amplifying an input signal in a hearing device, with which the natural hearing sense can be better taken into account.

According to the invention, this object is achieved by a method for amplifying an input signal in a hearing device by predetermining a channel-specific compression characteristic in a plurality of spectrally separated processing channels of the hearing device, which defines a relationship between an input level and an output level in the respective processing channel of the hearing device, and amplifying a respective input signal component the hearing device in each Processing channel as a function of a channel-specific operating compression characteristic,
Specifying a channel-specific input level threshold for each processing channel, setting the respective channel-specific operating compression characteristic corresponding to the predetermined channel-specific compression characteristic below the channel-specific input level threshold, and setting a respective profile of the channel-specific operating compression characteristic with a compression ratio greater than 8 above the channel-specific input level threshold.

Advantageously, the amplification of an input signal is channel-specific in a plurality of processing channels, each corresponding to a frequency band. In this case, the compression is determined channel-specifically by a respective compression characteristic which depends on the input signal or the proportion of the input signal in the respective channel. Thus, this results in a gain of the input signal, which does not depend on the output level, but on the nature of the input signal. This signal-specific amplification can be realized, which rather takes the natural hearing sense into account.

Preferably, the input level of each channel-specific input signal component is determined with a time constant that is substantially greater than 250 ms. It is therefore a relatively long time constant, d. H. slow processing, which avoids signal distortions.

In addition, a channel-specific output level limit for each processing channel can be specified, wherein the respective channel-specific compression characteristic does not fall below a fixed distance to the channel-specific output level limit. Such a distance to a predetermined output level limit has the advantage that the specific output level limit is not exceeded even if the input level is subject to a certain dynamics, in particular during a settling time.

This fixed distance of the compression characteristic to the channel-specific output level limit should be at least 15 dB. This is therefore favorable, since speech signals have on average a dynamics of +/- 15 dB. The distance should therefore not fall below 15 dB.

According to the invention, a frequency-independent overall level of the input signal components comprising all input signal components is measured, a time is determined at which the measured total level reaches a predetermined total level threshold value for the time in each processing channel the respective channel-specific input level threshold corresponding to the current level of the respective input signal component (whereby the specification of the channel-specific Input level threshold), and the channel-specific operating compression characteristics in all the processing channels are set accordingly. This has the advantage that at higher input levels, the level limit is not signal-specific. Rather, it is thus established that an input signal with a high overall level (broadband) is present, and then the compression or limitation takes place very specifically as a function of the channel or of the frequency.

Here, the channel-specific operating compression characteristic can be kept unchanged as long as the measured total level is greater than or equal to the total level threshold. Thus, if the overall level of the input signal remains very high, a new compression curve need not be constantly determined.

In addition, each channel-specific operating compression characteristic may correspond to the respective given channel-specific compression characteristic when the measured overall level is below the overall level threshold. At low overall levels of the input signal, therefore, the predetermined compression characteristic can be used in the respective channel, without having to determine this as a function of the input signal or input signal component.

In addition, it is favorable if, in each processing channel, the compression is close to 1 in a level interval immediately below the respective channel-specific input level threshold. This has the advantage of natural dynamics of the output signal in the range of the frequency-dependent or channel-specific input level threshold.

Furthermore, a minimum level value can be specified for each channel-specific input level threshold. This may have advantages if narrowband input signals are present. In this case, the level limitation or strong compression is not already at very low input levels.

FIG 1

den schematischen Aufbau einer Hörvorrichtung gemäß dem Stand der Technik;

FIG 2

ein Eingangs-Ausgangs-Pegeldiagramm mit einer Verstärkungsbegrenzung oberhalb einer frequenzabhängigen Eingangspegelschwelle;

FIG 3

spektrale Leistungsdichten am Eingang und Ausgang bei einem breitbandigen Signal; und

FIG 4

spektrale Leistungsdichten am Eingang und Ausgang bei identischem breitbandigen Eingangspegel wie in FIG 3 jedoch einem schmalbandigen Signal.

The present invention will now be explained in more detail with reference to the accompanying drawings, in which:

FIG. 1

the schematic structure of a hearing device according to the prior art;

FIG. 2

an input-output level diagram with a gain limit above a frequency-dependent input level threshold;

FIG. 3

Spectral power densities at the input and output at a broadband signal; and

FIG. 4

spectral power densities at input and output at identical broadband input level as in FIG. 3 but a narrowband signal.

The embodiments described in more detail below represent preferred embodiments of the present invention.

In a hearing device and in particular in a hearing aid, an input signal is typically split by an analysis filter bank into a plurality of input signal components, and the input signal components are processed in a frequency-specific manner in a plurality of channels. There is thus a specific amplification in each channel. At the end of the channel-specific processing, the individual channels are brought together in a synthesis filter bank, which finally results in a broadband output signal.

The proposed solution according to the invention relates to an input-side or input-dependent limitation of the gain to reduce the distortion in loud broadband input signals. The limitation of the gain is determined by a compression characteristic 10 according to FIG FIG. 2 achieved specifically for each channel. The compression characteristic 10 is frequency-dependent and thus channel-specific. From the compression characteristic curve 10 in the input-output level diagram, the output level L _{A of} the hearing device results as a function of the input level L _E at the input of the hearing device.

On the bisector 11 of the input-output level diagram, the output level L _A corresponds to the input level L _E. On the bisector 11 so no gain and the compression ratio is 1. The vertical distance from the bisector 11 to the compression curve 10 corresponds to the level-specific gain, which is caused by the compression curve 10.

In order to realize the input-side limitation of the amplification, a frequency-dependent or channel-specific input level threshold L _{S is} additionally specified for each channel. Such a channel-specific input level threshold L _S divides the operating compression characteristic curve 10 actually used in operation into two halves. Below the input level threshold L _S corresponds to the operating-compression characteristic 10 a predetermined compression characteristic. Above the input level threshold L _S , the operating compression characteristic curve 10 deviates from the predetermined characteristic curve 12 (dotted line in FIG FIG. 2 ). It continues here horizontally steadily. This corresponds to an infinitely high compression ratio. For the present invention, however, it is sufficient if the operating-compression characteristic above the channel-specific or frequency-dependent input level threshold with very low slope, namely with a compression ratio of more than 8. With such a high compression ratio of the input level related dynamic compression above the (fixed) frequency-dependent input level threshold L _S and a slow time constant for signal processing (much greater than 250 ms) result in compression ratios in which the static gain is reduced with a further increase in the input level beyond the input level threshold L _S.

In FIG. 2 Furthermore, a frequency-dependent output level limit L _{G is} shown. It indicates an output level that should not be exceeded at any input level. The output sound pressure generated by the hearing device or the hearing aid should be at least 15 dB below the channel-specific or frequency-dependent output level limit value L _{G in} accordance with the operating compression characteristic curve 10. This is because the language has an average dynamic range of 30 dB (+/- 15dB). Since the measurement of the input level is, for example, in the range of 1 ms, the operating point is fixed only after a certain time. This transient time can cause significant distortion if the level is not limited.

As already indicated, the vertical distance d of the operating compression curve 10 corresponds to the mid perpendicular 11 of the actual applied gain at the respective input level L _E. At low input levels, there is typically a higher gain than at higher input levels Input levels. At very high input levels it is even attenuated.

In the FIG. 2 shown operating compression curve 10 is channel-specific or frequency-dependent and applies here for the frequency f ₁ . For other frequencies, the respective compression characteristic may have a different course.

The channel-specific input level threshold L _S is
not fixed or predetermined. Rather, it is also calculated as a function of a broadband input sound pressure level (ie the frequency-independent total input level). For this purpose, for example, the input level of the respective input signal component is sampled in each channel exactly when the associated frequency-independent or broadband total input level reaches a predetermined frequency-independent level threshold (eg 15 dB below the output level limit value L _G ). It is the break point 13 of the operating compression curve 10 dynamically determined. Accordingly, the break point 13 and the associated input level threshold L _{S may be} low in some of the processing channels and higher in others.

The effect of the dynamic determination of the channel-specific input level threshold L _S as a function of the total input level can be determined on the basis of 3 and 4 be explained. The 3 and 4 represent spectral power densities L at the output 14, 15 and at the input 16, 17, respectively. FIG. 3 applies to a broadband signal BB (eg broadband noise) while FIG. 4 for a narrowband signal SB (eg sinusoidal tone). Both signals have added the same broadband total input level across all channels, ie the area under the dashed curve 16 corresponds to the area under the dashed curve 17. This total input level corresponds to the sum of the individual levels and represents the total energy of the input signal. For example is measured at a total input level of 90 dB.

The FIG. 2 can be seen schematically as a section through the 3 and 4 be considered at the frequency f ₁ . For a given level, the distance d 'between the input 17 and the output 15 results at the frequency f ₁ .

For a broadband noise according to FIG. 3 For example, the distance between spectral power density at the output 14 and at the input 16 is almost constant. This is because the input level L _E hardly varies over the frequency, so according to FIG. 2 almost always the same gain is applied, and thus the spectral power density at the output 14 remains almost constant.

Has the input signal according to FIG. 4 For example, at a frequency f2 a significantly higher spectral power density than at other frequencies, so changes according to FIG. 2 also the gain clearly above the frequency. Since it typically decreases at higher levels, the spacing of the curves 15 and 17 at the frequency f _{2 is} less than at other frequencies.

Is in accordance with a broadband input signal BB FIG. 3 reaches the threshold for the total input level, the levels of the individual channels according to curve 16 are at a medium level and it is in accordance with FIG. 2 applied a corresponding average gain. The energy increase between the input signal and the output signal represents the area between the curves 14 and 16.

In the narrowband signal SB, the threshold for the total input level is reached when the levels around the frequency f _{2 are} very high, while the levels outside this maximum are relatively low. Accordingly, the level maximum at the frequency f _{2 is} less amplified than outside this maximum at the lower level frequencies. The total energy increase results again from the area between the curves 15 and 17. Since in the narrowband signal predominantly low levels, resulting over a large part of the spectrum, a greater gain than the broadband signal BB, so that the area between the curves 15 and 17 is greater than the area between the curves 14 and 16. However, this means that the overall input level of a narrowband signal is amplified more than the overall input level of a wideband signal. Thus, the natural hearing is exploited, because it is a narrow-band signal amplified more than a broadband, the narrow-band amplified signal is then perceived no louder than the broadband amplified signal.

When using slower compression time constants thus obtained for the case of the high compression ratios above the input threshold levels L _S negligible signal distortion. After exceeding the input level thresholds, a sufficiently large distance between the resulting output levels and the frequency-dependent output level limit values L _G prevents distortion of very loud speech signals. Only in a dynamic view, it will be distorted for a short time during the settling time.

By using a frequency-independent total input level threshold, the input-side boundary is independent of the spectral distribution of the signal. However, different frequency-dependent input level thresholds L _{S are} determined, which are dependent on the current spectral distribution of the signal.

According to a development, only compression ratios near 1 are used in a defined input level interval 18 immediately below the frequency-dependent input level threshold L _S. The applied amplification factors are thus level-independent in this interval (same distance of the operating-compression characteristic 10 to the perpendicular bisector 11). In addition, the gain in this area should be significantly reduced compared to the static set gain factors at very low levels. This results in sum, as shown above, despite the identical broadband total input level threshold (= Gesamtpegelschwellwert) a higher output sound pressure level for narrowband signals than for broadband signals. The loudness of the thus limited output signals is thus much better adapted to the psychoacoustic boundary conditions.

Claims (6)

1. Method for boosting an input signal in a hearing apparatus by

   - prescribing one channel-specific compression characteristic (12) each in multiple spectrally separate processing channels of the hearing apparatus, which defines a relationship between an input level (L_E) and an output level (L_A) in the respective processing channel of the hearing apparatus,

   - boosting a respective input signal component of the hearing apparatus in each processing channel on the basis of a channel-specific operating compression characteristic (10),

   - prescribing a channel-specific input level threshold (L_S) for each processing channel,

   - stipulating the respective channel-specific operating compression characteristic (10) according to the prescribed channel-specific compression characteristic (12) below the channel-specific input level threshold (L_S), and

   - stipulating a respective profile of the channel-specific operating compression characteristic (10) with a compression ratio of greater than 8 above the channel-specific input level threshold (L_S),
   characterized

   - in that a frequency-independent overall level of the input signal comprising all the input signal components is measured, a time at which the measured overall level reaches a prescribed overall level threshold value is ascertained, the respective channel-specific input level threshold (L_S) is stipulated for the time in each processing channel according to the present level of the respective input signal component, and the channel-specific operating compression characteristics (10) are stipulated in all the processing channels accordingly.
2. Method according to Claim 1, wherein a channel-specific output level limit value (L_G) is prescribed for each processing channel and the respective channel-specific operating compression characteristic (10) does not become less than a fixed interval from the channel-specific output level limit value (L_G).
3. Method according to Claim 2, wherein the fixed interval is at least 15 dB.
4. Method according to one of the preceding claims, wherein the channel-specific operating compression characteristics (10) are maintained without alteration for as long as the measured overall input level is greater than or equal to the overall level threshold value.
5. Method according to one of the preceding claims, wherein each channel-specific operating compression characteristic (10) corresponds to the respective prescribed channel-specific compression characteristic (12) if the measured overall level is below the overall level threshold value.
6. Method according to one of the preceding claims, wherein a minimum level value is prescribed for each channel-specific input level threshold (L_S).

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