EP2124482A2 - Hearing device with an equalisation filter in a filter bank system - Google Patents

Abstract

The device has a filter bank system with a multi-layer analysis filter bank (AFB) and a multi-stage synthesis filter bank (SFB) to separate an input signal of the hearing apparatus into multiple sub-band signals by a set of filter bank channels and/or to again merge the sub-band signals. The filter bank system has an equalization filter (EQ) to compensate differences of complex frequency responses between the filter bank channels. The equalization filter compensates damping or amplification differences and group delay differences between the filter bank channels.

Description

The present invention relates to a hearing aid with a filter bank system which has a multi-stage analysis filter bank and / or a multi-stage synthesis filter bank to split an input signal of the hearing device through several filter bank channels into a plurality of subband signals and / or to re-assemble subband signals of a plurality of filter bank channels. The term "hearing device" is understood to mean any sound-emitting device which can be worn in or on the ear, in particular a hearing device, 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 shown using 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

Sound signals recorded with one or more microphones of a hearing device and in particular of a hearing device are frequently split into K subband signals by means of one or more frequency-selective digital analysis filter banks (AFB). The subband signals are then subjected to a subband-specific signal manipulation. Finally, the manipulated subband signals are resynthesized by means of a digital synthesis filter bank (SFB). The decomposition and resynthesis are performed by a filter bank composed of at least two cascaded stages or a partially at least two-stage (analysis) filter bank for decomposing the input signal into K subband signals with a reduced sampling rate. The filter bank system is here designed so that the K subband signals to be manipulated have different bandwidths B _i with i = 1,... I, where 2≤I <K.

The entire filter bank system thus consists of a multi-level AFB and a multi-level SFB. The individual filter banks can each be conventional complex-modulated filter banks.

The filter bank outlined above for generating subband signals of different bandwidths B _i, with i = 1,..., I and 2≤I <K effects a delay (group delay) of the K subband signals, which depends on the respective signal or channel bandwidth B _i . This results in between the subband signals or subband signal groups of different bandwidths B _i jumps in the total signal delay, which have a disturbing effect on the signal quality.

From the publication WO 98/02983 For example, a low-noise noise reduction filter is known. An analysis filter bank breaks an input signal into two output channels. The first channel signal is an estimate of a periodic component of the input signal, and the second channel signal is an estimate of a non-periodic component of the input signal. In the first channel, the signal undergoes a delay while the signal in the second channel passes through a noise reduction filter.

Furthermore, in Göckler, Heinz G .; Groth Alexandra: Multiratensysteme Sample rate conversion and digital filter banks, Wildburgstetten, Schlemmbachverlag 2004, p. 397 to 399 , discloses a maximum decimation M-channel analysis filter bank in tree structure. The filter bank has three stages.

The object of the present invention is therefore to improve the signal quality in the processing of signals in hearing devices using multi-stage filter banks.

According to the invention, this object is achieved by a hearing device with a filter bank system which has a multi-stage analysis filter bank and / or a multi-stage synthesis filter bank to split an input signal of the hearing device through a plurality of filter bank channels into a plurality of subband signals and / or subband signals of a plurality of filter bank channels reassemble, with the filter bank system equipped with at least one equalization filter for differences equalize the frequency responses between filter bank channels.

In an advantageous manner, it is possible with the equalization filter (equalizer) to compensate for the different group delays and, if appropriate, different magnitude profiles of the frequency responses of the filter bank channels or to smooth or smooth out time-lapse jumps.

Thus, discontinuities in the frequency response of the filter bank system can be eliminated and related disturbances suppressed.

Preferably, both the analysis filter bank and the synthesis filter bank are multi-level, and the equalization filter is arranged between two hierarchical levels of filters of the filter bank system. Alternatively, the equalization filter may be located at the bottom of the analysis filterbank or the synthesis filterbank. This only requires one or more equalizers that operate at the lowest sampling rate and thus require less processing power.

Still alternatively, the equalization filter may be located in the topmost stage of the synthesis filterbank. The advantage of this is that the group delay / magnitude frequency response transition over the maximum frequency width, namely the entire signal bandwidth can be distributed.

Preferably, the equalization filter is placed in the synthesis filterbank. It can also be used to equalize distortions that originate from the analysis filter bank.

Preferably, at least two pairs of adjacent filter banks are present in the filter bank system, which have different bandwidth in relation to each other, so that in each filter bank pair two filter bank channels different width next to each other, and in the wider of the two filter bank channels each have an equalization filter for group delay increase is arranged. This makes it possible to easily achieve a steady transition of the group delay at the subband boundaries.

FIG 1

den prinzipiellen Aufbau eines Hörgeräts gemäß dem Stand der Technik;

FIG 2

die Struktur einer gesamten Filterbank-Kaskade aus AFB und SFB mit Equalizer;

FIG 3

ein Gruppenlaufzeitdiagramm über mehrere Teilbänder der Filterbänke von FIG 2;

FIG 4

die Struktur eines Entzerrungsfilters realisiert als Kaskade von rekursiven Strukturen zweiter Ordnung;

FIG 5

eine Allpassstruktur mit minimaler Multipliziereranzahl;

FIG 6

einen Signalflussgrafen eines Allpasses ersten Grades;

FIG 7

einen Signalflussgrafen eines Allpasses zweiten Grades;

FIG 8

ein Gruppenlaufzeitdiagramm mit Sprungkompensation;

FIG 9

die Spezifikation eines komplexen Entzerrungsfilters und

FIG 10

die Spezifikation eines reellen Entzerrungsfilters.

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

FIG. 1

the basic structure of a hearing aid according to the prior art;

FIG. 2

the structure of an entire filter bank cascade of AFB and SFB with equalizer;

FIG. 3

a group delay diagram over several subbands of the filter banks of FIG. 2 ;

FIG. 4

the structure of an equalization filter realized as a cascade of second-order recursive structures;

FIG. 5

an allpass structure with minimum multiplier number;

FIG. 6

a signal flow graph of a first-degree allpass;

FIG. 7

a signal flow graph of a second-degree allpass;

FIG. 8

a group delay diagram with jump compensation;

FIG. 9

the specification of a complex equalization filter and

FIG. 10

the specification of a real equalization filter.

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

In FIG. 2 is a filter bank cascade consisting of a multi-stage analysis filter bank (AFB) and a multi-stage synthesis filter bank (SFB) shown. The exemplary filter bank is used for signal processing in a hearing device and in particular in a hearing aid. The input filter bank (FB1) of the AFB divides the input signal into four channels. The output-side filter banks FB2A, FB2B, FB2C and FB2D further divide the four channels into ultimately 24 channels. The lowest channel of the FB1 is split by the FB2A into twelve channels, while the remaining three channels of the FB1 are divided into four channels using the output-side filter banks FB2B, FB2C and FB2D. The input sampling rate of the FB1 is 4 kHz, for example. The sampling rate between the two filter bank stages f _Zw is 6 kHz in the example chosen. The sampling rates in the subband channels at the output of the AFB are in the high frequency groups that is after the filter banks FB2B, FB2C and FB2D each 3kHz. The sampling rate after the filter bank FB2A of the lower frequency group is 1.2 kHz. In this case, a downward scanning is advantageously carried out here.

Subsequent to the AFB, a subband specific signal manipulation is performed, which in FIG. 2 but not shown. For the sake of clarity, the AFB in FIG. 2 directly the SFB for the resynthesis of the signal. The SFB is constructed symmetrically to the AFB with regard to the filter banks in the individual stages. Accordingly, the filter banks FB3A, FB3B, FB3C and FB3D are located in the lowest stage of the SFB, which combine twelve or four subband signals into one signal. The four resulting signals at a sampling rate of 6 kHz are fed to the higher synthesis stage FB4, which composes the signals into an output signal with a sampling rate of 24 kHz.

The wider filter banks FB2A and FB3A in the lower frequency group also lead here to an increased group delay τ _g compared to the next higher frequency group with the narrower filter banks FB2B and FB3B. This can FIG. 3 be taken clearly. For the sake of clarity, only the effects of the filter banks FB3A, FB3B and FB3C of the synthesis filter bank are shown there. At the border between the two filter banks FB3A and FB3B, there would be a group delay jump, which is indicated by dashed lines. However, such a jump would lead to disturbances in the output signal.

According to the invention, the filter bank FB3B is therefore followed by an equalization filter (equalizer EQ). This equalization filter EQ increases the group delay of the filter bank FB3B at the upper (higher frequency) band edge to the value of the group delay of the filter bank FB3A at its lower band edge. Thus, the results in FIG. 3 Solid, continuous curve between the two filter banks FB3A and FB3B. Disturbances in the output signal due to group delay differences of the filter banks can thus largely be avoided. However, the equalization filter EQ can also be arranged at other locations in the AFB-SFB system. As a result, for example, the dotted transition of the group delay from the value of the filter bank FB3A to the value of the filter bank FB3C would be FIG. 3 possible (see below for details).

In accordance with the principles of the present invention, an AFB SFB system is generally provided with at least one equalizer (EQ) to reduce group delay differences and / or attenuation / gain differences between different bandwidth filter bank channels B _i . The equalization function should always refer to the case where the subband signals of the AFB-SFB filter bank are not subject to manipulation, ie a so-called "idle state" exists. The aim of the approximation method is not the absolute approximation of the properties of the filter bank channels of different bandwidth, but to extend the abrupt, limited to a very narrow band frequency ranges transitions of the transmission properties to a wider frequency band in order to avoid disturbing artifacts. In general, therefore, the equalization filter is used to increase group delays in certain subbands or to modify attenuations / amplifications in the desired manner. In a special case, this could also be based on the example of FIG. 3 the group delay of the filter bank FB3B at the upper band edge is increased from the value of the group delay of the filter bank FB3C to the value of the group delay of the filter bank FB3A at the lower band edge.

In the following, further exemplary embodiments for an arrangement of one or more equalization filters EQ in the filter bank system are shown:

For example, an equalization filter EQ can also be integrated into the AFB. In particular, it could be analogous to the example of FIG. 2 be connected between the output of the filter bank FB1 and the input of the filter bank FB2B. With several microphones, which also require several AFB, this would lead to an increased effort.

According to a further embodiment, an equalization filter could be provided at the lowest level of the subbands in the wider (3kHz) channel with the lowest center frequency. In this case, the transition region is extended only over one channel (the 3 kHz bandwidth), while it may extend over 3 x 3 kHz when the equalization filter is arranged at a higher level. Alternatively, one equalizing filter must be used in each of four adjacent 3 kHz channels. The advantage of using only one or two equalization filters at this lowest level is that they can work at the lowest sampling rate and thus generally require less computing power.

In another embodiment, the equalization filter EQ becomes the highest level of the cascaded filterbank system here at the output of the filter bank FB4, arranged. Although this requires a higher sampling rate and thus a higher outlay, the group delay or magnitude frequency response transition can be distributed over the maximum frequency width, ie the entire signal bandwidth (see dotted line in FIG FIG. 3 )

In another embodiment, the filter bank system has more than two different bandwidths. At each transition between adjacent channels of different bandwidth, an equalization filter EQ is provided. In this case, the equalization filter is to be arranged in each case in the channel with the larger bandwidth, since there it must increase the group delay. In the case of magnitude frequency equalization, the amplifying or attenuating equalizing filter EQ may also be arranged in the other channel.

As the embodiments presented above show, the introduction according to the invention of equalizers or equalization filters EQ in individual filter bank channels at a different hierarchical level avoids abrupt transitions of the attenuation / amplification and / or the group delay. Particular advantages arise when the smallest possible number of equalization filters EQ is used, in which they are placed at those points where they are most effective. But they can also be located where they cause the least amount of computation.

1. 1. Rekursive (IIR) Realisierung des Entzerrers EQ mit einer der beiden Direktformen (= 1. und 2. kanonische Form aus Karl-Dirk Kammeyer, Kristian Kroschel: "Digitale Signalverarbeitung, Filterung und Spektralanalyse mit MATLAB-Übungen", 6. Auflage, Teubner Verlag 2006, Kapitel 4.1, Seiten 78 ff .) mit hoher Koeffizientenempfindlichkeit. Eine weitere Realisierungsmöglichkeit besteht in der Kaskadenform (= 3. kanonische Form; hierzu ebenso K-D Kammeyer et al. a.a.O.) mit geringer Koeffizientenempfindlichkeit. FIG 4 zeigt eine derartige Struktur des Entzerrungsfilters EQ. Es verhält sich beispielsweise wie ein Allpass und stellt eine herkömmliche Kaskade aus rekursiven Filtern zweiter Ordnung dar. Die Filterkoeffizienten von EQ sind beispielsweise mit der MATLAP-Funktion tf2sos in diese Form umzurechnen. Aus dem Allpass sechster Ordnung ergeben sich so drei Sektionen (γ = 2, 3) zweiter Ordnung mit dem Verstärkungsfaktor g_•, dem Koeffizienten des FIR-Teils b_{0, •}, b_{1, •}, b_{2, •} und den Koeffizienten des IIR-Teils a_{1, •}, a_{2, •}. Schließlich lässt sich der Entzerrer EQ auch mit einer Parallelform (= 4. kanonische Form; vgl. ebenso K-D Kammeyer et al. a.a.O.) mit geringer Koeffizientenempfindlichkeit realisieren.
2. 2. Nichtrekursive (FIR) Realisierung des Entzerrers EQ mit einer der beiden Direktformen (= 1. und 2. kanonische Form) mit in diesem Fall geringer Koeffizientenempfindlichkeit, aber auch mit der Kaskadenform (= 3. kanonische Form) mit geringer Koeffizientenempfindlichkeit (vgl. hierzu ebenso K-D Kammeyer et al. a.a.O.).
3. 3. Ausführung des Entzerrers zur kombinierten Entzerrung von Betragsfrequenzgang und Gruppenlaufzeit: Realisierung als IIR-System oder als FIR-System mit nicht symmetrischer Impulsantwort (Koeffizienten) gemäß obiger Punkte 1 bzw. 2.
4. 4. Ausführung des Entzerrers zur ausschließlichen Entzerrung von Betragsfrequenzgängen von Filterbankkanälen: Realisierung als IIR-System oder als linearphasiges FIR-System mit symmetrischer Impulsantwort (Koeffizienten) gemäß obiger Punkte 1 bzw. 2.
5. 5. Ausführung des Entzerrers zur ausschließlichen Entzerrung der Gruppenlaufzeit von Filterbankkanälen: Realisierung als IIR-Allpass gemäß obigem Punkt 1. Außerdem kann der Entzerrer gemäß FIG 5 als sehr effizienter Allpass realisiert werden (vgl. K-D Kammeyer et al., Kapitel 4.3 "Allpässe"). Die Allpassstruktur von FIG 5 ist zwar bezüglich der Speicher nicht kanonisch, da 2n Speicherelemente für ein System n-ter Ordnung benötigt werden, sie kommt dafür aber mit der Minimalzahl von Multiplizierern, nämlich n+1, aus. Vom Standpunkt des Realisierungsaufwandes bietet diese Struktur gegenüber der kanonischen Form daher Vorteile.
   Der Entzerrer kann auch hier in Kaskadenform realisiert sein, wobei jeder Block erster oder zweiter Ordnung ein bzw. zwei Verzögerungsglieder und einen (zwei) Mulizplizierer benötigt. Ein entsprechender kanonischer Allpass erster Ordnung mit einem einzigen Multiplizierer ist in FIG 6 wiedergegeben, während ein kanonischer Allpass zweiter Ordnung mit zwei Multiplizierern in FIG 7 dargestellt ist.
6. 6. Die Angleichung der Gruppenlaufzeit kann stetig durch einen Allpass in dem Entzerrungsfilter EQ erfolgen. In FIG 8 ist der Gruppenlaufzeitsprung 10 dargestellt, der ohne das Laufzeitfilter EQ auftritt. Damit die Gruppenlaufzeit möglichst monoton verändert wird, sind die einzelnen Übergangsbereiche 11, 12, 13 und 14 der Filterübertragungsfunktionen H₀, H₁, H₂ der Filterbänke FB3A, FB3B und FB3C zu beachten. Soll die Gruppenlaufzeit anstelle des Sprungs 10 monoton verlaufen, so kann durch das Entzerrungsfilter EQ beispielsweise die Gruppenlaufzeit addiert werden, die sich in FIG 8 unter der gestrichelten Linie 15 ergibt, welche die Übergangsbereiche 12 und 13 verbindet (vgl. auch FIG 3). Möchte man des Weiteren die Gruppenlaufzeit für höhere Frequenzen möglichst niedrig halten, so kann der Übergangsbereich für die Gruppenlaufzeit weiter eingeschränkt werden. Der Verlauf der Gruppenlaufzeit kann dann entsprechend der durchgezogenen Linie 16 etwas steiler gehalten werden.

The equalization filter EQ, with which transitions of the transmission characteristics limited to a very narrow band frequency range can be extended to a wider frequency band, can be realized in a variety of ways. Below are some concrete implementation forms listed:

1. 1. Recursive (IIR) realization of the equalizer EQ with one of the two direct forms (= 1st and 2nd canonical form Karl-Dirk Kammeyer, Kristian Kroschel: "Digital Signal Processing, Filtering and Spectral Analysis with MATLAB Exercises ", 6th edition, Teubner Verlag 2006, chapter 4.1, pages 78 ff .) with high coefficient sensitivity. A further possibility of realization exists in the cascade form (= 3rd canonical form, here also KD Kammeyer et al., Supra) with low coefficient sensitivity. FIG. 4 shows such a structure of the equalization filter EQ. For example, it behaves like an all-pass and represents a conventional cascade of second-order recursive filters. The filter coefficients of EQ are to be converted into this form, for example, with the MATLAP function tf2sos. The sixth-order allpass produces three sections (γ = 2, 3) of the second order with the gain factor g _• , the coefficient of the FIR part b _{0, •} , b _{1, •} , b _{2, •} and the coefficients of the IIR Part a _{1, •} , a _{2, •} . Finally, the equalizer EQ can also be realized with a parallel form (= 4th canonical form, see also KD Kammeyer et al., Supra) with low coefficient sensitivity.
2. 2. Nonrecursive (FIR) realization of the equalizer EQ with one of the two direct forms (= 1st and 2nd canonical form) with in this case low coefficient sensitivity, but also with the cascade form (= 3rd canonical form) with low coefficient sensitivity (cf. KD Kammeyer et al., supra).
3. 3. Equalizer for combined equalization of magnitude frequency response and group delay: Implementation as IIR system or as FIR system with non-symmetric impulse response (coefficients) according to points 1 and 2 above.
4. 4. Execution of the equalizer for the exclusive equalization of magnitude frequency responses of filter bank channels: realization as an IIR system or as a linear phase FIR system with symmetric impulse response (coefficients) according to points 1 and 2 above.
5. 5. Execution of the equalizer for the exclusive equalization of the group delay of filter bank channels: realization as IIR all-pass according to item 1 above FIG. 5 be implemented as a very efficient all-pass (see KD Kammeyer et al., Chapter 4.3 "All-passes"). The allpass structure of FIG. 5 Although it is not canonical with respect to the memories, since 2n memory elements are needed for an nth order system, it does so with the minimum number of multipliers, n + 1. From the point of view of the implementation effort, this structure therefore offers advantages over the canonical form.
   The equalizer can also be realized here in cascade form, with each first or second order block requiring one or two delay elements and one (two) multipliers. A corresponding first order canonical allpass with a single multiplier is in FIG. 6 while a second-order canonical all-pass with two multipliers in FIG. 7 is shown.
6. 6. The equalization of the group delay can be done continuously by an allpass in the equalization filter EQ. In FIG. 8 the group delay 10 is shown, which occurs without the delay filter EQ. So that the group delay is changed as monotone as possible, the individual transition regions 11, 12, 13 and 14 of the filter transfer functions H ₀ , H ₁ , H _{2 of} the filter banks FB3A, FB3B and FB3C are observed. If the group delay is to be monotonic instead of jump 10, the group delay can be added by the equalization filter EQ, for example FIG. 8 below the dashed line 15, which connects the transition regions 12 and 13 (cf. FIG. 3 ). Furthermore, if one wishes to keep the group delay for higher frequencies as low as possible, then the transition range for the group delay can be further restricted. The course of the group runtime can then be kept slightly steeper according to the solid line 16.

The equalization filter EQ can be further optimized by designing the simplest possible all-pass according to the specification FIG. 8 contains approximately. This is in FIG. 9 First, the complex-valued specification (resulting from the processing of the signals by, for example, complex-modulated filter bank) of an all-pass normalized to the sampling rate f _zw applied in the subband. The dashed line 17 describes a drop in the additionally introduced group delay, which is technically necessary for a perfect superposition of at least the subbands, and the solid line 18 a steeper drop in the sense of the lowest possible group delay for higher frequencies. Instead of a complex equalizer according to FIG. 9 but may also be a real equalizer accordingly FIG. 10 be used. The artefacts created by the symmetrical parts do not disturb here. The construction of a real filter is, however, much easier than that of a complex filter, so that here the real filter is to be preferred.

The above-described implementation forms, individually or in combination with each other, enable an equalizer to be implemented in individual filter bank channels in one or more hierarchical levels in order to avoid abrupt transitions of the attenuation / amplification and / or the group delay.

Claims (8)

Hearing device with a filter bank system which has a multi-stage analysis filter bank (AFB) and / or a multi-stage synthesis filter bank (SFB) to split an input signal of the hearing device through a plurality of filter bank channels into a plurality of subband signals and / or to reassemble subband signals of a plurality of filter bank channels, characterized in that - The filter bank system is equipped with at least one equalization filter (EQ) to compensate for differences in complex frequency responses between filter bank channels. Hearing apparatus according to claim 1, wherein with the equalization filter (EQ) group delay differences between the filter bank channels are compensated. Hearing apparatus according to claim 1 or 2, wherein with the equalization filter (EQ) attenuation or gain differences between the filter bank channels are compensated. Hearing apparatus according to one of the preceding claims, wherein both the analysis filter bank (AFB) and the synthesis filter bank (SFB) are constructed in multiple stages and the equalization filter (EQ) is arranged between two hierarchical levels of filters of the filter bank system. Hearing apparatus according to one of the preceding claims, wherein the equalization filter (EQ) is arranged in the synthesis filter bank (SFB). Hearing apparatus according to one of the preceding claims, wherein the equalization filter (EQ) in the lowest stage of the analysis filter bank (AFB) or the synthesis filter bank (SFB) is arranged. Hearing apparatus according to one of claims 1 to 5, wherein the equalization filter (EQ) is arranged in the uppermost stage of the synthesis filter bank (SFB). Hearing apparatus according to one of claims 1 to 6, wherein in the filter bank system at least two pairs of adjacent filter banks are present, each having adjacent filter banks in relation to each other channels of different bandwidth, so that in each filter bank pair two filter bank channels of different widths are adjacent, and in the wider of the two filter bank channels each equalization filter (EQ) is arranged for group delay increase.

Images