AU2013202444B2

AU2013202444B2 - Method for restricting the output level in hearing apparatuses

Info

Publication number: AU2013202444B2
Application number: AU2013202444A
Authority: AU
Inventors: Oliver Dressler
Original assignee: Sivantos Pte Ltd
Current assignee: Sivantos Pte Ltd
Priority date: 2012-04-05
Filing date: 2013-04-04
Publication date: 2014-11-20
Anticipated expiration: 2033-04-04
Also published as: DK2648424T3; AU2013202444A1; EP2648424B1; EP2648424A2; DE102012205651A1; EP2648424A3; DE102012205651B4

Abstract

Abstract Method for restricting the output level in hearing apparatuses 5 Psychoacoustic boundary conditions are to be better taken into account in the amplification of input signals of a hear ing apparatus. To this end, a method is defined for amplifying an input signal in a hearing apparatus by predetermining each channel-specific compression characteristic curve (12) in a 0 number of spectrally-separated processing channels of the hearing apparatus, which defines a relationship between an in put level (LE) and an output level (LA) in the respective pro cessing channel of the hearing apparatus, and amplifying a re spective input signal portion of the hearing apparatus in each 5 processing channel as a function of a channel-specific operat ing compression characteristic curve (10). A channel-specific input level threshold (Ls) is predefined here for each pro cessing channel. Finally, the respective channel-specific op erating compression characteristic curve (10) is then defined 0 according to the predetermined channel-specific compression characteristic curve (12) below the channel-specific input level threshold (Ls) and the respective curve of the channel specific operating compression characteristic curve (10) is defined with a compression ratio of greater than 8 above the 5 channel-specific input level threshold (Ls). (Prior art) -5 d 11l

Description

1 Method for restricting the output level in hearing apparatuses TECHNICAL FIELD The present invention relates to a method for amplifying an 5 input signal in a hearing apparatus by predefining each chan nel-specific compression characteristic curve in a number of spectrally-separated processing channels of the hearing appa ratus, which defines a relationship between an input level and an output level in the respective processing channel of the 0 hearing apparatus and amplifying a respective input signal portion of the hearing apparatus in each processing channel as a function of a channel-specific operating compression charac teristic curve. The term "hearing apparatus" is understood here to mean any auditory stimulus-producing device which can 5 be worn in or on the ear, in particular a hearing device, a headset, earphones or suchlike. BACKGROUND Hearing devices are wearable hearing apparatuses which are 0 used to provide hearing assistance to the hard-of-hearing. In order to accommodate the numerous individual requirements, various designs of hearing devices are available such as be hind-the-ear (BTE) hearing devices, hearing device with exter nal earpiece (RIC: receiver in the canal) and in-the-ear (ITE) 5 hearing devices, for example also concha hearing devices or completely-in-the-canal (ITE, CIC) hearing devices. The hear ing devices listed as examples are worn on the outer ear or in the auditory canal. Bone conduction hearing aids, implantable or vibrotactile hearing aids are also available on the market. 0 With these devices the damaged hearing is stimulated either mechanically or electrically. -7 ' 00 ) 7Q 0 2 The key components of hearing devices are principally an input transducer, an amplifier and an output transducer. The input transducer is normally a sound transducer e.g. a microphone and/or an electromagnetic receiver, e.g. an induction coil. 5 The output transducer is most frequently realized as an elec troacoustic transducer, e.g. a miniature loudspeaker, or as an electromechanical transducer, e.g. a bone conduction receiver. The amplifier is usually integrated into a signal processing unit. This basic configuration is illustrated in FIG 1 using 0 the example of a behind-the-ear hearing device. One or more microphones 2 for picking up ambient sound are incorporated into a hearing device housing 1 to be worn behind the ear. A signal processing unit 3 which is also integrated into the hearing device housing 1 processes and amplifies the micro 5 phone signals. The output signal from the signal processing unit 3 is transmitted to a loudspeaker or receiver 4, which outputs an acoustic signal. The sound may be transmitted to the device wearer's eardrum by way of an acoustic tube which is fixed in the auditory canal by means of an earmold. Power 0 for the hearing device and in particular for the signal pro cessing unit 3 is supplied by means of a battery 5 which is also integrated in the hearing device housing 1. The performance of a hearing device is by standard (see IEC 5 60118-7:2005) determined by the achievable output sound pres sure level at an input level of 90 dB SPL (Sound Pressure Lev el). The so-called OSPL 90 reproduction curve resulting there from must be adjusted optimally in order on the one hand to prevent an excessively loud output level and excessive distor 0 tions of the output signal and on the other hand to rule out operation in the saturation range of the receiver in the event of inadequate speech intelligibility. -7 ' 00 ) 7Q 0 3 Two methods of restricting the maximum output sound pressure level are generally available, such as described in Dillon H.: "Hearing aids" Turramurra, Australia, Boomerang Press (2001). On the one hand, so-called "peak clipping" cuts off signal 5 peaks. Alternatively, an output level-controlled dynamic com pression (compression limiting) can be implemented with a high compression rate, which shows the relationship between the in put level range and the output level range. In most instances, the output level-controlled dynamic compression is used. The 0 signal peaks are only cut off in hearing devices covering very significant hearing losses. Common to all methods is the comparison of the output sound pressure level with a specific level threshold. The respective 5 restriction algorithms are then effective if this level threshold is exceeded. For restriction by means of output lev el-controlled dynamic compression, both a frequency-dependent and also a frequency-independent level threshold can be prede termined. The achievable maximum output sound pressure level 0 is optimized in individual frequency ranges with the suitable selection of frequency-dependent level thresholds. The signal processing in digital hearing devices usually takes place in a number of (e.g. 48 or 64) channels. A specific fre 5 quency band is assigned to each of these channels. An input signal portion is then processed in each of the channels in a frequency-dependent and/or channel-specific manner. The frequency-dependent and/or channel-specific level thresh 0 olds (converted into the frequency bands of the respective channel signal processing) for the restriction generally lie below a frequency-independent, i.e. broadband level threshold, to which the broadband overall level of the output signal is -7 ' 00 ) 7Q 0 4 related. Aside from the channel-specific output level re striction, the broadband output level restriction can also take effect, which is applied in the signal flow after the frequency-dependent and/or channel-specific level restriction. 5 As a result, narrowband signals (e.g. pure tones) in a lower broadband level are however restricted as broadband (e.g. low noise) signals. This results in only the frequency-independent level threshold being effective for very loud broadband input signals. The distortions associated therewith are very bother 0 some in the case of loud signals. It is further known, assuming the same loudness, that the lev el of a narrowband signal has to be higher than the level of a broadband signal. Therefore a pure signal at the frequency 1 5 kHz with the level 78 dB SPL is for instance equally as loud as a uniformly excited noise with the level of 60 dB SPL, as apparent from E. Zwicker, H. Fastl: "Psychoacoustics, Facts and Models", Springer (1999). This is contrary to the above behavior in an output level restriction by dynamic compression 0 with fixed frequency-dependent thresholds for the output lev el. This restriction then renders the loudness of a broadband signal significantly higher than that of a narrowband signal. Thus, a need exist to provide a method for amplifying an input 25 signal in a hearing apparatus, with which better allowance can be made for the natural hearing perception. SUMMARY According to an aspect of the present disclosure, there is 30 provided a method for amplifying an input signal in a hearing apparatus by predefining a channel-specific compression char acteristic curve in each of a number of spectrally-separated processing channels of the hearing apparatus, which defines a relationship between an input level and an output level in the 9296978 5 respective processing channel of the hearing apparatus, prede fining a channel-specific input level threshold for each pro cessing channel, defining the respective channel-specific op erating compression characteristic curve according to the pre 5 defined channel-specific compression characteristic curve be low the channel-specific input level threshold, defining a re spective curve of the channel-specific operating compression characteristic curve with a compression ratio of greater than 8 above the channel-specific input level threshold, and ampli 0 fying a respective input signal portion of the hearing appa ratus in each processing channel as a function of a channel specific operation compression characteristic curve. The amplification of an input signal advantageously therefore 5 takes place in a channel-specific manner in a number of pro cessing channels which correspond to a frequency band respec tively. Here the compression is defined in a channel-specific manner by a compression characteristic curve in each instance, which depends on the input signal and/or on the portion of the 0 input signal in the respective channel. An amplification of the input signal therefore results, which does not depend fix edly on the output level, but instead on the nature of the in put signal. A signal-specific amplification can thus be real ized, which makes more allowance for the natural hearing per 25 ception. The input level of each channel-specific input signal portion is preferably determined with a time constant which is essen tially larger than 250 ms. A relatively long time constant, 30 i.e. a slow processing, therefore exists, as a result of which signal distortions are prevented. 9296978 6 Furthermore, a channel-specific output level limit value can be predefined for each processing channel, wherein the respec tive channel-specific compression characteristic curve does not reach a fixed distance from the channel-specific output 5 level limit value. Such a distance from a predetermined output level limit is advantageous in that the specific output level limit is also then not exceeded if the input level lies there below particularly during a settling time of a specific dynam ic. 0 This fixed distance between the compression characteristic curve and the channel-specific output level limit value should amount to at least 15 dB. This is therefore favorable since speech signals average a dynamic of +/-15 dB. The distance 5 should therefore not fall below 15 dB. According to a further aspect, a frequency-independent overall level of the input signal encompassing all input signal por tions is measured, a point in time is determined, at which the 0 measured overall level reaches a predefined overall level threshold value, for the point in time in each process channel at which the respective channel-specific input level threshold accordingly defines the momentary level of the respective in put signal portion (as a result of which the channel-specific 5 input level threshold is predefined) and the channel-specific operating compression characteristic curves are defined ac cordingly in all processing channels. This is advantageous in that with higher input levels, the level restriction does not take place in a non-specific manner in terms of signals. In 0 stead, it is therefore defined that an input signal exists with a high overall level (broadband) and the compression and/or restriction then takes place very specifically as a function of the channel and/or frequency. -7 ' 00 ) 7Q 0 7 In this case the channel-specific operating compression char acteristic curve can be kept unchanged as long as the measured overall level is greater than or equal to the overall level 5 threshold value. Thus if the overall level of the input signal remains very high, a new compression characteristic curve need not be determined continuously. Furthermore, each channel-specific operating compression char 0 acteristic curve can correspond to the respective predefined channel-specific compression characteristic curve if the meas ured overall value lies below the overall level threshold val ue. With low overall levels of the input signal, the prede fined compression characteristic curve can therefore be used 5 in the respective channel without this having to be determined as a function of the input signal and/or input signal portion. It is furthermore favorable if in each processing channel the compression in a level interval directly below the respective 0 channel-specific input level threshold lies close to 1. This is advantageous in terms of a natural dynamics of the output signal in the region of the frequency-dependent and/or chan nel-specific input level threshold. 5 Furthermore, a minimal level value can be predefined for each channel-specific input level threshold. This is advantageous in some instances if narrowband input signals exist. In this case, the level restriction and/or strong compression does not take place with very small input levels. 30 BRIEF DESCRIPTION OF THE DRAWINGS The present invention is now explained in more detail with the aid of the appended drawings, in which: -7 ' 00 ) 7Q 0 8 FIG 1 shows the schematic layout of a hearing apparatus ac cording to the prior art; 5 FIG 2 shows an input-output level diagram with an amplifi cation restriction above a frequency-dependent input level threshold; FIG 3 shows spectral power densities at the input and out 0 put for a broadband signal; and FIG 4 shows spectral power densities at the input and out put with an identical broadband input level as in FIG 3 but nevertheless with a narrowband signal. 5 The embodiments explained in more detail below represent pre ferred exemplary embodiments of the present invention. DETAILED DESCRIPTION 0 An input signal in a hearing apparatus and in particular in a hearing device is typically broken down into a number of input signal portions by an analysis filter bank and the input sig nal portions are further processed in a number of channels in a frequency-specific manner. A specific amplification there 5 fore takes place in each channel. The individual channels are combined in a synthesis filter bank at the end of the channel specific processing, as a result of which a broadband output signal finally results. 0 The inventively proposed solution relates to an input side or input-dependent restriction of the amplification in order to reduce the distortions in the case of loud broadband input signals. The amplification is restricted by a compression -7 ' 00 ) 7Q 0 9 characteristic curve 10 according to FIG 2 especially for each channel. The compression characteristic curve 10 is frequency dependent and thus channel-specific. The output level LA Of the hearing apparatus results from the compression characteristic 5 curve 10 in the input-output level diagram as a function of the input level LE at the input of the hearing apparatus. The output level LA corresponds to the input level LE on the angle bisector 11 of the input-output level diagram. No ampli 0 fication therefore takes place on the angle bisector 11 and the compression ratio amounts to 1. The vertical distance of the angle bisector 11 from the compression characteristic curve 10 corresponds to the level-specific amplification caused by the compression characteristic curve 10. 5 In order now to realize the input-side restriction in amplifi cation, a frequency-dependent and/or channel-specific input level threshold Ls is also predefined for each channel. Such a channel-specific input level threshold Ls divides the operating 0 compression characteristic curve 10 actually used during oper ation into two halves. Below the input level threshold Ls, the operating compression characteristic curve 10 corresponds to a predetermined compression characteristic curve. Above the in put level threshold Ls, the operating compression characteris 5 tic curve 10 deviates from the predetermined characteristic curve 12 (dotted line in FIG 2). It proceeds here horizontally continuously further. This corresponds to an infinitely high compression behavior. It is sufficient for the present inven tion for the operating compression characteristic curve to 0 proceed above the channel-specific and/or frequency-dependent input level threshold with a very minimal gradient, namely with a compression ratio of more than 8. With a high compres sion ratio of the input level-related dynamic compression of W) 0Q' 7Q 0 10 this type above the (fixed) frequency-dependent input thresh old Ls and a slow time constant for the signal processing (es sentially greater than 250 ms), compression ratios result in which the static amplification is reduced with a further in 5 crease in the input level above the input level threshold Ls. A frequency-dependent output level limit value LG is also shown in FIG 2. It identifies an output level which is not to be ex ceeded with any input level. The output sound pressure gener 0 ated by the hearing apparatus and/or the hearing device is to lie according to the operating compressing characteristic curve 10 at least 15 dB below the channel-specific or frequen cy-dependent output level limit value LG. This is due to the fact that speech averages a dynamic range of 30 dB (+/- 15dB). 5 Since the measurement of the input level lies for instance in the range of 1 ms, the working point is only established after a specific time. In this settling time, it may result in sig nificant distortions if the level is not restricted. 0 As already indicated, the vertical distance d of the operating compression characteristic curve 10 from the perpendicular bi sector 11 corresponds to the actually applied amplification with the respective input level LE. With low input levels, a higher amplification typically takes place than with higher 5 input levels. With very high input levels, it is even attenu ated. The operating compression characteristic curve 10 shown in FIG 2 is channel-specific and/or frequency-dependent and ap 0 plies here to the frequency fi. The respective compression characteristic curve may possess another curve for other fre quencies. -7qQQq)7Q 1 11 The channel-specific input level threshold Ls need not be fix edly set and/or predefined. Instead, it can also be calculated as a function of a broadband input sound pressure level (i.e. of the frequency-independent overall input level). To this 5 end, the input level of the respective input signal portion is then accurately scanned for instance if the associated fre quency-independent and/or broadband overall input level lies [below] a predefined frequency-independent level threshold (e.g. 15 dB below the output level limit threshold LG) . The 0 break-point 13 of the operating compression characteristic curve 10 is dynamically determined in this way. Accordingly, the break-point 13 and/or the associated input level threshold Ls may be low in some of the processing channels and higher in others. 5 The effect of the dynamic definition of the channel-specific input level threshold Ls as a function of the overall input level can be explained with the aid of FIG 3 and 4. FIG 3 and 4 each represent spectral power densities 1 at the output 14, 0 15 and at the input 16, 17. FIG 3 applies to a broadband sig nal BB (e.g. broadband noise), while FIG 4 applies to a nar rowband signal SB (e.g. pure tone). Both signals have added the same broadband overall input level across all channels, i.e. the area below the dashed curve 16 corresponds to the ar 5 ea below the dashed curve 17. This overall input level corre sponds to the total of the individual level and represents the overall energy and/or overall output of the input signal. For instance an overall input level of 90 dB is measured. 0 FIG 2 can then be considered schematically as a cross-section through FIG 3 and 4 at frequency fi. For a specific level, the distance d' between input 17 and output 15 then results at frequency fi. -7 ' 00 ) 7Q 0 12 With a broadband noise according to FIG 3, the distance be tween the spectral density at output 14 and at input 16 is al most constant for instance. This is because the input level LE 5 hardly varies across the frequency so that in accordance with FIG 2, the same amplification is almost always applied and the spectral power density at the output 14 thus also remains al most constant. 0 If the input signal in accordance with FIG 4 nevertheless has a significantly higher spectral power density at a frequency f2 for instance than at other frequencies, the amplification thus also changes in accordance with FIG 2 across the frequen cy. Since it typically reduces at higher levels, the distance 5 between curves 15 and 17 is less at frequency f 2 than at other frequencies. If the threshold value for the overall input level is reached in a broadband input signal BB in accordance with FIG 3, the 0 levels of the individual channels according to curve 16 are on an average level and a corresponding average amplification is applied in accordance with FIG 2. The area between the curves 14 and 16 represents the increase in energy between the input signal and output signal. 5 With the narrowband signal SB, the threshold value is achieved for the overall input level, if the levels around frequency f 2 are very high, while the levels outside of this maximum are comparatively low. Accordingly, the level maximum at frequency 0 f 2 is less amplified than outside of this maximum at frequen cies with a lower level. The overall increase in energy is produced again from the area between curves 15 and 17. Since low levels predominantly exist with the narrowband signal, a W) 0Q' 7Q 0 13 larger amplification results across a large part of the spec trum than with the broadband signal BB, so that the area be tween curves 15 and 17 is greater than the area between curves 14 and 16. This nevertheless means that the overall input lev 5 el of a narrowband signal is more amplified than the overall input level of a broadband signal. Natural hearing perception is thus used, since a narrowband signal is amplified more than a broadband signal, wherein the narrowband amplified signal is then also not perceived louder than the broadband amplified 0 signal. If slower compression time constants are used, negligible sig nal distortions thus result in the event of high compression ratios above the input level thresholds Ls. After the input 5 level thresholds are exceeded, a sufficiently dimensioned dis tance of the resulting output level from the frequency dependent output level limit values LG prevents a distortion of very loud speech signals. Distortions briefly result during the settling time only when considered dynamically. 0 The use of a frequency-independent overall input level thresh old produces the input side restriction independently of the spectral distribution of the signal. Nevertheless, different frequency-dependent output level thresholds Ls are determined, 5 which are dependent on the current spectral distribution of the signal. According to one development, only compression ratios close to 1 are used in a defined input level interval 18 directly below 0 the frequency-dependent input level threshold Ls. The applied amplification factors are thus level-independent in this in terval (same distance of the operating characteristic curve 10 from the perpendicular bisector 11). Furthermore, the amplifi -7 ' 00 ) 7Q 0 14 cation in this range should be significantly attenuated in comparison with the statically set amplification factors at very low levels. A higher output sound pressure level results overall for narrowband signals than for broadband signals, as 5 displayed above, despite the identical broadband overall input level threshold (=overall level threshold value). The loudness of the output signals restricted thereby is thus significantly better matched to the psychoacoustic boundary conditions. 0 -7 ' 00 ) 7Q 0

Claims

1. A method for amplifying an input signal in a hearing appa ratus, the method comprising: 5 predefining a channel-specific compression characteristic curve in each of a number of spectrally-separated processing channels of the hearing apparatus, which defines a relation ship between an input level and an output level in the respec tive processing channel of the hearing apparatus; 0 predefining a channel-specific input level threshold for each processing channel; defining the respective channel-specific operating com pression characteristic curve according to the predetermined channel-specific compression characteristic curve below the 5 channel-specific input level threshold; defining a respective curve of the channel-specific oper ating compression characteristic curve with a compression ra tio of greater than 8 above the channel-specific input level threshold; and 0 amplifying a respective input signal portion of the hear ing apparatus in each processing channel as a function of a channel-specific operating compression characteristic curve.

2. The method as claimed in claim 1, wherein the input level 25 of each channel-specific input signal portion is determined with a time constant which is significantly greater than 250 Ms.

3. The method as claimed in claim 1 or 2, wherein a channel 30 specific output level limit value is predefined for each pro cessing channel and the respective channel-specific operating compression characteristic curve does not reach a fixed dis tance from the channel-specific output level limit value. 9296978 16

4. The method as claimed in claim 3, wherein the fixed dis tance amounts to at least 15 dB.

5 5. The method as claimed in one of the preceding claims, wherein a frequency-independent overall level of the input signal encompassing all input signal portions is measured, a point in time is determined at which the measured overall lev el reaches a predetermined overall level threshold value, for 0 the point in time at which the respective channel-specific in put level threshold is determined in each processing channel in accordance with the momentary level of the respective input signal portion, and the channel-specific operating compression characteristic curves are defined accordingly in all pro 5 cessing channels.

6. The method as claimed in claim 5, wherein the channel spe cific operating compression characteristic curve is kept un changed as long as the measured overall input level is greater 0 than or equal to the overall level threshold value.

7. The method as claimed in claim 5 or 6, wherein each chan nel-specific operating compression characteristic curve corre sponds to the respectively predetermined channel-specific com 5 pression characteristic curve, if the measured overall level lies below the overall level threshold value.

8. The method as claimed in any one of the preceding claims, wherein in each processing channel the compression ratio in a 0 level interval directly below the respective channel-specific input level threshold lies close to 1. -7 ' 00 ) 7Q 0 17

9. The method as claimed in one of the preceding claims, wherein a minimal level value is predefined for each channel specific input level threshold. 5

10. A method being substantially as herein disclosed with ref erence to an embodiment as that embodiment is disclosed in one or more of the accompanying drawings. 0 Siemens Medical Pte. Ltd. Patent Attorneys for the Applicant SPRUSON & FERGUSON -7 ' 00 ) 7Q 0