EP0793897B1 - Hearing aid - Google Patents

Abstract

PCT No. PCT/EP95/02033 Sec. 371 Date May 9, 1997 Sec. 102(e) Date May 9, 1997 PCT Filed May 29, 1995 PCT Pub. No. WO96/17493 PCT Pub. Date Jun. 6, 1996A hearing aid includes a microphone (1), a signal-transmission unit (2, 3,) for forming or otherwise processing the signal, an output amplifier (4) to which an earphone (10) is connected, and a battery as the power supply. The output amplifier (4) is implemented essentially as a SIGMA - DELTA amplifier and is connected to a pulse generator (8) which produces a high-frequency pulsed clock signal in the 1 MHz region. A series-connected low-pass filter (15) is also provided. The input signal to the signal converter is a representation, produced by signal processing in the transmission unit, of the low-frequency input signal to the hearing aid, this signal being converted in the signal converter into a binary signal. The output signal (14) thus appears, after passing through the low-pass filter, essentially as an amplified copy of the low-frequency input signal.

Description

The invention relates to a hearing aid with a microphone, a transmission part for signal processing and an output amplifier with connected Listener.

Output amplifiers for hearing aids should have low distortion have a low energy requirement, even with high output power.

Class B amplifiers are more efficient than A amplifiers. Amplifiers of this type have also hitherto been customary in hearing aids.

Output amplifiers in the form of switching amplifiers have an even better one Efficiency since the losses in the switches can theoretically be zero.

Known switching amplifiers use pulse width modulation.

Examples of such D-amplifiers are e.g. in the European patent application o 590 903 Al from Exar Corporation and in US-A 5,247,581 from Exar Corporation and US-A 4,689,819 and US-A 4,592,087 to Industrial Research Products Inc. discloses and describes in detail.

Such D-amplifiers work in principle as follows:

The rectangular pulse sequence of an oscillator lying in the ultrasound range is an integrator supplied, which also the output voltage of a low frequency signal is supplied by a microphone via an amplifier train arrives and serves as a preload. The output signal of the integrator is then a triangular pulse train, the zero crossings through the   Bias voltage supplied to the integrator varies in the hearing frequency range will. That is, this low-frequency bias makes the zero crossings of the triangular signal from one symmetrical to the axis of symmetry Course without bias signal variable to asymmetrical conditions shifted, the asymmetry in terms of sign and size a continuously changing function of the amplitude of the low frequency Input signal is.

These zero crossings are then used to control the timing and polarity the output signal of a polarity-reversing, symmetrical CMOS switching driver stage used the duration of positive and negative Switching pulses according to the time shift between the zero crossings of the integrator output signal varies, and thus a pulse-modulated Output signal to the listener with a frequency spectrum in the Low frequency range that gives an amplified image of the output signal of the microphone.

Such D-amplifiers working with pulse width modulation have a very good efficiency and have almost no cross modulation.

A disadvantage of the D-amplifier with pulse width modulation is that the pulse width changes either continuously or in very small steps should be achieved when a high signal to noise ratio is reached shall be.

The known class D output amplifiers use a continuous one Modulation, i.e. a continuous variation in pulse width and need hence a continuous microphone output signal as an input signal. If the signal processing preceding the output amplifier is time-discrete and / or amplitude discretely, then this digital signal must first   e.g. converted into a holding network or a digital / analog converter will. This represents an additional effort that can hardly be sold.

From EP-A 0495328 e.g. known as a sigma-delta converter particularly suitable as an A / D converter with discrete components. Such However, circuits are for use in hearing aids with highly integrated ones digital circuits less suitable.

Furthermore, EP-A0597523 discloses a fast D / A converter which is made of a sigma-delta converter and a downstream asynchronous Sigma-delta modulator consists of the output signal of the sigma-delta converter generates an ambivalent, asynchronously modulated signal, which is then fed to a low pass filter.

Again, the effort for the output amplifier is a fully digitized one Hearing aid too high. In addition, it does not allow a high signal / noise ratio achieve.

From WO 89/04583 a hearing aid is known which consists of an ear part to be carried and a part connected by a cable to the body supporting signal processing part, in which over an A / D converter a digital signal processing and a subsequent D / A converter Adaptation of the transfer function of the hearing aid to the auditory deficit Carrier should be reached.

The effort involved here, in particular the use of an A / D converter, a signal processor and a subsequent D / A converter much too high and not usable for a fully digitized hearing aid. In addition, an extremely high signal / noise ratio can be achieved with such a circuit do not reach.  

Finally, EP-A 0578021 discloses a hearing aid, but not one Signa-Delta converter contains a normal A / D converter, one Signal processing and a D / A converter.

These circuit parts are then followed by a modulator that generates a PWM signal, that still has to be fed to a low-pass filter. Here too is the Effort too high, apart from the fact that with a fully digital hearing aid the use of normal analog-digital converters and after signal processing subsequent digital-to-analog converter all possible positive Results of digital signal processing become quite illusory.

The invention therefore goes a completely different way, the use of D / A converters of the usual type in the output amplifier of a fully digital Avoids hearing aid.

The invention is therefore intended to provide a hearing aid device with a novel, essential simpler output amplifiers are proposed, in which a relative high signal / noise ratio can be achieved with extremely low power requirements and high output power with minimal distortion and anything Lack of cross-modulation and control of the output amplifier with a digital input signal. The output amplifier can completely be constructed as a digital highly integrated CMOS circuit.

This is achieved according to the invention with the features of claim 1.

Further features of the invention can be found in the further claims.  

The invention will now be described on the basis of an exemplary embodiment described in more detail with the accompanying drawings.

Fig. 1

ein Prinzipschaltbild eines Hörhilfsgerätes mit einem Ausgangsverstärker gemäß der Erfindung ;

Fig. 2

einen in dem Ausgangsverstärker des Hörhilfsgerätes verwendeten Signalkonverter und

Fig. 3

Impulsdiagramme zur Erläuterung der Arbeitsweise des Ausgangsverstärkers des Hörhilfsgerätes.

In the drawings:

Fig. 1

a schematic diagram of a hearing aid with an output amplifier according to the invention;

Fig. 2

a signal converter used in the output amplifier of the hearing aid and

Fig. 3

Pulse diagrams to explain the operation of the output amplifier of the hearing aid.

1 shows, for example, a hearing aid with a novel output amplifier, however, its use is not limited to use in hearing aids is limited, but is generally applicable to digital amplifiers where it comes down to a high ratio of useful signal to interference signal.

In the hearing aid device shown purely schematically in FIG. 1, the acoustic Signal recorded by a microphone 1 and in a low-pass filter Antialiasing filter is limited to a frequency range that is common in hearing aids. This low-frequency signal is now in a signal processor 3rd subjected to signal processing. Among them is e.g. to understand that the analog input signal is either further processed analogously in the manner that the amplifier characteristic of the signal processor to that for the respective Hearing damage or hearing loss of its wearer regarding all necessary variables is adjusted.

Such variable, which are dependent on the frequency, are e.g. the Amplification of the individual stages, the limit level, the compression threshold, the automatic gain control with its response and Fall times, a combination of compression and expansion, or whatever a non-linear course of the amplification of individual stages or overall all levels, as well as the output sound pressure level.

On the other hand, digital signal processing is probably preferred Provide. In this case, the signal processor would have to have a digital-to-analog converter on the input side included, for which a separate clock generator for the clock would be required. This is the general state of the art. Of course then all the variable functions mentioned above are in digital technology representable.  

A new output amplifier then follows the signal processor 3. This consists essentially of a signal converter 4, which is essentially a Σ-Δ converter. This signal converter first contains a subtraction stage 5 with two inputs, namely a positive input and a negative Input, with the positive input at the output of the signal processor 3 connected. A low-pass filter 6 follows this subtraction stage 5 In the simplest version, the low-pass filter 6 could be an integrator. On a comparator 7 with holding network is connected to this integrator 6. The output of this comparison stage is via a feedback connection connected to the negative input of subtracting stage 5. In addition, is a High-frequency clock generator 8 is provided, which is a high-frequency clock pulse signal with a frequency in the range of about 1 MHz to the comparison stage 7 issues. The output of the signal converter 4 is via a low-pass function connected to the handset 10.

A clock generator required for the signal processor 3 is essential lower frequency is preferably by the high frequency clock generator 8 synchronized. This can be done simply by frequency division, for example can be achieved with a factor M. A typical clock frequency for the signal processor 3 could be about 32 kHz.

The mode of operation of the signal converter 4 should be based on FIGS. 2 and 3 are explained.

The high-frequency clock signal 11 of the clock pulse generator 8 is, as already mentioned, forwarded to comparison level 7. The digital input signal 12 in FIG. 3 (an extremely simplified representation) is the subtracting level at their positive input fed. The output signal 14 of the signal converter 4 reaches the negative input of the via a feedback connection Subtracting stage and is subtracted there from the input signal 12.  

The resulting output signal is the integrator 6 (which here Represents low-pass filter) and integrated there to the output signal 13. This signal 13 is synchronized in the comparison stage 7 with holding network with the edges of the high-frequency clock signal into the output signal 14 converted, which has only two possible values, here the simplicity are shown as +1 and -1 for the sake of convenience.

The input signal 12 should initially have the value -0.5. The integrated Signal 13 then rises from -1.5 to zero, resulting in a first output pulse transition from -1 to +1. The integrated signal then drops again to -1.5, after which the output signal 14 again assumes the value -1.

The subsequent rise in the input signal 12 to the value zero a steeper increase in the integrated signal 13 to the value 0.5. For the duration of the input signal level 0, one then obtains via the integration the corresponding signal values of the output signal 14 between -1 and +1, where the values -1 are the lower values of the integrated signal and the values +1 correspond to the upper value of the integrated signal.

In the same way, the other values of the input signal of 0.3, 0.6 and 1.0 via integration into corresponding pulses of the output signal 14 converted. I.e. the ratio changes in the output signal 14 from positive values to negative values per time unit depending from the input signal 12.

It is obvious that this is a very simplified, strong stretched representation. A clock frequency of about 1 MHz could be drawn do not represent. In addition, the amplitude changes are extreme represented simply as rough steps.  

When converting a low-frequency analog signal into a digital one Signal by time-discrete and / or amplitude-discrete conversion quantized the analog signal. The stages of the input signal shown in FIG. 3 12 therefore represent a corresponding amplitude step quantized analog signal.

While usually with a pulse width modulation of the usual kind with clock pulse frequencies of e.g. 100 kHz is sufficient Case to achieve a large ratio of useful signal to interference signal much higher clock pulse frequencies required, for example in Can be in the range of 1 MHz.

It is obvious that the output signal 14 of the signal converter 4 in addition the desired increased low-frequency component a strong high-frequency component Contains signal portion, which of course is an unwanted interference signal represents e.g. must be removed by a passive low-pass filter.

If you use this output amplifier in a hearing aid, then you can the inductance of the voice coil of the receiver and the low-pass characteristics the mechanical and acoustic system of the hearing aid and human ear take over this low-pass function completely, so that a separate low-pass filter appears unnecessary.

This new output amplifier, especially suitable for hearing aids has a number of advantages. All pulse edges are with a known one Clock frequency synchronized, which can also be used to the essential for the upstream signal processor, at essential synchronize lower clock frequency working clock pulse generator.  

In addition, the input signal of the output amplifier can be a digital signal be, and the output amplifier can be designed as a pure digital circuit will. I.e. but that the entire circuit is constructed as a digital circuit can be, only at the input of the signal processor 3 Analog / digital Vandler would have to be provided. This results in the further one Possibility of using the entire circuit in C-MOS technology as a highly integrated Build circuit.

Claims (5)

1. Hearing aid with a microphone (1), a transmission section (2, 3) for signal processing, an output amplifier (4) with connected earphone (10), said amplifier consisting essentially of a signal converter realized as a Σ-Δ-converter with a connected clock generator (8) generating a high-frequency clock signal and with a downstream low-pass filter function (15), as well as with a battery for power supply, characterized in that the input signal of the signal converter is a digital image of the input signal of the hearing aid consisting of several bits/bytes, in that this input signal can be converted in the signal converter into an output signal (14) that has only two possible signal values, and that this output signal essentially appears as an amplified image of the low-frequency input signal after passing through the low-pass function.
2. Hearing aid in accordance with claim 1, characterized in that the signal converter (4) being connected with its input side to the transmission section (2, 3), essentially consists of a subtraction stage (5) with a positive and negative input, a low-pass filter (6) and a comparator circuit (7) with holding network controlled by a clock pulse generator (8) with high-frequency clock pulses (11), whereby the positive input of the subtraction stage (5) is connected to the transmission section (2, 3) and the negative input of the subtraction stage to the output of the comparator stage (7) by way of a feedback loop.
3. Hearing aid in accordance with claim 1, characterized in that the clock frequency of the clock pulses generated by the clock generator (8) lies in the range of 1 MHz.
4. Hearing aid in accordance with claim 1, characterized in that the clock signal output to the signal processor (3) by a clock generator for clock generation can be synchronized by the high-frequency clock signal (11) of the clock generator (8)
5. Hearing aid in accordance with claim 1, characterized in that the low-pass function is realized by the electrical, acoustic and mechanical characteristics of the earphone (10) and possibly the human ear.