(11) CH 653 508 A5

(51) Int.Cl⁴: H04 R 25/00

(19) Swiss Confederation

Swiss Office for Intellectual Property

Invention Patent for Switzerland and Liechtenstein

Patent Protection Agreement between Switzerland and Liechtenstein of December 22, 1978

(12) Swiss Patent A5

(21) Application number: 2766/81

(22) Filing date: April 28, 1981

(24) Patent granted: December 31, 1985

(45) Patent published: December 31, 1985

(73) Patent holder: Gfeller AG, Bern

(72) Inventor: Meyer, Serge Eduard Hector, Bern

(74) Representative: Bovard AG, Bern 25

(54) Hearing Aid

(57) The speech signals, generated by a microphone (11), are fed to the input of a preamplifier (12). The preamplifier has a control input (13). The amplification gain of the preamplifier can be changed by feeding a control direct current to the control input. The amplified speech signals are fed to a power amplifier (15) via a volume controller (14). An earphone (17) is connected to the output of the power amplifier via a limiter (16). One input of a subtraction stage (19) is connected to the input of the limiter; and another input is connected to the output of the limiter. The differential signal, occurring at the output of the subtraction stage, is amplified by a control amplifier (20) and rectified by a rectifier (21). The rectified differential signal is fed to an integrator (22), which outputs a d.c. control signal, which depends on the extent of the clipping of the speech signal fed to the earphone, for the preamplifier (12). If the audio level is clipped, the hearing aid, described above, generates a significantly lower non-linear distortion factor than the hearing aids with the peak power known to date.

[see Figure]

voltage controlled preamplifier

volume controller

power amplifier

limiter

integrator

control amplifier

subtraction stage

CH 653 508 A5

[page 2 column 1]

Patent Claims

1. Hearing aid device comprising a microphone (11; 47), a voltage controlled preamplifier, a power amplifier (15; 46) and a means (16; 53) for clipping the voltage, sent by the power amplifier to an earphone (17; 51), characterized by switching elements (19, 20, 21, 22; 55, 56, 59) for generating a control direct current for the preamplifier (12; 45) as a function of the extent of the clipping of the voltage sent to the earphone.

2. Hearing aid device, as claimed in claim 1, characterized in that the switching elements comprise a subtraction stage (19), which exhibits two inputs and one output, said one input being connected to the input side and said other input being connected to the output side of the clipping means; a control amplifier (20) for amplifying the differential signal, occurring at the output of the subtraction stage; a rectifier (21) for rectifying the amplified differential signal; and an integrator (22), which is connected to the rectifier, for generating the control direct current, which depends on the differential signal, for the voltage controlled preamplifier (12).

3. Hearing aid device, as claimed in claim 1, characterized in that the clipping means (16) comprises two antiparallel connected diodes (27); that the switching elements comprise a resistor (28); a control amplifier (20), which is connected to the resistor; a rectifier (21); and an integrator (22), which is connected to the rectifier, for generating the control direct current, which depends on a differential signal occurring at the resistor, for the voltage controlled preamplifier (12); and that the said resistor for generating the said differential signal is connected in series to the antiparallel connected diodes.

4. Hearing aid device, as claimed in claim 3, wherein the final stage of the power amplifier (15) exhibits a transistor (24) which operates in the class A mode, characterized in that a potentiometer (18) is connected in parallel to the earphone (17) in order to switch on the start of the clipping operation; that the series connection comprising the resistor and the diodes exhibits a capacitor (29) that is connected in series; and that the whole series connection is connected, on the one hand, to the wiper of the potentiometer (18) and, on the other hand, to the ground lead of the power amplifier.

5. Hearing aid device, as claimed in claim 3, wherein the final stage of the power amplifier (15) exhibits a transistor (24) which operates in the class A mode, characterized in that a series connection comprising the resistor (28), the antiparallel connected diodes (27), a capacitor (29), and a controllable resistor (18') for switching on the start of the clipping operation is connected parallel to the earphone (17) in an alternating current manner.

6. Hearing aid device, as claimed in claim 1, wherein the final stage of the power amplifier (15; 46) exhibits two transistors (30, 31), which operate in the class A mode; and the earphone (32; 51) exhibits a winding with a center tap (33; 52), characterized in that the switching elements exhibit a controllable resistor (34; 53) for switching on the start of the clipping operation; a diode (42; 55), attached to the central tap; a resistor (23; 56) and an integrator (22; 59) for generating the control direct current, which depends on the extent of the clipping operation, for the voltage controlled preamplifier (12; 45); and that the controllable resistor (34; 53) is connected between the central tap and the one connecting terminal (54) of the supply voltage source of the hearing aid.

7. Hearing aid device, as claimed in claim 6, characterized in that there is a capacitor (44; 58) for smoothing the differential signal occurring at the controllable resistor (34; 53).

[page 2 column 2]

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The invention relates to a hearing aid, as claimed in the preamble of patent claim 1.

Most of the existing hearing aids, in particular those exhibiting a push-pull parallel amplifier as the final stage, are provided with a limiter, which makes sure that the audio level, generated by the earphone of the hearing aid, does not exceed an adjustable maximum value. The clipping operation, also called peak clipping, abbreviation PC, of the voltage, fed to the hearing aid, may be carried out by means of transistors, resistors, and in many cases with the aid of antiparallel coupled diodes. Owing to this clipping process, the original signal is deformed or rather distorted. The more extensive the clipping operation, the more the signal is deformed and, thus, the greater the distortions. That is, the non-linear distortion factor increases significantly. For the hearing impaired, who use such an existing hearing aid, the result is not only a desired decrease in the dynamics but also - as a major drawback - a decrease in the discrimination because of the non-linear distortion contents.

The object of the invention is to provide a hearing aid that is conformity with the type described in the introductory part and in which the non-linear distortion factor, introduced by clipping, is effectively reduced without decreasing the advantages of the audio clipping.

The inventive hearing aid is characterized by the features, disclosed in the characterizing part of patent claim 1.

The invention is explained in detail below with reference to the drawings, where

Figure 1 is a block diagram of one embodiment of the inventive hearing aid.

Figure 2 is a graphical representation of a sine wave signal prior to the clipping operation, after the clipping operation and of the differential signal between the two aforesaid signals.

Figure 3 is a schematic drawing of the final stage in a current feedback circuit as a first alternative of the inventive hearing aid.

Figure 4 is a schematic drawing of the final stage in a current feedback circuit as a second alternative of the inventive hearing aid.

Figure 5 is a wiring diagram of a push-pull final stage of another embodiment of the inventive hearing aid.

Figure 6 is a graphic representation of a sine wave signal prior to the clipping operation and of a differential signal occurring at a resistor of the embodiment, according to Figure 5, and

Figure 7 is a schematic drawing of an embodiment of the inventive hearing aid with integrated current circuits.

In the embodiment of the inventive hearing aid, depicted in Figure 1, the sound waves striking a microphone 11 are converted into electric signals and amplified by means of a voltage controlled preamplifier 12. The preamplifier 12 has a control input 13; and an amplification gain of the preamplifier may be changed as a function of a direct current, fed into the control input 13. The larger the direct current that is fed in, the smaller the amplification gain is. A portion of the output voltage of the preamplifier 12 is fed to the input of a power amplifier 15 by way of a volume controller 14. The output voltage of the power amplifier 15 is fed by means of a limiter 16 to an earphone 17. The limiter 16 is allocated a potentiometer 18, which permits the threshold of the start of the clipping operation and/or the maximum audio level to be adjusted.

[page 3 column 1]

Furthermore, there is a subtraction stage 19 with two inputs and one output. The one input of the subtraction stage 19 is connected to the input of the limiter 16; and the other input of the subtraction stage 19 is connected to the output of the limiter 16. At the output of the subtraction stage 19 there occurs a differential signal, which is produced by subtracting the signal, fed to the second input, from the signal of the subtraction stage that is fed to the first input.

Figure 2a shows a period of a sine wave signal, which has not been clipped yet and which is fed to the input of the limiter 16 and to the first input of the subtraction stage 19. Figure 2b shows the clipped signal, which occurs at the output of the limiter and which, on the one hand, is fed to the earphone 17 and, on the other hand, to the second input of the subtraction stage 19. Figure 2c shows the differential signal, which occurs at the output on the subtraction stage 19 and which is obtained by subtracting the signal, shown in Figure 2b, from the signal, shown in Figure 2a.

The differential signal is fed to the input of a control amplifier 20 and then rectified by means of a rectifier 21. The rectified differential signal is fed to an integrator 22, which outputs a control direct current to the control input 13 of the preamplifier 12 as a function of the clipping operation.

If the output voltage of the power amplifier 15 falls below the response threshold of the limiter 16 (said response threshold being adjustable with the potentiometer 18), the output of the subtraction stage does not show any differential voltage so that no control voltage is fed to the control input 13 of the preamplifier 12; and the preamplifier 12 operates at the full degree of amplification.

If the peaks of the output voltage of the power amplifier 15 exceed the response threshold of the limiter 16, the output of the subtraction stage 19 exhibits a differential signal, which depends on the extent to which the response threshold has been exceeded. As a function thereof, a control voltage is fed to the control input 13 of the preamplifier 12. Thus, the amplification gain is reduced. The response time of the configuration, shown in Figure 1, depends on the time constants of the integrator 22 and may range by choice from 2 to 10 ms. If a high intensity sound wave of a frequency of, for example, 500 Hz suddenly impinges on the microphone 11, then in a response time of 2 ms the peaks of a maximum of two half waves are significantly clipped in the limiter 16. Thereafter the subsequent periods are already reduced by the controllable preamplifier 12 and are no longer so significantly clipped. That means in other words that only the first two peaks of the output signal of the power amplifier 15 exhibit a large non-linear distortion; and the following periods exhibit a significantly smaller non-linear distortion because the output voltage of the power amplifier 15 is reduced by means of the controllable preamplifier.

The power amplifier 15 may include a power transistor in a class A circuit or two transistors in the push-pull circuit. In the latter case the output voltage may be clipped by the limited supply voltage of the push-pull final stage. Thus, the rectified differential signal can be picked off directly with relatively simple means (a feature that will be described below with reference to Figure 5). In this case the subtraction stage 19, the control amplifier 20 and the rectifier 21 may be dispensed with. Then the rectified differential signal is fed directly to the integrator 22 by way of a resistor 23, a state that is shown with the dashed-dotted lines in Figure 1.

[page 3 column 2]

Figure 3 is a wiring diagram of the final stage of a class A amplifier in a current feedback circuit, where the final stage comprises a transistor 24 and an emitter resistor 25. The earphone 17 is connected directly to the collector of the transistor 24. The collector of the transistor 24 is bridged by a capacitor 26 in order to prevent the final stage from oscillating at high frequencies. The limiter comprises two anti-parallel coupled diodes 27, which are connected in series to a resistor 28 and to a coupling capacitor 29. In the embodiment shown in Figure 3, the subtraction stage 19, shown in Figure 1, is formed by the resistor 28. Through this resistor flow only the currents, which are allowed to pass through the diodes 27, and which generate the voltage drops (depicted in Figure 2c) at the resistor 28. The signals, occurring at the resistor 28, already match the differential signal, which in turn is fed to the control amplifier 20, thereafter to the rectifier 21 and the integrator 22.

The potentiometer 18 for adjusting the response threshold of the clipping operation, is connected in parallel to the earphone 17; and the one lead of the coupling capacitor 29 is connected to the tapping point of the potentiometer 18.

Figure 4 shows a wiring diagram similar to the one depicted in Figure 3, but with the one difference that the potentiometer 18' for adjusting the response threshold of the clipping operation is connected in series to the series connection comprising the coupling capacitor 29, the antiparallel coupled diodes 27 and the resistor 28. The potentiometer 18' acts as the controllable resistor. Even in this circuit arrangement the differential signal may be picked off directly at the resistor 28 and fed to the control amplifier 20.

Figure 5 is a wiring diagram of a push-pull stage of the power amplifier 15 from Figure 1. The push-pull stage comprises the two transistors 30 and 31, which operate in a class B mode. The base of the transistor 31 is fed an input signal, which is 180 deg. out of phase in relation to the input voltage of the transistor 30. An earphone 32 having a center tap 33 is connected to the collectors of the two transistors 30 and 31, whereas the center tap 33 is connected to the supply voltage via a controllable resistor 34. The earphone 32 is bridged by means of a capacitor 35, thus decreasing the amplification of the high frequencies.

In Figure 6a the solid curve 36 is a graphical representation of the useful signal, fed to the earphone 32. For the sake of simplicity a sine wave signal is shown. The amplitude of this signal is not so high yet that the limiter has an impact. The solid curve 36b already shows the effect of the limiter on an enlarged signal; and upon application of the circuit described below, the dotted curve 37b shows the signal applied to the earphone 32. The dashed curve 37 is a graphical representation of that signal corresponding to 36b that would be effective at the earphone 32 if the resistor 34 were short circuited.

The solid curve 38 in Figure 6b is a graphical representation of the voltage, occurring between the center tap 33 of the earphone and the negative lead 39 of the supply source. U₀ stands for the voltage between the center tap 33 of the earphone 32 and the connecting terminal 39. The curve 38 is the sum of the signal alternating voltages, superimposed on the direct voltage U₀ and represented by the curves 40 and 41. According to Figure 5, the center tap 33 of the earphone 32 is connected, as indicated by the dashed line in Figure 1, to the integrator 22 by way of a diode 42 and the resistor 23. The integrator 22 can be, for example, a capacitor (not illustrated), which is charged up to the voltage U₀, using means that are also not illustrated.

[page 4 column 1]

The diode 42 exhibits a threshold voltage Usch; and, when the signal, represented by the curve 38, exceeds the threshold voltage (see Figure 6b), a signal, represented by the curve 43 in Figure 6c, is fed to the capacitor, acting as the integrator. Thus, the result is that the voltage of the said capacitor decreases as a function of the amplitude of the signal, represented by the curve 38. At the same time the signal, depicted in Figure 6c, is smoothed out, so that the voltage at the said capacitor can be fed as the control signal to the control input 13 of the preamplifier 12. The reduction in the amplification gain of the preamplifier 12 begins when the signal, represented by means of the curve 38 in Figure 6b, drops below the voltage, labelled Ue.

Between the center tap 33 of the earphone 32 and the connecting terminal 39 there may be a capacitor 44, as shown by the dashed line in Figure 5. Thus, the signal, illustrated by means of the curve 38 in Figure 6b, is pre-smoothed; and the mean value of the voltage, occurring between the center tap 33 and the connecting terminal 39, decreases as the modulation of the transistors 30 and 31 increases.

The following table shows the advantage of the above described hearing aid as compared to the prior art hearing aids exhibiting a simple clipping operation. The values of the non-linear distortion factors (given in %) are measured at a signal frequency of 700 Hz and an output sound pressure of 107 dB.

Clipping dB	Class A amplifier of the prior art hearing aid	Class A amplifier of the inventive design	Push-pull amplifier of the prior art hearing aid	Push-pull amplifier of the inventive design
10	5%	5%	8%	3%
20	24%	15%	25%	6%
30	42%	24%	38%	21%

[page 4 column 2]

Figure 7 shows the wiring diagram of another embodiment of the inventive hearing aid. This embodiment comprises an integrated voltage controlled preamplifier 45, for example the integrated current circuit LD 511 P, and an integrated power amplifier 46 with a push-pull final stage, for example the integrated current circuit LC 549. The speech signals, generated by a microphone 47, are fed [to the input connection 9] to the preamplifier 45 by way of a capacitor 48. The amplified signal is picked off at the output 2 of the preamplifier 45 and fed to the input connection 2 of the power amplifier 46 by means of a potentiometer 49 in order to control the volume and by means of a capacitor 50. Connected to the output terminals 5 and 8 of the power amplifiers 46 is an earphone 51 with a center tap 52. The center tap 52 in turn is connected to the positive connecting terminal 54 of a voltage source (not illustrated) by means of a controllable resistor 53 for adjusting the response threshold of the clipping operation; and said center tap is connected to the control input 8 of the preamplifier 45 by means of a diode 55 and a resistor 56. A capacitor 58, which is connected between the center tap 52 and the negative connecting terminal 57 of the current source (not illustrated), smoothes to some extent the signal, which is described above with reference to Figure 6b. Furthermore, a capacitor 59, which is connected to the control input 8 of the preamplifier 45, acts as an integrator so that direct voltage is fed, as the control voltage, to the control input 8 of the preamplifier 45.

The above described embodiments with push-pull final stages exhibit approximately half the response time as compared to amplifiers in the class A operating mode owing to the full wave rectification of the signal at identical time constants of the integrators (22).

The major advantage of the above described hearing aid is that the amplification gain of the preamplifier is decreased only after reaching the response threshold, at which the clipping operation begins, so that the clipped signal, fed to the earphone, exhibits significantly less non-linear distortion. Thus, in addition to the desired decrease in dynamics, a significant improvement in the discrimination is obtained without decreasing the advantages or reducing the efficiency of the audio clipping operation.

Figure 1

voltage controlled preamplifier

volume controller

power amplifier

limiter

integrator

control amplifier

subtraction stage

Figure 2

prior to clipping

after clipping

difference

Figure 3	Figure 4	Figure 5
to the rectifier	to the rectifier	to the integrator

Figure 7
preamplifier	power amplifier