EP0280909B1 - Circuit arrangement for the detection of oscillations - Google Patents

Abstract

A circuit arrangement for suppressing oscillations, such as acoustic feedback in a hearing aid, has a circuit which recognizes the presence of oscillations in a useful signal, an oscillatory frequency search circuit, and an oscillation modifying circuit controlled by the search circuit. The oscillation modifying circuit suppresses oscillations by filtering. Drift effects are avoided by a frequency clamp-on sub-circuit in the search circuit, which retains the frequency of the recognized oscillation at the modifying circuit, even when the oscillatory signal at the input of the search circuit disappears.

Description

The present invention relates to a circuit arrangement according to the preamble of patent claim 1.

With electronic devices that include a microphone and a loudspeaker in close proximity to one another, there is often the risk of acoustic feedback effects. In particular in the case of hearing aids, the sound transducers (microphone and receiver) of which are only slightly separated from one another, tones occur due to this phenomenon, e.g. Whistle on, which are bothersome.

In the past, efforts were made to reduce the susceptibility to vibrations, particularly in the case of hearing aids, mainly by narrowing the auditory canal and producing sound-insulating earmolds. Electrical countermeasures at most cut or shift the frequency band instead of attacking the vibration signal itself.

Circuits have recently been developed (for example by RIM-Elektronik, Munich, and US Pat. No. 4,232,192) which recognize vibrations as such and then suppress them. Such circuits tap the useful signal between the input converter and a power amplifier connected upstream of the output converter and amplify it by means of an additional amplifier. The amplified signal is compared in a comparator stage with a threshold voltage and fed into a phase locked loop (so-called phase locked loop or PLL for short). An input signal is evaluated as a vibration if it exceeds an amplitude threshold specified by a comparator. The PLL is activated when all thresholds are exceeded. The PLL then "searches" for the frequency of the overshoot signal and outputs a suppression signal to a notch filter connected upstream of the power amplifier, which suppresses the frequency range of the overshoot signal.

However, it is not only vibrations that exceed the predetermined amplitude threshold. A variety of useful signals, e.g. Speech signals can have high amplitudes. It is therefore advantageous to evaluate further vibration characteristics in order to recognize a vibration as such.

Another starting point was chosen in US Pat. No. 4,091,236 and FR-A 23 70 276 (= DE-PS 27 47 821). Here a period constancy of the input signal is used as a recognition criterion. However, since many useful signals also have constant periods, this solution is also unsatisfactory.

The object of the present invention is to construct a vibration detection circuit which only emits a vibration detection signal when the input signal has been assessed as positive in a plurality of test processes, so that incorrect evaluations in the vibration detection circuit are avoided.

The object is achieved by the characterizing features of claim 1.

According to the invention, the vibration detection circuit comprises elements which test the useful signal with regard to three vibration characteristics. Since vibrations are long-lasting AC voltages with a large amplitude and relatively high frequency, the vibration detection circuit checks the amplitude size of the input signals, the time interval between individual signal swings (frequency) and the duration of a series of successive signal swings (group length).

Further details of the invention will become apparent from the following description of an embodiment with reference to the drawing and in conjunction with the further dependent claims.

• Figur 1 ein Prinzipschaltbild einer schallverarbeitenden Einrichtung, insbesondere eines Hörgerätes, mit einer entsprechend der Erfindung ausgebildeten Schaltungsanordnung zum Unterdrücken von Schwingungen aufgrund von akustischen Rückkopplungseffekten,
• Figur 2 eine erfindungsgemäß ausgebildete Schwingungserkennungsschaltung, die eine auf akustischen Rückkopplungseffekten beruhende Schwingung im Nutzsignal als solche erkennt,
• Figur 3 ein Pulsdiagramm der in der Schwingungserkennungsschaltung verarbeiteten Signale,
• Figur 4 eine der Schwingungserkennungsschaltung nachgeschaltete Schwingungsfrequenzsuchschaltung,
• Figur 5 eine von der Schwingungsfrequenzsuchschaltung gesteuerte Beeinflussungsschaltung, die hier z.B. als Notchfilter ausgebildet ist,
• Figur 6 eine Schwingungserkennungsschaltung mit zugeordneter Schwingungsfrequenzsuchschaltung, wobei Erkennungsimpulse mittels eines Zählers erzeugt werden (Modifikation der Anordnung der Figuren 2 und 4),
• Figur 7 eine digital ausgebildete Schwingungserkennungsschaltung mit zugeordneter Schwingungsfrequenzsuchschaltung (Modifikation der Anordnung der Figuren 2 und 4).

Show it:

• Figure 1 is a schematic diagram of a sound processing device, in particular a hearing aid, with a corresponding circuit arrangement designed to suppress vibrations due to acoustic feedback effects,
• FIG. 2 shows an oscillation detection circuit designed according to the invention, which detects an oscillation in the useful signal based on acoustic feedback effects as such,
• FIG. 3 shows a pulse diagram of the signals processed in the vibration detection circuit,
• FIG. 4 shows an oscillation frequency search circuit connected downstream of the oscillation detection circuit,
• FIG. 5 shows an influencing circuit controlled by the oscillation frequency search circuit, which is designed here, for example, as a notch filter,
• FIG. 6 shows a vibration detection circuit with an associated vibration frequency search circuit, detection pulses being generated by means of a counter (modification of the arrangement in FIGS. 2 and 4),
• FIG. 7 shows a digitally designed oscillation detection circuit with an associated oscillation frequency search circuit (modification of the arrangement in FIGS. 2 and 4).

Figure 1 shows a circuit arrangement comprising the invention, which can be installed, for example, in a hearing aid. It comprises a microphone 1 as an acoustic input converter, which converts acoustic input signals SE, SR into electrical signals SO, an acoustic output converter 2 (which is designed either as a loudspeaker or, particularly in the case of hearing aids, as a so-called receiver), a power amplifier 3 connected upstream of the output converter 2 and a vibration suppressing circuit 4.

The vibration suppression circuit 4 is designed as an electrical feedback circuit. It suppresses electrical signals which are generated due to acoustic feedback effects and which generally lead to undamped vibrations in the circuit. The feedback effect is indicated in FIG. 1 with a dashed arrow line between acoustic output transducer 2 and microphone 1.

An acoustic useful signal SE together with an acoustic feedback signal SR is converted into a corresponding electrical signal S0 in the microphone 1. The output signal S5 of the vibration suppression circuit 4 is subtracted from this signal S0 in a subtractor 5 from the signal S0. The remaining signal S1 is amplified in a non-inverting power amplifier 3 to a signal S2. The signal S2 is converted back into an acoustic signal SA in the output converter 2. At the same time, the signal S2 is fed to the vibration suppression circuit 4 as an input signal.

The vibration suppression circuit 4 consists, functionally broken down, of a vibration detection circuit 6 according to the present invention, a vibration frequency search circuit 7 and an influencing circuit 8. In the vibration suppression circuit 4, the input signal S2 is passed to the vibration detection circuit 6. It is also fed to the influencing circuit 8. In the vibration detection circuit 6 it is checked whether the signal S2 contains a vibration based on acoustic feedback effects. If an oscillation is present, an oscillation detection signal S3 is emitted. The signal S3 sets the oscillation frequency search circuit 7 into operation, a sequence of signals S4 being emitted by the oscillation frequency search circuit 7 until the oscillation detection signal S3 at the output of the oscillation detection circuit 6 disappears. That when the Vibration detection signal S3 at the output of the oscillation frequency search circuit 7 signal S4 is held by the search circuit 7 until a new oscillation occurs. The signals S4 control the influencing circuit 8 in the sense that frequency ranges, which are to be assigned to a detected oscillation, are largely suppressed in the entire captured frequency spectrum of the signal S0 by means of a filter. As already mentioned, the output signal S5 of the influencing circuit 8 is the output signal of the vibration suppression circuit 4.

FIG. 2 shows a vibration detection circuit 6 designed according to the invention and implemented with analog components. Since vibrations are long-lasting AC voltages with a large amplitude and relatively high frequency, the vibration detection circuit 6 checks the input signal S2 for these properties. In a first stage, the amplitude of the input signal S2 is compared with a first threshold value UT1 by means of a first comparator 9. If the amplitude of S2 exceeds the threshold value UT1, an exceeding signal S21 is produced which is designed as a square wave voltage with time intervals t₀ to t _n . (The signals S2 and S21 as well as the further evaluation signals S22, S23, S24, which are processed and passed on by the vibration detection circuit 6, are shown in the pulse diagram of FIG. 3).

The subsequent stage comprises, as the first comparator, an RC element with an ohmic resistor 10, diode 10ʹ and capacitor 11 and a second comparator 12. The capacitor 11 is quickly charged by the exceeding signal S21 via the diode 10ʹ and with a predetermined time constant via the resistor 10 unload. This charge and discharge signal is identified in FIGS. 2 and 3 by S22. The time constant, together with the threshold value UT2 of the second comparator 12, determines a first time interval TI1, which defines the minimum frequency to which the vibration detection circuit 6 responds. Will one selected small time constant, then the vibration detection circuit 6 responds essentially only to high-frequency signals. With low-frequency signals, the capacitor 11 has sufficient time to discharge below the threshold value UT2 of the second comparator 12. These low-frequency signals are therefore not detected. This ensures that the oscillation detection circuit 6 only reacts to signals that result from acoustic feedback effects, while portions of the useful signal (eg voice signal) that occur periodically at a lower frequency are not taken into account.

If the oscillation criteria "large amplitudes" and "high frequencies" were met in the first two stages, overshoot signals S23 are released to a second comparator 13 to 16. The overshoot duration signals S23 are square-wave voltages which have the same duration as the threshold value overshoots of the charge and discharge signals S22. The overshoot duration signals S23 thus reflect how long a large, high-frequency input signal lasts (T1). The second comparator comprises a diode 13, an RC element 14, 15 and a third comparator 16. With the overshoot signal S23, the capacitor 15 is charged via the resistor 14. Resistor 14 and capacitor 15 are dimensioned so that the charging time constant is large, e.g. 0.5 to 2 seconds. If the overshoot time signal S23 drops only briefly, the capacitor 15 is immediately completely discharged via the diode 13. However, if the overrun duration signal S23 lasts longer than a predetermined second time interval TI2, the capacitor 15 charges up to such an extent that the voltage exceeds the threshold value UT3 of the subsequent third comparator 16. In this case, the input signal S2 fulfills all vibration detection criteria and the signal S3 emitted by the comparator 16 is regarded as a vibration detection signal.

FIG. 4 shows an oscillation frequency search circuit 7. The oscillation frequency search circuit 7 is between the oscillation detection circuit 6 and influencing circuit 8 arranged and controls the influencing circuit 8 such that detected vibrations are suppressed. A first device 17 of the search circuit 7 generates digital frequency-determining signals S33 and is controlled by the vibration detection signals S3. The main component of the first device 17 is a counting device 18, which comprises a counter 19, a counting direction switch 20 and a reset element 21, also called "power-on reset". In addition, the first device 17 comprises an oscillator 22 and an associated AND gate 23. The counter 19 also serves as a holding device for the frequencies of the detected vibration, as will be explained in more detail below.

When the device is switched on, the reset element 21 ensures that the output signals S32 on all four output lines of the counter 19 are in the zero state (also referred to as the “low” state). This 0000 value is digitally incremented by one each time a step pulse S31 ("high" state) is registered at the input of the counter 19. After all four output lines have been switched to "high", the original zero state is restored at the next step pulse S31 and the step sequence is repeated. However, a pulse S31 is only generated if a vibration detection pulse S3 is present at the AND gate 23 connected upstream of the counter 19. If this is the case, then the pulses S31 'generated by means of oscillator 22 are forwarded as step pulses S31. The oscillator 22 therefore determines the speed at which the counter 19 is advanced.

The counter 19 continues to switch the output pulses S32 until the vibration detection signal S3 disappears. (Signal S3 disappears when the oscillation has been suppressed by the influencing circuit 8). When signal S3 disappears, counter 19 receives no further pulses S31 and remains in the set state until a new one Vibration detection signal S3 occurs. The counter 19 thus stores the set state and therefore, together with the AND gate 23, serves as a holding device for holding the frequency of the detected vibration at the influencing circuit 8. It is advantageous to design the oscillation frequency search circuit 7 as a holding device, since the vibration suppression circuit 4 does not drift can and a return of the suppressed vibration is avoided.

The first device 17 also includes a counting direction switch 20 at the output of the counter 19. This has the effect that a sudden change in the digital frequency-determining signals S33 from 111 to 000 is avoided by counting down every second sequence by inversion of the output pulses S32 from 111 to 000 becomes. This is advantageous in that the filter arranged in the influencing circuit 8 for suppressing the oscillation frequency when the counting direction is reversed does not jump from one end to the other end of the frequency spectrum, but instead moves back and forth in the frequency spectrum.

A second device 24 of the oscillation frequency search circuit 7 samples the frequency-determining signals S33 of the first device 17 (output of the counting direction switch 20) and controls the influencing circuit 8 via the control signals S4. For this purpose, a decoder 25 transmits the eight signal possibilities arriving via three lines to eight different lines. These eight control signals S4 control the influencing circuit 8 in the sense that they determine which frequency range in the controllable frequency spectrum is filtered by the influencing circuit 8.

The decoder 25 controls the influencing circuit 8 by means of a discretely variable resistor 26. FIG. 5 shows the resistor 26 as part of the influencing circuit 8. If there is oscillation, the control signals S4 passed through one or more of the lines of resistor 26. Each line comprises at least one transistor 27, an ohmic resistor 28 and an inverter 29, the resistors 28 having different resistance values. If there is no vibration (S33 = 000), all transistors 27 are turned on (by inversion of the control signals S4). In contrast, with a value S33 = 111, all the transistors 27 block. The resistance values of the resistors 28 are preferably selected so that the influencing circuit 8 drives eight adjacent frequency ranges within the band 1 kHz to infinity. It is also advantageous if at least one transistor-resistor combination makes it possible to control a frequency range above the acoustic limit of human hearing, so that only this range is filtered after the device is switched on and before an oscillation occurs.

The influencing circuit 8 also comprises further ohmic resistors 30, capacitors 31 and an amplifier 32, which are arranged in the form of a bandpass filter. Such a filter is e.g. known from the book "Semiconductor Circuit Technology" by Tietze and Schenk (Springer-Verlag Berlin, Heidelberg, 7th edition (1985), pages 419-421). Since the bandpass filter is designed as a negative feedback of the power amplifier 3, the circuit 8 simulates a notch filter, which forms a suction circuit at the resonance frequency. The bandwidth and gain of the filter are independent of the discretely variable resistor 26. The resonance frequency can thus be varied by changing the resistance values in the resistor 26 without influencing the bandwidth or amplification. The output resistor 33 determines the weight of the feedback S5 on the subtractor 5 (see FIG. 1).

Further variants of the influencing circuit 8, which are not described in detail here, are possible. For example, the circuit 8 could be designed as a CR high-pass filter, phase shifter, phase switch or gain reducer.

There are also modification options for circuits 6 and 7. Figure 6 shows e.g. a variant 6ʹ of the oscillation detection circuit 6 and a variant 7ʹ of the oscillation frequency search circuit 7. Here, the second comparator 13 to 16 of the oscillation detection circuit 6 is replaced by a digital arrangement which comprises an inverter 34, a digital counter 35 and an AND gate 36.

The input signal is checked in the same way as in the exemplary embodiment corresponding to FIG. 2 according to the vibration characteristics "large amplitude" and "high frequencies". However, an overshoot signal S23 is processed digitally in order to determine whether the large, high-frequency input signal is long-lasting. The counter 35 has two signal inputs: an input for the overshoot signal S21 and a reset input which, together with the inverter 34, continuously resets the counter 35 to the zero state, except when an overshoot signal S23 occurs. As long as an overshoot signal S23 is present, the counter 35 counts the overshoot signals S21. After a certain number of exceeding signals S21 has occurred, the input signal is regarded as a recognized oscillation. The counter 35 then generates, together with the AND gate 36, vibration detection signals S3. These vibration detection signals S3 are indexing pulses and can be given directly to the counter 19 of the search circuit 7ʹ. This circuit variant therefore does not require an oscillator 22 in the oscillation frequency search circuit.

FIG. 7 shows a further modification of the circuits in FIGS. 2 and 4, in which the first and second comparison elements (10 to 12, 13 to 16) are replaced by digital arrangements. The counter 37 is reset to a 000 level each time an exceeding signal S21 occurs. This process corresponds to a rapid charging of the capacitor 11 in the analog variant of FIG. 2. The counter 37, the output Q of which is in the "low" state, begins in the direction of a second one To count the counter reading at a speed predetermined by the oscillator 38. This counting process corresponds to the discharge of the capacitor 11 in the analog variant. The "low" state at the output Q of the counter 37 is converted by means of an inverter 39 into a "high" state at the input of the AND gate 40, as a result of which the indexing pulses of the oscillator 38 are passed on. If a further exceeding signal S21 arrives before the counter 37 reaches its second counter reading, the output Q of the counter 37 remains at "low". However, when the counter 37 reaches the second counter reading, the output Q of the counter 37 is switched to "high" and the AND gate 40 blocks the switching pulses of the oscillator 38. Reaching the second counter reading corresponds to discharging the capacitor 11 up to the threshold value UT2 in Figure 3.

The counter 35 measures according to the second comparator 13 to 16 of the analog variant, the time duration T 1 of a series of successive exceeding signals S21. This counter 35 is only reset to a 000 state if there is no exceeding signal S21 at the AND gate 41: as soon as an exceeding signal S21 occurs, both inputs of the AND gate 41 are switched to a "high" state, which is what Reset input of counter 35 results in a "low" state. The counter 35 counts by means of switching pulses from the oscillator 38 until the exceeding signals S21 disappear or until a predetermined, very high counter reading is reached. The very high meter reading is comparable to the slow charging of the capacitor 15 up to a threshold value UT3 in the analog variant. Vibration detection signals S3 are generated by counter 35 as in the modification described above (see FIG. 6).

Claims (13)

1. Circuit arrangement for the detection of oscillations due to acoustic feedback effects with a threshold value comparator arrangement which on the exceeding of the amplitude of an input signal (S2) over a specified amplitude threshold value generates exceeding signals (S21) of the duration of the exceeding of the threshold value, characterized by

   - a first threshold value comparator (9) for the evaluation of the amplitude of the input signals (S2),

   - a first comparison element, consisting of a second threshold value comparator (12) with an amplitude threshold value (UT2) and a charge and discharge device (10, 10', 11) for the detection of the time intervals (t₀ to t_n) between individual signals, and

   - a second comparison element, consisting of a third threshold value comparator (16) with an amplitude threshold value (UT3) and a charge and discharge device (13, 14, 15) for the detection of the duration (T₁) of a signal group when a series of input signals (S2) succeed one another closely.
2. Circuit arrangement according to claim 1, characterized in that the first comparison element (10 to 12; 37 to 40) supplies a continuous exceeding signal (S23) as long as the time intervals (t₀ to t_n) between individual signals fall short of a specified first time interval (TI1), and in that the second comparison element (13 to 16; 34 to 36, 38, 41) supplies an oscillation detection signal (S3) as long as the continuous exceeding signal (S23) endures beyond a specified second time interval (TI2).
3. Circuit arrangement according to claims 1 and 2, characterized in that at the output of the first threshold value comparator (9) of the threshold value comparator arrangement, the threshold value comparator (9) generating the exceeding signals (S21), the second threshold value comparator (12) is arranged as first comparison element, the charge and discharge device (10, 10', 11) of which can be charged with the occurrence of the exceeding signals (S21) and can be discharged during the time intervals between the exceeding signals (S21) with specified first time constant, and the generated charge and discharge signals (S22) can be compared with the amplitude threshold value (UT2) of the second threshold value comparator (12) by means of the comparison element, whereby it generates a continuous exceeding signal (S23) for the duration of the exceeding of the amplitude threshold value (UT2) of the second threshold value comparator (12).
4. Circuit arrangement according to claim 1 to 3, characterized in that at the output of the second threshold value comparator (12) the third threshold value comparator (16) is arranged as element for the evaluation of a series of successive exceeding, the charge and discharge device (13 to 15) of which can be charged during the period of time of the occurrence of a continuous exceeding signal (S23) with specified second time constant and can be discharged at the end of each continuous exceeding signal and in that the third threshold value comparator (16) generates the oscillation detection signal (S3) when the charge and discharge signal (S24) of the charge and discharge device (13 to 15) exceeds the amplitude threshold value (UT3) of the third threshold value comparator (16).
5. Circuit arrangement according to claims 1 to 4, characterized in that the respective time interval (TI1; TI2) is specified by a time constant in connection with a threshold value (UT2, UT3).
6. Circuit arrangement according to claims 1 to 5, characterized in that the first time interval (TI1) is specified by the time constant of a discharge device (10, 11; 37, 38) for the exceeding signals (S21) after preceding charge in a charge device (10', 11; 37) and the threshold value (UT2) is that of a threshold value comparator (12; 37, 40) for the charge and discharge signals (S22).
7. Circuit arrangement according to claims 1 to 5, characterized in that the second time interval is specified by the time constant of a charge device (14, 15; 35, 38) for the continuous exceeding signals (S23) and the threshold value (UT3) is that of a threshold value comparator (16; 35, 41) for the charge signals (S24) of the continuous exceeding signals.
8. Circuit arrangement according to one of claims 6 or 7, characterized in that the charge or discharge device (10 to 11, 13 to 15) comprises a capacitor (11, 15) which determines the time constant together with a charge or discharge resistor (10, 14).
9. Circuit arrangement according to claim 8, characterized in that the threshold value is an analog signal with which the charge and discharge signal of the capacitor (11, 15) can be compared.
10. Circuit arrangement according to claim 6 or 7, characterized in that the charge or discharge device (35, 37, 38) comprises a counter (35, 37), the counting speed of which is determined by the time constant.
11. Circuit arrangement according to claim 10, characterized in that the threshold value is specified by a count.
12. Circuit arrangement according to one of claims 6, 10 or 11, characterized in that at the output of a threshold value comparator (9) generating the exceeding signals (S21) a first counter (37) is arranged as element for the evaluation of the time intervals (t₀ to t_n) between the exceeding signals, which with the occurrence of each exceeding signal (S21) is set on to a first count and in dependence upon counting pulses of a counting pulse generator (38) counts in the direction of a second count and generates the continuous exceeding signal (S23) until the second count has been reached.
13. Circuit arrangement according to claim 12, characterized in that at the output of the first counter (37) a second counter (35) is arranged as element for the evaluation of a series of successive exceeding signals (S21), which before the occurrence of a continuous exceeding signal (S23) is held on to a first count and after the occurrence of a continuous exceeding signal in dependence upon counting pulses of a counting pulse generator (38) counts in the direction of a second count and generates the oscillation detection signal (S3) when the second counter (35) reaches the second count and until the second counter is reset to the first count.

Images