Oscillation detection

Abstract

The invention relates to oscillation detection and, more particularly, concerns a method and apparatus for identifying oscillation in a signal due to feedback, permitting appropriate action to be taken to suppress the oscillation.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to oscillation detection and, more particularly, concerns a method and apparatus for identifying oscillation in a signal due to feedback. The present invention may be used in conjunction with the method and apparatus for suppressing oscillation in a signal described in applicant's copending application entitled ‘Oscillation Suppression’ (Attorney ref. 30-518-9119).

BACKGROUND OF THE INVENTION

[0002] In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date:

[0003] (i) part of common general knowledge; or

[0004] (ii) known to be relevant to an attempt to solve any problem with which this specification is concerned.

[0005] Acoustic amplifiers are used in many common applications such as telephones, radios, headsets, hearing aids, and public address systems. Typically, such an application comprises a microphone or other input transducer to pick up sounds and convert them into an electrical signal, an electronic amplifier to increase the power of the electrical signal, and a speaker or other output transducer to convert the amplified electrical signal back into sound.

[0006] If the input and output transducers are close enough, the output acoustic signal may be picked up by the input transducer and fed back into the amplifier with a delay, the delay being the time taken for the sound to travel from the output transducer to the input transducer (plus any delay due to the electrical processing of the signal). This is ‘acoustic feedback’. Electrical feedback can also occur if the electrical signal at the output is coupled back to the input, for example by inductive or capacitive coupling. Further, mechanical feedback can also occur if vibrations are transmitted from the output transducer to the input transducer via the body or case of the amplification system. Under feedback conditions, the device can then become unstable and the components begin to ring. The ringing then self-reinforces and increases in intensity to drive the components into saturation. FIG. 1 illustrates a feedback loop, showing diagrammatically the components in an acoustic amplifier circuit, namely microphone 1, amplifier 2 and speaker 3, with feedback loop 4 representing the output signal feeding back to the input transducer.

[0007] All forms of feedback may result in instability or oscillation of the output signal from the amplifier under certain conditions. Oscillation and instability are undesirable because they distort the signals being amplified and can result in very loud unpleasant sounds. In the case of hearing aids, this can lead to problems both for the wearer and for those around. The conditions for oscillation are that the total gain around the loop must be greater than 1, so that the signal is fed back into the system with a greater intensity each time, and the total delay around the loop must be a whole number of periods of the oscillation frequency, so that the input and output signals add constructively. Equivalently, the total phase change around the loop must be a multiple of 2&pgr; radians for the oscillation frequency. These criteria are set out in equations 1 to 3 below.

Loop Gain>1  (eq. 1)

Loop Delay=N×period  (eq. 2)

Loop Phase Change=2N&pgr; radians  (eq. 3)

[0008] (where N is a positive integer)

[0009] Any electronic system containing a microphone and speaker in close proximity may suffer from acoustic feedback. In hearing aids, this often results in the wearer experiencing unpleasant audible effects such as loud whistling tones at certain frequencies, usually high frequencies.

[0010] The traditional procedure for increasing the stability of a hearing aid is to reduce the gain at high frequencies, as suggested in, for example, U.S. Pat. No. 4,689,818. This may be done by setting the maximum gain value for each frequency, or automatic high frequency (HF) gain roll-off may be used. Controlling feedback by modifying the system frequency response, however, means that the desired high-frequency response of the instrument must be sacrificed in order to maintain stability.

[0011] Efforts have been undertaken to reduce tie susceptibility of hearing aids to feedback oscillation by improving the fit and insulating properties of the ear mould. Efforts have also been undertaken from an electrical standpoint, from attenuation and notch filtering, as disclosed in U.S. Pat. No. 4,088,835, to estimation and subtraction of the feedback signal, as disclosed in U.S. Pat. No. 5,016,280, to frequency shifting or delaying the signal, as disclosed in U.S. Pat. No. 5,091,952. Many different approaches to an electrical solution with continuous monitoring of the feedback path have been documented in the relevant literature.

[0012] A technique commonly used to suppress feedback in public address systems is a frequency shift, in which the input signal is altered by a few Hertz prior to being output at the receiver. This approach has not been particularly successful in hearing aids because a large frequency shift is required to achieve a significant increase in gain. In hearing aids, the distance between microphone and receiver is much smaller than in public address systems, and thus a feedback signal with only a small frequency shift may still be relatively closely in phase with the input.

[0013] Signal phase can also be altered by using a time-varying delay [1]. While this can provide 1-2 dB of additional useable gain, it can also result in an audible ‘warbling’ effect. All pass filters have also been used to modify the phase response of the feedback loop, but it can be difficult to achieve satisfactory phase at all frequencies. Methods have been proposed to push danger regions in the phase response to frequencies outside the primary audio range where suppression can be applied without loss of sound quality [2] [3]. These techniques still assume that the feedback path is constant however, and no suggestion has been made that an adaptive implementation may be developed.

[0014] The most common gain altering approaches attempt to reduce the system gain only in narrow bands where feedback is likely to occur. This has been attempted with a variety of notch filter implementations [1] [4] [5]. Adaptive notch filtering has allowed 3-5 dB of additional useable gain. Two of the biggest problems with notch filtering techniques have been the inability to accurately track the variations in the feedback path with a narrow band, and the effects on normal spectral content with a broader band. In addition, the notch filter can actually contribute an additional phase change to the loop and shift the frequency of oscillation as soon as it is applied.

[0015] Substantial increases in useable gain have been achieved by inserting an additional feedback path, based on an estimation of the real feedback path, but 180 degrees out of phase. Early adaptive implementations of such systems performed continuous estimation of the feedback path by inserting noise signals with appropriate statistical properties at the receiver and correlating the output with the input at the microphone [1] [6]. These reported up to 10 dB of additional useable gain [7] but, since the noise ‘test’ signals were audible and unpleasant for most wearers, this particular technique never became particularly widespread.

[0016] More recent feedback cancellation systems of this type rely on sounds in the environment to perform their correlation [8]. To avoid artefacts and incorrect suppression of speech however, the estimation time has to be longer than in systems using unnatural sounds to perform correlation. This means that sudden changes in the feedback path can result in several seconds of whistling before successful cancellation occurs. If implemented in conjunction with another technique to handle sudden changes, this approach can allow at least 10 dB of additional useable gain [9]. The benefits and limitations of such systems are discussed in [10].

[0017] Nearly all of the techniques discussed here require some knowledge of the frequency of oscillation. However, as a result of the nature of direct and multiple reflected acoustical paths between microphone and speaker (or the changing acoustic properties of the ear/earmould/hearing aid coupling with regard to hearing aids) the frequency of acoustic feedback is unpredictable and may extend over a substantial portion of the audio frequency spectrum (between 20 and 20,000 Hz). As a result, it is desirable to have a circuit that can quickly identify an oscillation and its frequency.

[0018] U.S. Pat. Nos. 4,232,192 and 4,079,199 propose systems using a phase locked loop (PLL) adapted to recognize an oscillation when it occurs. As is known, however, when the input signal falls off, a PLL tends to become unstable and to drift. The result of the drift is an undesirable periodic, acoustic noise signal.

[0019] U.S. Pat. No. 4,845,757 describes another oscillation recognition circuit. This circuit detects oscillations by looking for long-lasting alternating voltages having relatively large amplitude and relatively high frequency. This is problematic in many applications because it means that the signal may contain feedback oscillations for some time before they are identified by such a circuit.

[0020] There remains a need in the art to provide an improved or at least an alternative way of detecting oscillations in a signal in a reliable, effective and rapid manner, and to apply appropriate suppression to the signal upon detection.

SUMMARY OF THE INVENTION

[0021] The invention provides, in accordance with a first aspect, a method of identifying oscillation in a signal due to feedback, the method comprising the steps of:

[0022] converting the signal at each of a series of successive time windows into the frequency domain;

[0023] calculating for each of a plurality of frequency bands the change in signal phase from a time window to a subsequent time window; and

[0024] comparing, for some or all of said frequency bands, the results of the calculation step to one or more defined criteria to provide a measure of whether oscillation due to feedback is present in the signal.

[0025] This affords a technique for automatically monitoring whether the change in phase over time in a frequency band is sufficiently constant to indicate the presence of an oscillation in the signal. The successive time windows represent time intervals selected for desired performance, and are preferably of 1-100 ms duration. The windows may be discrete, or successive such windows may overlap.

[0026] Preferably, the method includes the step of further calculating, for each of the frequency bands, the change in signal amplitude from a time window to a subsequent time window, and comparing the result of the further calculation step to one or more further defined criteria, to provide a further measure as to whether oscillation due to feedback is present in the signal. This provides an additional level of discrimination.

[0027] The signal-conversion into the frequency domain may be carried out by way of a Fast Fourier Transform technique.

[0028] In a preferred form, for each frequency band, for each time window, the signal phase from one or more previous time windows is compared with that from the current time window to calculate a change of phase, and this phase change is then compared with a previous phase change to provide a measure of the change in phase change.

[0029] Preferably, the signal phase change is calculated from each time window to the next successive time window, to provide a continuous monitoring of the change in phase change in that frequency band. Alternatively, other approaches may be employed to monitor the phase change over successive time windows, such as a statistical sampling technique.

[0030] A counter may be employed, the counter incremented if the value of the change in phase change is within a prescribed limit, the counter being reset if it is not, the measure of whether oscillation due to feedback is present in the signal being provided by the counter reaching a value Mp.

[0031] If signal amplitude monitoring is employed, the method may include the step of, for each frequency band, for each time window, comparing the amplitude from at least a previous window with that of the current window to calculate a change in amplitude.

[0032] A counter may be employed, the counter being incremented if the value of the amplitude change is greater than zero, the counter being reset if it is not, the further measure of whether oscillation due to feedback is present in the signal being provided by the counter reaching a value Ma.

[0033] The value of Mp and/or Ma is selected as appropriate, dependent on the specific application and the level of sensitivity required to achieve the desired performance.

[0034] In one form of the invention, Mp is equal to Ma.

[0035] Preferably, on determination that oscillation due to feedback is present in the signal, a selected 20 method for suppressing oscillation is applied to the signal in that frequency band.

[0036] The suppression technique employed may include the step of adding a random phase to the signal in at least one of said frequency bands for a prescribed period of time. Alternatively, the suppression technique may be selected from the group of: applying a phase shift; applying a notch filter; subtracting a signal from the input signal; and applying a gain attenuation.

[0037] The invention provides, in accordance with a second aspect, apparatus for identifying oscillation in a signal in a system having an input transducer and an output transducer, comprising:

[0038] means for converting the signal into the frequency domain;

[0039] means for analysing the converted signal at each of a succession of time windows over a number of frequency bands, to determine the amplitude and phase of the signal in each frequency band;

[0040] means for calculating the change in signal phase for each frequency band from a time window to a subsequent time window; and

[0041] means for comparing the change in phase with one or more defined criteria to provide a measure of whether oscillation is present in the signal.

[0042] Preferably, means are included for further calculating, for each of the frequency bands, the change in signal amplitude from one time window to a subsequent time window, and means for comparing the result of the further calculation step to one or more further defined criteria, to provide a further measure as to whether oscillation is present in the signal.

[0043] The converting means may comprise a Fast Fourier Transform (FFT) unit.

[0044] The apparatus may include means for comparing, for each frequency band and for each time window, the signal phase from one or more previous time windows with that from the current window to calculate a change of phase, and means for comparing this phase change with a previous phase change to provide a measure of the change in phase change.

[0045] Preferably, the means for comparing is arranged to calculate the signal phase change from each time window to the next successive time window, to provide continuous monitoring of the change in phase change in that frequency band.

[0046] In one form of the invention, a counter is included, arranged to be incremented if the value of the change in phase change is within a prescribed limit, and to be reset if it is not, the measure of whether oscillation is present in the signal being provided by the counter reaching a value Mp.

[0047] If means are included for calculating the change in signal amplitude from one time window to a subsequent time window, this may comprise means for comparing, for each frequency band and for each time window, the amplitude from at least one previous window with that of the current window, to calculate a change in amplitude.

[0048] A counter may be arranged to be incremented if the value of the amplitude change is greater than zero, and to be reset if it is not, the further measure of whether oscillation is present in the signal being provided by the counter reaching a value Ma.

[0049] In a preferred form, the apparatus is provided in combination with a means for suppressing oscillation, the suppressing means arranged to be triggered in accordance with the measure of whether oscillation is present in the signal.

[0050] The apparatus may include means for reconverting the signal to a waveform signal to be fed to the output transducer.

[0051] The invention differs from previous techniques because it relies on continuous monitoring of signal phase information as the primary criterion for oscillation detection, thus allowing oscillation conditions to be identified before the amplitude of the signal at a particular frequency becomes unstably high, ideally before audible ringing occurs.

[0052] The present invention therefore provides a feedback detection system that continually monitors an input signal and recognises the presence of an oscillation quickly and accurately. Further, the invention provides alteration of the feedback loop in a manner that disrupts the feedback oscillation conditions and suppresses the oscillation without significantly affecting the system frequency response.

[0053] In the preferred method of carrying out the invention, short samples or windows of the input signal are analysed into a number of frequency bands via a Fast Fourier Transform (FFT), the amplitude and phase of each frequency component is calculated and then checked against the following oscillation criteria:

[0054] 1. The change in phase from one window to the next must be constant within an acceptable small variation for at least Mp successive windows.

[0055] 2. (Optional) The amplitude of the frequency component should be increasing from one window to the next for at least Ma successive windows.

[0056] The invention is based on the realisation that if an oscillation is present in a frequency band it will either dominate the band or be attenuated by destructive interference. Thus any band containing an oscillation that is not attenuated will have a reasonably constant change in phase from one window to the next. In addition, any band that is feeding back to the input will experience an increase in amplitude. By monitoring each frequency band with regards to at least the first of these criteria, the technique can be used to identify oscillation, often before the amplitude becomes uncomfortably loud. In addition, by using these two criteria in conjunction the system can avoid misdiagnosing loud sounds or most oscillating musical tones as feedback.

[0057] It should be noted that the feedback detection method may be used with any suitable approach to feedback suppression.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] The present invention will become more apparent by describing in detail a preferred non limiting embodiment with reference to the attached drawings, in which:

[0059] FIG. 1 is a block diagram schematically illustrating a feedback loop;

[0060] FIG. 2 is a block diagram of an apparatus according to the present invention;

[0061] FIG. 3 is a flow diagram illustrating the logic and process of feedback detection;

[0062] FIG. 4 is a flow diagram illustrating the logic and process of feedback suppression; and

[0063] FIGS. 5 and 6 are block diagrams of alternative architectures of apparatus according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0064] An acoustic system 10 in accordance with the invention, such as a hearing aid, is schematically depicted in FIG. 2. A microphone 11 converts an acoustic signal, such as the speech, into an analogue electrical signal proportional to the acoustic signal, which signal is then converted by an A/D converter 12 into a digital signal. The output of A/D converter 12 is connected to the input of a Discrete Fourier Transform (DFT) unit—such as a Fast Fourier Transform (FFT) unit 13—for analysing the frequency components of the signal, and unit 14 enables analysis of 64 frequency bands across the spectrum of the signal. A suitable unit is the Toccata Plus integrated circuit designed and developed by the Dspfactory, operating with 16 kHz sampling rate and using 128 point windows of 8 millisecond duration with 50% overlap to yield 64 linearly spaced frequency bands at 125 Hz intervals from 0 to 8000 Hz. Module 20 is a feedback detector arranged to monitor the phase and amplitude of the signal in each frequency band in the spectrum (adjusted if appropriate, as explained further below) during successive sampling windows at short intervals, such as successive 8 millisecond windows with 50% overlap, calculated every 4 milliseconds. The apparatus includes a counter for each frequency band, which can be incremented or reset at each successive time window.

[0065] For each time window, the measured phase from the previous window is subtracted from the phase in the current window to calculate the change in phase at a particular frequency band. This change in phase is compared to the previous change in phase. If the values are within a defined variation (ie the change in the phase change is within the threshold) then the counter is incremented, otherwise the counter is reset. Further, the amplitude in the current window is compared with the amplitude in the previous window. If the current amplitude is less than the previous amplitude, then the counter is reset. The feedback detector is programmed to respond—by triggering feedback suppression—to the counter reaching a value M. The present invention contemplates that either the change in phase change criterion (counter reaches Mp) or the change in amplitude criterion (counter reaches Ma) may be considered for suppression triggering, or both.

[0066] The example represented in FIG. 3 illustrates, for a time window, the process of detection using the change in phase change criterion. For each of the 64 bands, the state of the band is determined (30). If that band is already being suppressed (31), no calculations are performed. Otherwise, the phase is calculated (32), and the previous phase value calculated for that band (which value has been stored—see below) is subtracted from the current phase value (33) to provide a current value of phase change. The next step (34) is to subtract the previous phase change value from the current phase change value, to output a value of change of phase change. This value is then checked (35) and (37), and if it is within a prescribed threshold for phase change variation, the counter is imcremented by 1 (41). The subtraction of 2&pgr; radians (36) and second check (37) ensure that the value of the change of phase change is checked, irrespective of whether the change has increased or decreased. If the value is not within the threshold, the counter is reset to 0 (38), the current phase and phase change value is saved (39), and the next band is selected (40).

[0067] If the counter has been incremented (41), a check is made to determine if it has reached a value Mp (42), thereby indicating an oscillation has been detected (43) and flagging that band for suppression (see below). If not, the current phase and phase change values are saved (39), and the next band is selected (40). It is to be noted that the bands can be checked in parallel or sequentially within each time window.

[0068] In simulations carried out by the inventors, where both criteria for detection have been employed, Ma=Mp=12 gives good performance. Using Ma=Mp simplifies the detection apparatus and method, as a common counter can be used. If only one criterion is to be employed in detecting feedback, the Ma or Mp value may be increased to avoid false triggering of feedback suppression.

[0069] Once the counter for any frequency band exceeds the required values of Ma and/or Mp, this frequency band is deemed to be in oscillation, and an apply phase module 21 is triggered (see FIG. 2). Apply phase module 21 generates a complex number with random phase and amplitude 1 for each window, and multiplies the real gain value at module 22 for the frequency band by this complex number before the gain is applied to the signal via gain unit 23 to provide an adjusted spectrum 24. The loop illustrated in FIG. 2 indicates that the phase of the gain multipliers depends on the apply phase unit, which depends on the feedback detector unit. Apply phase module 21 continues to apply random phase to the gain for about one second, to allow the conditions which created the feedback path to change.

[0070] The example represented in FIG. 4 illustrates the process of suppression for a time window, appropriate for the example embodiments illustrated in FIGS. 5 and 6. Firstly, the state of a selected band is checked (40), to determine whether it is flagged for suppression (41). If not, the next band is selected (47). If it is flagged for suppression, the magnitude of the signal at that band is obtained (42) and multiplied by the real part of the generated random complex number (43), the resulting new real component being saved (44). Further, the magnitude of the signal is multiplied by the corresponding imaginary part of the generated random complex number (45), and the resulting new imaginary component saved (46).

[0071] The signal passes through MPO unit (Maximum Power Output) 25 (see FIG. 2), and is then reconverted into a time domain waveform by inverse FFT module 26. A D/A converter 27 then converts the digital signal to an electrical analogue signal before supplying it to the hearing aid output terminal to drive speaker 28.

[0072] It is to be noted that the ‘magnitude of the signal’ in a band referred to above in the context of FIG. 4 may be the output spectrum value (for the example embodiments shown in FIGS. 5 and 6), or may be the gain value (for the example embodiment shown in FIG. 2), and the invention may be implemented using either approach, the selection depending at least in part on the hardware employed for the processing. In the alternative architectures of FIGS. 5 and 6 the random phase is applied to the output spectrum rather than to the gains, in both embodiments the gain values are applied to the signal by gain unit 23 before feedback detector 20. In FIG. 6, MPO unit 25 is omitted, to illustrate that the invention can be implemented without it.

[0073] Feedback detector 20 and apply phase module 21 do not necessarily have to be applied together. An alternative form of feedback suppression, such as application of a notch filter, may be applied to a signal in which feedback oscillation has been identified by feedback detector 20. Other types of feedback suppression which might be employed include gain attenuation at the frequency band in question, applying a time varying phase change, or subtraction of the signal at the frequency band in question. Similarly, an alternative form of feedback detector, such as a phase locked loop (PLL) circuit, may be employed, apply phase module 21 being used to apply a random phase to the signal in that particular frequency band once feedback has been detected.

[0074] It has been found in simulations carried out by the inventors that application of both feedback detector 20, combining the monitoring of both phase change and amplitude, along with the application of apply phase module 21, can result in suppression of all feedback oscillation in 60-100 milliseconds.

[0075] Modifications and improvements to the invention will be readily apparent to those skilled in the art. Such modifications and improvements are intended to be within the scope of this 20 invention. For example, in accordance with the invention, the signal spectrum may be split into a plurality of discrete frequency bands, or alternatively neighbouring bands may overlap. The word ‘comprising’ and forms of the word ‘comprising’ as used in this description and in the claims does not limit the invention claimed to exclude any variants or additions.

Claims

1. A method of identifying oscillation in a signal due to feedback, the method comprising the steps of:

converting the signal at each of a series of successive time windows into the frequency domain;

calculating for each of a plurality of frequency bands the change in signal phase from a time window to a subsequent time window; and

comparing, for some or all of said frequency bands, the results of the calculation step to one or more defined criteria to provide a measure of whether oscillation due to feedback is present in the signal.

2. The method of claim 1, including the step of further calculating, for each of the frequency bands, the change in signal amplitude from a time window to a subsequent time window, and comparing the result of the further calculation step to one or more further defined criteria, to provide a further measure as to whether oscillation due to feedback is present in the signal.

3. The method of claim 1, in which the step of signal conversion into the frequency domain is carried out by way of a Fast Fourier Transform technique.

4. The method of claim 1, in which the number of frequency bands is around 64.

5. The method of claim 1, in which said successive time windows are in the range of 1 to 100 ms.

6. The method of claim 1, in which for each frequency band, for each time window the signal phase from one or more previous time windows is compared with that from the current window to calculate a change of phase, and this phase change is then compared with a previous phase change to provide a measure of the change in phase change.

7. The method of claim 6, in which the signal phase change is calculated from each time window to the next successive time window, to provide a continuous monitoring of the change in phase change in that frequency band.

8. The method of claim 6, in which a counter is employed, the counter being incremented if the value of the change in phase change is within a prescribed limit, the counter being reset if it is not, the measure of whether oscillation due to feedback is present in the signal being provided by the counter reaching a value Mp.

9. The method of claim 2, in which for each frequency band, for each time window the amplitude from at least one previous window is compared with that of the current window to calculate a change in amplitude.

10. The method of claim 9, in which a counter is employed, the counter being incremented if the value of the amplitude change is greater than zero, the counter being reset if it is not, the further measure of whether oscillation due to feedback is present in the signal being provided by the counter reaching a value Ma.

11. The method of claim 10 in which a second counter is employed, said second counter being incremented if the value of the change in phase change is within a prescribed limit, the second counter being reset if it is not, the measure of whether oscillation due to feedback is present in the signal being provided by the second counter reaching a value Mp and, wherein Mp=Ma.

12. The method of claim 1, in which, on determination that oscillation due to feedback is present in the signal, a selected method for suppressing oscillation is applied to the signal in that frequency band.

13. The method of claim 12 in which the suppression technique includes the step of adding a random phase to the signal in at least one of said frequency bands for a prescribed period of time.

14. The method of claim 12 in which the suppression technique is selected from the group of: applying a phase shift; applying a notch filter; subtracting a signal from the input signal; and applying a gain attenuation.

15. Apparatus for identifying oscillation in a signal in a system having an input transducer and an output transducer, comprising:

means for converting the signal into the frequency domain;

means for analysing the converted signal at each of a succession of time windows over a number of frequency bands, to determine the amplitude and phase of the signal in each frequency band;

means for calculating the change in signal phase for each frequency band from a time window to a subsequent time window; and

means for comparing the change in phase with one or more defined criteria to provide a measure of whether oscillation is present in the signal.

16. The apparatus of claim 15, including means for further calculating, for each of the frequency bands, the change in signal amplitude from one time window to a subsequent time window, and means for comparing the result of the further calculation step to one or more further defined criteria, to provide a further measure as to whether oscillation is present in the signal.

17. The apparatus of claim 15, wherein the converting means comprises a Fast Fourier Transform (FFT) unit.

18. The apparatus of claim 15, including means for comparing, for each frequency band and for each time window, the signal phase from one or more previous time windows with that from the current window to calculate a change of phase, and means for comparing this phase change with a previous phase change to provide a measure of the change in phase change.

19. The apparatus of claim 18, wherein said means for comparing is arranged to calculate the signal phase change from each time window to the next successive time window, to provide continuous monitoring of the change in phase change in that frequency band.

20. The apparatus of claim 18, including a counter arranged to be incremented if the value of the change in phase change is within a prescribed limit, and to be reset if it is not, the measure of whether oscillation is present in the signal being provided by the counter reaching a value Mp.

21. The apparatus of claim 16, in which the means for further calculating comprise means for comparing, for each frequency band and for each time window, the amplitude from at least one previous window with that of the current window, to calculate a change in amplitude.

22. The apparatus of claim 21, including a counter arranged to be incremented if the value of the amplitude change is greater than zero, and to be reset if it is not, the further measure of whether oscillation is present in the signal being provided by the counter reaching a value Ma.

23. The apparatus of claim 15, in combination with a means for suppressing oscillation, the suppressing means arranged to be triggered in accordance with the measure of whether oscillation is present in the signal.

24. The apparatus of claim 15, including means for reconverting the signal to a waveform signal to be fed to the output transducer.