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. The method involves using an FFT device or similar to convert a 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. For additional discrimination, the change in signal amplitude from a time window to a subsequent time window may also be calculated for each of the frequency bands, and the result compared with one or more further defined criteria. The invention has particular application in hearing aid devices.

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 M p .  
     
     
         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 M a .  
     
     
         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 M p  and, wherein M p =M a .  
     
     
         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 M p .  
     
     
         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 M a .  
     
     
         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.