A method of determining a system parameter sp(n) of an adaptive algorithm in an adaptive feedback cancellation algorithm in an audio processing system, the audio processing system comprising
- a) a microphone system comprising
- a1) a number P of electric microphone paths, each microphone path MPi, i=1, 2, . . . , P, providing a processed microphone signal, each microphone path comprising
- a1 .1) a microphone Mi for converting an input sound to an input microphone signal yi;
- a1.2) a summation unit SUMi, for receiving a feedback compensation signal {circumflex over (v)}i;
- and the input microphone signal or a signal derived therefrom and providing a compensated signal ei;
- and
- a1.3) a beamformer filter gi for making frequency-dependent directional filtering of the compensated signal ei, the output of said beamformer filter gi providing a processed microphone signal ēi, i=1, 2, . . . , P;
- a2) a summation unit SUM(MP) connected to the output of the microphone paths i=1, 2, . . . , P, to perform a sum of said processed microphone signals ēi, i=1, 2, . . . , P;
- thereby providing a resulting input signal;
- b) a signal processing unit for processing said resulting input signal or a signal originating therefrom to a processed signal;
- c) a loudspeaker unit for converting said processed signal or a signal originating therefrom to an output sound, said processed signal or said signal originating therefrom being termed the loudspeaker signal u;
- said microphone system, signal processing unit and said loudspeaker unit forming part of a forward signal path;
- and
- d) an adaptive feedback cancellation system comprising a number of internal feedback paths IFBPi, i=1, 2, . . . , P, for generating an estimate of a number P of unintended feedback paths, each unintended feedback path at least comprising an external feedback path from the output of the loudspeaker unit to the input of a microphone Mi, i=1, 2, . . . , P, and each internal feedback path comprising a feedback estimation unit for providing an estimated impulse response hest,i of the ith unintended feedback path, i=1, 2, . . . , P, using said adaptive feedback cancellation algorithm, the estimated impulse response hest,i constituting said feedback compensation signal {circumflex over (v)}i being subtracted from said microphone signal yi or a signal derived therefrom in respective summation units SUMi of said microphone system to provide error signals ei, i=1, 2, . . . , P;
- the forward signal path, together with the external and internal feedback paths defining a gain loop;
- the method comprising:
- S1) determining an expression of an approximation of the square of the magnitude of the feedback part of the open loop transfer function, πest(ω,n), where ωis normalized angular frequency, and n is a discrete time index, where the feedback part of the open loop transfer function comprises the internal and external feedback paths, and the forward signal path, exclusive of the signal processing unit, and wherein the approximation defines a first order difference equation in πest(ω,n), from which a transient part depending on previous values in time of πest(ω,n) and a steady state part can be extracted, the transient part as well as the steady state part being dependent on the system parameter sp(n) at the current time instance n;
- S2a) determining the slope per time unit α for the transient part,
- S3a) expressing the system parameter sp(n) by the slope α;
- S4a) detatinining the system parameter sp(n) for a predefined slope-value α pd;
- or
- S2b) determining the steady state value πest(ω,∞) of the steady state part,
- S3b) expressing the system parameter sp(n) by the steady state value πest(ω,∞);
- S4b) determining the system parameter sp(n) for a predefined steady state value πest(ω,∞)pd.