the oscillation 4 result no feedback is detected 5 cross fading back to system module 1 takes place alternatively the system can also continue operating with system module 2 if the oscillation detector 19 initiates a new u201c request u201d i e feedback whistle is suspected it is possible to switch back from system module 2 to system module 1 in the event of feedback whistle this transition again results in a change in the oscillation frequency or no change in the case of a regular input signal according to a second detection situation no sinusoidal signal is present at the input the system is unstable and feedback whistle is occurring the system then reacts as follows 1 the oscillation detector 19 responds 2 cross fading from system module 1 to system module 2 takes place 3 the modification detector 20 detects that the oscillation frequency is changing 4 result the harmonic signal is the result of instability therefore feedback is present 5 as in the previous situation the system can continue operating with system module 2 and only cross fade if necessary or it can fade back again immediately after feedback checking in order to also cover the eventuality that feedback whistle arises e g after a sine wave has been applied to the system as a wanted signal and this oscillation has already been detected as u201c non feedback whistle u201d system module switching can be repeated within a certain time interval as long as the oscillation detector 19 responds in a further embodiment the oscillation detector 19 only detects whether an oscillation is present without knowing the frequency of the oscillation in this case the modification detector 20 must undirectedly analyze the overall signal for signal.