Abstract: The invention concerns a digital filter. The filter comprises at least a first and a second 5 delayed summation line, whereby one of the delay lines comprise a warped filter FIR filter and the other line is a FIR filter, whereby: at least two delayed summation lines are present, midpoint elements from the first delayed summation line are used as input to the second delayed summation line, output from the first delayed summation line is delayed with a delay corresponding to the total delay of the second delayed summation line, the delayed output from the first delayed summation line is added to output from the second delayed summation line to form a new output.

Abstract: The invention concerns an audio system comprising a microphone, audio signal processing means, an output transducer and mans for detecting a possible feedback tone and the corresponding frequency of the feedback tone in the audio system between the output transducer and the microphone. According to the invention means for counteracting feedback are provided. Further, mans are provided for changing the phase of the audio signal at a given frequency.