Patent ID: 8144888

Claim:
An active noise reducing filter apparatus for actively reducing noise d from a primary noise source x, comprising: a secondary source signal connector for connecting to at least one secondary source, such as a loudspeaker, wherein said secondary source generates secondary noise y to reduce said primary noise d; a sensor connector for connecting to at least one sensor, such as a microphone, for measuring said primary and secondary noise as an error signal e; a first delay connected to said sensor connector for delaying said error signal e and a time reversed secondary path filter G* for providing a delayed filtered error signal e′; a reference signal connector for receiving at least one reference signal x, said reference signal x being coherent with said primary noise d; a first control filter W a connected to said reference signal connector for receiving said reference signal x and for calculating a control signal from said reference signal for providing a secondary source signal u; a second delay connected to said reference signal connector for receiving said reference signal x and for calculating a delayed reference signal x′; and an adaptation circuit arranged to adapt said first control filter W a based on said delayed reference signal x′ and an error signal e″ wherein the filter apparatus further comprises: a third delay connected to said first control filter W a for receiving said control signal and for calculating a delayed control signal y′; a second control filter W b connected to said second delay for receiving said delayed reference signal x′ and for calculating an auxiliary control signal y″; wherein said adaptation circuit is connected to said second control filter for adapting said second control filter W b while receiving said error signal e″ as a sum of said auxiliary control signal y″ and an auxiliary noise signal d′, said auxiliary noise signal d′ constructed from a difference of said delayed filtered error signal e′ and said delayed control signal y′; wherein for adapting said first control filter W a said adaptation circuit is arranged to update said first control filter W a by a copy of said updated second control filter W b ; and a flatness improving preconditioning circuit for preconditioning the reference signals.