System for suppressing ambient noise in a hands-free device

Abstract

In order to suppress as much noise as possible in a hands-free device in a motor vehicle, for example, two microphones (M, M) are spaced a certain distance apart, the output signals (MS, MS) of which are added in an adder (AD) and subtracted in a subtracter (SU). The sum signal (S) of the adder (AD) undergoes a Fourier transform in a first Fourier transformer (F), and the difference signal (D) of the subtracter (SU) undergoes a Fourier transform in a second Fourier transformer (F). From the two Fourier transforms R(f) and D(f), a speech pause detector (P) detects speech pauses, during which a third arithmetic unit (R) calculates the transfer function Hof an adaptive transformation filter (TF). The transfer function of a spectral subtraction filter (SF), at the input of which the Fourier transform R(f) of the sum signal (S) is applied, is generated from the spectral power density Sof the sum signal (S) and from the interference power density Sgenerated by the adaptive transformation filter (TF). The output of the spectral subtraction filter (SF) is connected to the input of an inverse Fourier transformer (IF), at the output of which an audio signal (A) can be picked up in the time domain which is essentially free of ambient noise.