Abstract
A minimum variance distortionless response (MVDR) beamformer can be an effective multi-microphone noise reduction strategy' provided that a vector of transfer functions from the desired speech signal at a reference microphone to the other microphones, i.e. a vector of the relative transfer functions (RTFs), is known. When using a local microphone array (LMA) and an external microphone (XM), this RTF vector has two distinct parts: an RTF vector for that of only the LMA and a single RTF component for the XM, with the reference microphone on the LMA. Whereas a priori assumptions can be made for the RTF vector for the LMA, the RTF for the XM must be estimated as the XM position is generally unknown. This paper investigates a procedure for estimating this unknown RTF by making use of the a priori RTF vector for the LMA, thereby completing the RTF vector for use of the MVDR beamformer. It is shown that such a procedure results in an Eigenvalue Decomposition (EVD) of a 2×2 matrix for a system of M microphones in the LMA and one XM. The resulting performance is evaluated within the context of a monaural MVDR beamformer.