Directional Audio Coding (DirAC) is a well-established and efficient way to capture and reproduce a spatial sound event. In a recording room, DirAC requires four spatially coincident microphones to estimate the desired parameters, i.e., direction-of-arrival and diffuseness of sound: one omnidirectional and three figure-of-eight microphones pointing along the axes of a three-dimensional Cartesian coordinate system. In most consumer applications only two dimensional scenes need to be reproduced, implying that only two figure-of-eight microphones are required. Furthermore, instead of directional microphones, arrays of omnidirectional microphones are considered for economic reasons. Therefore, we investigate various two-dimensional microphone configurations with respect to their usability for DirAC. We derive theoretical limits for the correct estimation of both direction-of-arrival and diffuseness for the most suitable planar arrays. Furthermore, we suggest a way to equalize the systematic bias for the direction-of-arrival estimation, introduced by the discrete planar arrays.
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