We address the problem of integrating directional analysis of sound into the filterbank of a spatial audio coder, with the purpose of processing and coding with some degree of inde- pendence the plane waves traveling in different directions. A plane wave represents an elementary waveform in the spatio-temporal analysis of the sound field, the same way a complex exponential is an elementary waveform in the time domain analysis of signals. Since a two-dimensional separable filterbank is not flexible enough for this purpose, we propose a non-separable approach based on the quin- cunx filterbank with diamond-shaped filters, cascaded with a base transform filterbank. This solution provides an in- vertible and critically sampled decomposition of the spatio- temporal spectra into subbands representing the different di- rections of wave propagation.