Efficient format conversion in audio coding

Abstract

The present disclosure relates to a method of downmixing a plurality of input audio channels. The method include obtaining, for each of the input audio channels, a plurality of frequency coefficients in a plurality of corresponding frequency bins, and applying, for at least one frequency bin, a downmix matrix to a first array formed by the frequency coefficients of the plurality of input audio channels for the respective frequency bin to obtain a second array formed by the frequency coefficients of a plurality of intermediate audio channels for the respective frequency bin. The method further involves determining a third array including only the non-zero entries of the downmix matrix, and determining a fourth array including, for each entry of the third array, an entry indicative of a position of the respective entry of the third array within the downmix matrix. Applying the downmix matrix to the first array involves multiplying, for the at least one frequency bin, the entries of the third array with corresponding entries of the first array for the respective frequency bin, the corresponding entries of said first array being determined on the basis of the fourth array. The present disclosure further relates to a corresponding apparatus for downmixing a plurality of input audio channels and to a corresponding program storage device configured for downmixing a plurality of input audio channels.