When the number of available loudspeakers or transmission channels is smaller than the number of channels in an audio format, downmixing is required. If the audio in the channels contain nonaligned interdependent sounds, the downmixed signal may have perceptible spectral biases, such as that produced by a comb filter. A time–frequency domain, phase-adaptive downmixing technique is proposed to reduce such spectral effects. The technique aligns the phases of the input channel pairs or groups having a high measured normalized interchannel coherence prior to the downmixing. Simulations and listening tests were conducted to show the conditions in which the proposed method provides benefit with respect to the legacy methods. Computational evaluations showed that the method may be implemented in real time for a large number of channels using reasonable hardware. The target for the phase processing is weighted with the input channel amplitudes, and the phase coefficients are regularized over time and frequency to avoid processing artifacts.
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