Transform-domain decorrelation in Dolby Digital Plus

The Dolby Digital Plus (DDPlus) multichannel audio codec employs channel coupling for parsimonious transmission of high frequency components of the signal, wherein the transform coefficients of discrete channels beyond a coupling-begin frequency are transmitted as a mono-downmix. The decoder reconstructs individual channels by appropriate panning with frequency-banded gains. While channel coupling is a valuable parametric coding tool for low bit-rate encoding of multichannel audio, the resulting high inter-channel coherence can affect audio post-processors that utilize downmixing, such as headphone virtualizers. To mitigate these effects, a new low-complexity spatial audio coding tool is proposed, whose primary objective is to restore phase diversity in the decoder's output. In this new approach, a decorrelation signal is synthesized directly from the decoded real-valued and critically-sampled transform (MDCT) coefficients, avoiding the expense of computing the imaginary counterpart (MDST) or signal transformation to a different domain (such as QMF). The decorrelation signal is then adaptively mixed with the dry signal and inverse-transformed as in a conventional decoder. The degree of mixing depends on spatial parameters that are either sent in the bitstream or estimated in the decoder from the discrete (not coupled) low-frequency spectral coefficients. Listening tests demonstrate the significant performance benefits obtained in either mode of operation of the proposed tool.