MDCT-based complex prediction stereo coding
The invention provides methods and devices for stereo encoding and decoding using complex prediction in the frequency domain. In one embodiment, a decoding method, for obtaining an output stereo signal from an input stereo signal encoded by complex prediction coding and comprising first frequency-domain representations of two input channels, comprises the upmixing steps of: (i) computing a second frequency-domain representation of a first input channel; and (ii) computing an output channel on the basis of the first and second frequency-domain representations of the first input channel, the first frequency-domain representation of the second input channel and a complex prediction coefficient. The upmixing can be suspended responsive to control data.
1. A decoder system for providing a stereo signal by complex prediction stereo decoding, the decoder system comprising:
an upmix stage adapted to generate the stereo signal based on first frequency-domain representations of a downmix signal and a residual signal, each of the first frequency-domain representations comprising first spectral components representing spectral content of the corresponding signal expressed in a first subspace of a multidimensional space, the upmix stage comprising:
a module for computing a second frequency-domain representation of the downmix signal based on the first frequency-domain representation of the downmix signal, the second frequency-domain representation comprising second spectral components representing spectral content of the downmix signal expressed in a second subspace of the multidimensional space that includes a portion of the multidimensional space not included in the first subspace;
a weighted summer for computing a side signal by combining the first frequency-domain representation of the residual signal, the first frequency-domain representation of the downmix signal weighted by a real-valued part of a complex prediction coefficient, and the second frequency-domain representation of the downmix signal weighted by an imaginary-valued part of the complex prediction coefficient; and
a sum-and-difference stage for computing the stereo signal based on the first frequency-domain representation of the downmix signal and the side signal;
a first frequency-domain modifier stage arranged upstream of the upmix stage and operable in an active mode, in which it processes a frequency-domain representation of at least one signal, and a passive mode, in which it acts as a pass-through; and
a second frequency-domain modifier stage arranged downstream of the upmix stage and operable in an active mode, in which it processes a frequency-domain representation of at least one signal, and a passive mode, in which it acts as a pass-through.
2. The decoder system of claim 1 , wherein at least one of the first and the second frequency-domain modifier stages is a temporal noise shaping, TNS, stage.
3. The decoder system of claim 2 , further adapted to receive, for each time frame, a data field associated with that frame and to operate, responsive to a value of the data field, the first frequency-domain modifier stage in its active mode or its pass-through mode and the second frequency-domain modifier stage in its active mode or its pass-through mode.
4. The decoder system of claim 1 , further comprising:
a dequantization stage arranged upstream of the upmix stage, for providing the first frequency-domain representations of the downmix signal and the residual signal based on a bit stream signal.
5. A decoding method for providing a stereo signal by complex prediction stereo decoding, comprising:
receiving first frequency-domain representations of a downmix signal and a residual signal, and a complex prediction coefficient, wherein each of the first frequency-domain representations comprises first spectral components representing spectral content of the corresponding signal expressed in a first subspace of a multidimensional space;
computing a second frequency-domain representation of the downmix signal based on the first frequency-domain representation of the downmix signal, the second frequency-domain representation comprising second spectral components representing spectral content of the downmix signal expressed in a second subspace of the multidimensional space that includes a portion of the multidimensional space not included in the first subspace;
computing a side signal by combining the first frequency-domain representation of the residual signal, the first frequency-domain representation of the downmix signal weighted by a real-valued part of the complex prediction coefficient, and the second frequency-domain representation of the downmix signal weighted by an imaginary-valued part of the complex prediction coefficient;
computing the stereo signal based on the first frequency-domain representation of the downmix signal and the side signal; and either applying temporal noise shaping, TNS, to the first frequency-domain representation of the downmix signal and/or the first frequency-domain representation of the residual signal; or
applying TNS to at least one signal of the stereo signal.
6. A computer-program product comprising a computer-readable medium storing instructions which when executed by a general-purpose computer perform the method set forth in claim 5 .