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 coding, 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, wherein the upmix stage:
computes a second frequency-domain representation of the downmix signal based on the first frequency-domain representation thereof, the second frequency-domain representation comprising second spectral components representing spectral content of the signal expressed in a second subspace of the multidimensional space that includes a portion of the multidimensional space not included in the first subspace, wherein the second spectral components of the downmix signal are determined by applying a Finite Impulse Response (FIR) filter to the first spectral components of the downmix signal;
computes a first frequency-domain representation of a side signal(S), the first frequency-domain representation of the side signal(S) comprising first spectral components representing spectral content of the side signal expressed in the first subspace of the multidimensional space, on the basis of the first and second frequency-domain representations of the downmix signal, the first frequency-domain representation of the residual signal, and a complex prediction coefficient (a) encoded in a bit stream signal received by the decoder system; wherein each spectral component represents a range of frequencies, and wherein each of the first spectral components of the side signal is determined from spectral components of the downmix signal and the residual signal representing the same range of frequencies as the first spectral component of the side signal; and
computes the stereo signal on the basis of the first frequency-domain representation of the downmix signal and the side signal,
wherein the upmix stage is adapted to apply independent bandwidth limits for the downmix signal and the residual signal.
2. The decoder system of claim 1 , wherein an impulse response of the FIR filter is determined depending on a window function applied to determine the first frequency domain representation of the downmix signal.
3. The decoder system of claim 1 , wherein the bandwidth limits to be applied are signaled by two data fields, indicating for each of the signals a highest frequency band to be decoded.
4. The decoder system of claim 3 , adapted to receive an MPEG bit stream in which each of said data fields is encoded as a value of a max_sfb parameter of the MPEG bit stream.
5. The decoder system of claim 1 , further comprising:
a dequantization stage arranged upstream of the upmix stage, for providing said first frequency-domain representations of the downmix signal and residual signal based on a bit stream signal.
6. The decoder system of claim 1 , wherein:
the first spectral components have real values expressed in the first subspace;
the second spectral components have imaginary values expressed in the second subspace.
7. The decoder system of claim 1 , wherein the first spectral components are obtainable by one of the following:
a discrete cosine transform, DCT, or
a modified discrete cosine transform, MDCT.
8. The decoder system of claim 1 , further comprising at least one temporal noise shaping, TNS, module arranged upstream of the upmix stage; and
at least one further TNS module arranged downstream of the upmix stage; and
a selector arrangement for selectively activating either:
(a) said TNS module(s) upstream of the upmix stage, or
(b) said further TNS module(s) downstream of the upmix stage.
9. The decoder system of claim 6 , wherein:
the downmix signal is partitioned into successive time frames, each associated with a value of the complex prediction coefficient; and
computing a second frequency-domain representation of the downmix signal is deactivated responsive to the absolute value of the imaginary part of the complex prediction coefficient being smaller than a predetermined tolerance for a time frame.
10. The decoder system of claim 1 , said stereo signal being represented in the time domain and the decoder system further comprising:
a switching assembly arranged between said dequantization stage and said upmix stage, operable to function as either:
(a) a pass-through stage, or
(b) a sum-and-difference stage,
thereby enabling switching between directly and jointly coded stereo input signals;
an inverse transform stage adapted to compute a time-domain representation of the stereo signal; and
a selector arrangement arranged upstream of the inverse transform stage, adapted to selectively connect this to either:
(a) a point downstream of the upmix stage, whereby the stereo signal obtained by complex prediction is supplied to the inverse transform stage; or
(b) a point downstream of the switching assembly and upstream of the upmix stage, whereby a stereo signal obtained by direct stereo coding is supplied to the inverse transform stage.
11. A decoding method for upmixing an input stereo signal by complex prediction stereo coding into an output stereo signal, wherein:
said input stereo signal comprises first frequency-domain representations of a downmix signal and a residual signal and a complex prediction coefficient; and
each of said first frequency-domain representations comprises first spectral components representing spectral content of the corresponding signal expressed in a first subspace of a multidimensional space,
the method being performed by an upmix stage, and comprising:
computing a second frequency-domain representation of the downmix signal based on the first frequency-domain representation thereof, the second frequency-domain representation comprising second spectral components representing spectral content of the signal expressed in a second subspace of the multidimensional space that includes a portion of the multidimensional space not included in the first subspace, wherein computing a second frequency-domain representation of the downmix signal includes determining the second spectral components of the downmix signal by applying a Finite Impulse Response (FIR) filter to the first spectral components of the downmix signal; and
computing a first frequency-domain representation of a side signal(S), the first frequency-domain representation of the side signal(S) comprising first spectral components representing spectral content of the side signal expressed in the first subspace of the multidimensional space, on the basis of the first and second frequency-domain representations of the downmix signal, the first frequency-domain representation of the residual signal, and the complex prediction coefficient; wherein each spectral component represents a range of frequencies, and wherein each of the first spectral components of the side signal is determined from spectral components of the downmix signal and the residual signal representing the same range of frequencies as the first spectral component of the side signal, and
wherein independent bandwidth limits are applied for the downmix signal and the residual signal.
12. A computer-program product comprising a non-transitory computer-readable medium storing instructions which when executed by a general-purpose computer perform the method set forth in claim 11 .