MDCT-based complex prediction stereo coding

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, the upmix stage comprising:

a module for 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 the module is adapted to determine 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;

a weighted summer for computing a side signal 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 encoded in a bit stream signal received by the decoder system; and

a sum-and-difference stage for computing the stereo signal on the basis of 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 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 at least one of said first and second frequency-domain modifier stages is a temporal noise shaping, TNS, stage.

4. The decoder system of claim 3 , further adapted to receive, for each time frame, a data field associated with that frame and to operate, responsive to the 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.

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;

optionally, the first spectral components are obtainable by one of the following:

a discrete cosine transform, DCT, or

a modified discrete cosine transform, MDCT, and

optionally, the second spectral components are obtainable by one of the following:

a discrete sine transform, DST, or

a modified discrete sine, transform, MDST.

7. The decoder of claim 6 , wherein:

the downmix signal is partitioned into successive time frames, each associated with a value of the complex prediction coefficient; and

the module for computing a second frequency-domain representation of the downmix signal is adapted to deactivate itself, responsive to the absolute value of the imaginary part of the complex prediction coefficient being smaller than a predetermined tolerance for a time frame, so that it generates no output for that time frame.

8. The decoder system of claim 5 , 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.

9. The decoder system of claim 5 , wherein the module for computing a second frequency-domain representation of the downmix signal comprises:

an inverse transform stage for computing a time-domain representation of the downmix signal and/or of the side signal on the basis of the first frequency-domain representation of the respective signal in the first subspace of the multidimensional space; and

a transform stage for computing the second frequency-domain representation of the respective signal on the basis of the time-domain representation of the signal,

wherein, preferably, the inverse transform stage performs an inverse modified discrete cosine transform, MDCT, and the transform stage performs a modified discrete sine transform, MDST.

10. The decoder system of claim 9 , 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, for use in joint stereo coding; or

(b) a sum-and-difference stage, for use in direct stereo coding;

a further inverse transform stage arranged in the upmix stage, for computing a time-domain representation of the side signal;

a selector arrangement arranged upstream of the inverse transform stages, adapted to selectively connect these to either:

(a) a further sum-and-difference stage which is in turn connected to a point downstream of the switching assembly and upstream of the upmix stage; or

(b) a downmix signal obtained from the switching assembly and a side signal obtained from the weighted summer.

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 channel and a residual channel 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 including the steps of:

computing a second frequency-domain representation of the downmix channel 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;

computing the side channel 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;

and further comprising either the step, to be performed prior to the step of upmixing, of applying temporal noise shaping, TNS, to said first frequency-domain representation of the downmix signal and/or said first frequency-domain representation of the residual channel;

or the step, to be performed after the step of upmixing, of applying TNS to at least one channel of said output stereo signal.

12. 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 11 .