MDCT-based complex prediction stereo coding

• US 10,283,126 B2
• Filed: 12/20/2017
• Issued: 05/07/2019
• Est. Priority Date: 04/09/2010
• Status: Active Grant

• Current Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Original Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Inventors: Purnhagen, Heiko, Carlsson, Pontus, Villemoes, Lars
• Primary Examiner(s): Jamal, Alexander
• Application Number:
  US15/849,622
• Publication Number:
  US 20180137866A1
• Time in Patent Office: 503 Days
• Field of Search: 381 22, 381 19, 381 20, 381 21, 381 23, 704500, 704501, 704504
• CPC Class Codes: G10L 19/008 Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
• CPC Class Codes: G10L 19/0212 using orthogonal transformation
• CPC Class Codes: G10L 19/06 Determination or coding of the spectral characteristics, e.g. of the short-term prediction coefficients
• CPC Class Codes: G10L 19/167 Audio streaming, i.e. formatting and decoding of an encoded audio signal representation into a data stream for transmission or storage purposes
• CPC Class Codes: G10L 19/18 Vocoders using multiple modes
• CPC Class Codes: G10L 25/12 the extracted parameters being prediction coefficients
• CPC Class Codes: H04S 2400/01 Multi-channel, i.e. more than two input channels, sound reproduction with two speakers wherein the multi-channel information is substantially preserved
• CPC Class Codes: H04S 3/008 in which the audio signals are in digital form, i.e. employing more than two discrete digital channels data reduction aspects thereof based on psychoacoustics G10L19/02

Abstract

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.

Claims

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 (M) and a residual signal (D), 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;

a weighted summer for 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 a complex prediction coefficient (α

) 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

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,

wherein the upmix stage is further operable in a pass-through mode, in which said downmix and residual signals are supplied to the sum-and-difference stage directly.

2. The decoder system of claim 1, wherein said downmix signal and residual signal are segmented into time frames,

said upmix stage being further adapted to receive, for each time frame, a two-bit data field associated with that frame and to operate, responsive to the value of the data field, in its active mode or its pass-through mode.

3. The decoder system of claim 1, wherein said downmix and residual signal are segmented into time frames,

said upmix stage being further adapted to receive in an MPEG bit stream, for each time frame, an ms_mask_present field associated with that frame and to operate, responsive to the value of the ms_mask_present field, in its active mode or its pass-through mode.

4. The decoder system of claim 1, wherein:

the first spectral components have real values expressed in the first subspace; and

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.

5. The decoder of claim 4, 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.

6. The decoder of claim 5, wherein the downmix signal time frames are further partitioned into frequency bands, each accompanied by 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 frequency band of a time frame, so that it generates no output for that frequency band.

7. A decoding method for providing a stereo signal by complex prediction stereo coding, the method comprising the steps of:

receiving 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;

receiving a control signal; and

responsive to the value of said control signal;

(a) upmixing said downmix and residual signals, using an upmix stage, to obtain said stereo signal, by performing the sub-steps of;

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;

computing a first frequency-domain representation of a side signal, the first frequency-domain representation of the side signal 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 encoded in a bit stream signal;

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

computing said stereo signal by applying a sum-and-difference transform to the first frequency-domain representation of the downmix signal and the side signal, or

(b) suspending upmixing.

8. The decoding method of claim 7, 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.

9. The decoding method of claim 8, wherein:

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

said computing a second frequency-domain representation of the downmix signal is suspended 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 no output is generated for that time frame.

10. The decoding method of claim 9, wherein:

the downmix signal time frames are further partitioned into frequency bands, each accompanied by a value of the complex prediction coefficient; and

said computing a second frequency-domain representation of the downmix signal is suspended responsive to the absolute value of the imaginary part of the complex prediction coefficient being smaller than a predetermined tolerance for a frequency band of a time frame, so that no output is generated for that frequency band.

11. The decoding method of claim 7, wherein said stereo signal is represented in the time domain and the method further comprises:

omitting, responsive to said bit stream signal being coded by direct stereo coding or joint stereo coding, said step of upmixing; and

inversely transforming said bit stream signal to obtain said stereo signal.

12. The decoding method of claim 11, further comprising the step of:

omitting, responsive to said bit stream signal being coded by direct stereo coding or joint stereo coding, said steps of transforming said time-domain representation of the downmix signal and of computing a side signal; and

inversely transforming the frequency-domain representations of each of the channels encoded by the bit stream signal the to obtain said stereo signal.

13. 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 7.