Switching between coding schemes

• US 20060173675A1
• Filed: 03/11/2003
• Published: 08/03/2006
• Est. Priority Date: 03/11/2003
• Status: Active Grant

• Current Assignee: Intellectual Ventures I LLC (Intellectual Ventures LLC)
• Original Assignee: Intellectual Ventures I LLC (Intellectual Ventures LLC)
• Inventors: Ojanpera, Juha
• Granted Patent: US 7,876,966 B2
• Time in Patent Office: Days
• US Class Current: 704/203
• CPC Class Codes: G10L 19/022 Blocking, i.e. grouping of samples in time; Choice of analysis windows; Overlap factoring
• CPC Class Codes: G10L 19/18 Vocoders using multiple modes
• CPC Class Codes: G10L 19/22 Mode decision, i.e. based on audio signal content versus external parameters

Abstract

Methods and units are shown for supporting a switching from a first coding scheme to a Modified Discrete Cosine Transform (MDCT) based coding scheme calculating a forward or inverse MDCT with a window (h(n)) of a first type for a respective coding frame, which satisfies constraints of perfect reconstruction. To avoid discontinuities during the switching, it is proposed that for a transient frame immediately after a switching, a sequence of windows (h₀(n),h₁(n),h₂(n)) is provided for the forward and the inverse MDCTs. The windows of the window sequence are shorter than windows of the first type. The window sequence splits the spectrum of a respective first coding frame into nearly uncorrelated spectral components when used as basis for forward MDCTs, and the second half of the last window (h₂(n)) of the sequence of windows is identical to the second half of a window of the first type.

Claims

1. Method for supporting a switching from a first coding scheme to a second coding scheme at an encoding end of a hybrid coding system, both coding schemes coding signals on a frame-by-frame basis, which second coding scheme is a Modified Discrete Cosine Transform based coding scheme calculating at the encoding end a Modified Discrete Cosine Transform with a window (h(n)) of a first type for a respective coding frame, a window (h(n)) of said first type satisfying constraints of perfect reconstruction, said method comprising:

providing for each first coding frame, which is to be encoded based on said second coding scheme after a preceding coding frame has been encoded based on said first coding scheme, a sequence of windows (h₀(n),h₁(n),h₂(n)), wherein said window sequence splits the spectrum of a respective first coding frame into nearly uncorrelated spectral components when used as basis for forward Modified Discrete Cosine Transforms, and wherein the second half of the last window (h₂(n)) of said sequence of windows is identical to the second half of a window (h(n)) of said first type; and

calculating for a respective first coding frame a forward Modified Discrete Cosine Transform with each window (h₀(n),h₁(n),h₂(n)) of said window sequence and providing the resulting samples as encoded samples of said respective first coding frame.

2. Method according to claim 1,

wherein the shape of said windows (h(n)) of said first type is determined by a function, in which one parameter is the number of samples per coding frame;

wherein in the first half of a respective first coding frame at least one subframe is defined, to which a respective window (h₁(n)) of a second type is assigned by said window sequence, the shape of a window (h₁(n)) of said second type being determined by the same function as the shape of a window (h(n)) of said first type, in which function the parameter representing the number of samples per coding frame is substituted by a parameter representing the number of samples per subframe;

wherein a window (h₁(n)) associated to said at least one subframe is overlapped respectively by one half by a preceding window (h₀(n)) and a subsequent window (h₂(n)) of said sequence of windows, said preceding window (h₀(n)) and said subsequent window (h₂(n)) having at least for the samples in said at least one subframe a shape corresponding to the shape of said window (h₁(n)) of said second type;

wherein the sum of the values of said windows (h₀(n),h₁(n),h₂(n)) of said window sequence is equal to ‘

one’

for each sample of said coding frame which lies within said first half of said coding frame and outside of said at least one subframe; and

wherein the values of said windows (h₀(n),h₁(n), h₂(n)) of said window sequence are equal to ‘

zero’

for each sample which lies outside of said first coding frame.

3. Method according to claim 2, wherein the length of said last window (h₂(n)) of said window sequence is equal to the length of said coding frame, wherein the length of any other window (h₀(n),h₁(n)) but said last window (h₂(n)) of said window sequence corresponds to an even number of samples, said length of said last window (h₂(n)) of said window sequence being larger than said length of said other windows (h₀(n),h₁(n)) of said window sequence and said length of said last window (h₂(n)) of said window sequence being an integer multiple of said length of said other windows (h₀(n),h₁(n)) of said window sequence, wherein an offset is defined which is equal to half of the difference between said length of said last window (h₂(n)) of said window sequence and said length of said other windows (h₀(n),h₁(n)) of said window sequence, wherein the number of said other windows (h₀(n),h₁(n)) of said window sequence corresponds to the smallest even number equal to or larger than the largest integer smaller than the quotient between said offset and said length of said other windows (h₀(n),h₁(n)) of said window sequence, wherein a last one of said at least one subframe is centered at said offset and wherein said last window (h₂(n)) of said window sequence has values unequal to zero for samples equal to and larger than said offset.

4. Method according to claim 1, wherein an input signal is first subjected to a Linear Prediction analysis, which Linear Prediction analysis provides samples of coding frames for processing by said first coding scheme or said second coding scheme, and signals which are employed in a mode selection for selecting said first or said second coding scheme for a respective coding frame.

5. Method according to claim 1, wherein said signals provided by said first coding scheme and said second coding scheme are multiplexed to a bitstream together with an indication which coding scheme has been applied to a specific coding frame.

11. Hybrid encoder (40) comprising means (401-405) for realizing the steps of the method of claim 1.

12. Transform encoder component (403) for a hybrid encoder (40) comprising means for realizing the steps of the method of claim 1.

13. Hybrid decoder (41) comprising means (411-415) for realizing the steps of the method of claim 1.

15. Hybrid coding system comprising a hybrid encoder (40) with means (401-405) for realizing the steps of the method of one of claims 1 to 5, and a hybrid decoder (41) with means (411-415) for realizing the steps of the method of claim 6.

16. Method according to claim 1, wherein said signals provided by said first coding scheme and said second coding scheme are multiplexed to a bitstream together with an indication which coding scheme has been applied to a specific coding frame.

6. Method for supporting a switching from a first coding scheme to a second coding scheme at a decoding end of a hybrid coding system, both coding schemes coding input signals on a frame-by-frame basis, which second coding scheme is a Modified Discrete Cosine Transform based coding scheme calculating at the decoding end an Inverse Modified Discrete Cosine Transform with a window (h(n)) of a first type for a respective coding frame and overlap-adding the resulting samples with samples resulting for a preceding coding frame to obtain a reconstructed signal, a window (h(n)) of said first type satisfying constraints of perfect reconstruction, said method comprising:

providing for each first coding frame, which is to be decoded based on said second coding scheme after a preceding coding frame has been decoded based on said first coding scheme, a sequence of windows (h₀(n),h₁(n),h₂(n)), wherein said window sequence splits the spectrum of a respective first coding frame into nearly uncorrelated spectral components when used as basis for forward Modified Discrete Cosine Transforms, and wherein the second half of the last window (h₂(n)) of said sequence of windows is identical to the second half of a window (h(n)) of said first type; and

calculating for a respective first coding frame an Inverse Modified Discrete Cosine Transform with each window (h₀(n),h₁(n),h₂(n)) of said window sequence and providing the first half of the resulting samples as reconstructed frame samples without overlap adding.

7. Method according to claim 6,

wherein the shape of said windows (h(n)) of said first type is determined by a function, in which one parameter is the number of samples per coding frame;

wherein in the first half of a respective first coding frame at least one subframe is defined, to which a respective window (h₁(n)) of a second type is assigned by said window sequence, the shape of a window (h₁(n)) of said second type being determined by the same function as the shape of a window (h(n)) of said first type, in which function the parameter representing the number of samples per coding frame is substituted by a parameter representing the number of samples per subframe;

wherein a window (h₁(n)) associated to said at least one subframe is overlapped respectively by one half by a preceding window (h₀(n)) and a subsequent window (h₂(n)) of said sequence of windows, said preceding window (h₀(n)) and said subsequent window (h₂(n)) having at least for the samples in said at least one subframe a shape corresponding to the shape of said window (h₁(n)) of said second type;

wherein the sum of the values of said windows (h₀(n),h₁(n),h₂(n)) of said window sequence is equal to ‘

one’

for each sample of said coding frame which lies within said first half of said coding frame and outside of said at least one subframe; and

wherein the values of said windows (h₀(n),h₁(n), h₂(n)) of said window sequence are equal to ‘

zero’

for each sample which lies outside of said first coding frame.

8. Method according to claim 7, wherein the length of said last window (h₂(n)) of said window sequence is equal to the length of said coding frame, wherein the length of any other window (h₀(n),h₁(n)) but said last window (h₂(n)) of said window sequence corresponds to an even number of samples, said length of said last window (h₂(n)) of said window sequence being larger than said length of said other windows (h₀(n),h₁(n)) of said window sequence and said length of said last window (h₂(n)) of said window sequence being an integer multiple of said length of said other windows (h₀(n),h₁(n)) of said window sequence, wherein an offset is defined which is equal to half of the difference between said length of said last window (h₂(n)) of said window sequence and said length of said other windows (h₀(n),h₁(n)) of said window sequence, wherein the number of said other windows (h₀(n),h₁(n)) of said window sequence corresponds to the smallest even number equal to or larger than the largest integer smaller than the quotient between said offset and said length of said other windows (h₀(n),h₁(n)) of said window sequence, wherein a last one of said at least one subframe is centered at said offset and wherein said last window (h₂(n)) of said window sequence has values unequal to zero for samples equal to and larger than said offset.

9. Method according to claim 6, comprising as a preceding step demultiplexing a received bitstream into a first bitstream which is provided for processing by said first coding scheme, a second bitstream which is provided for processing by said second coding scheme, and a third bitstream which is provided for selecting the currently required coding scheme.

10. Method according to claim 6, wherein samples resulting in a processing by said first coding scheme and said second coding scheme are subjected to a Linear Prediction synthesis.

14. Transform decoder component (413) for a hybrid decoder (41) comprising means for realizing the steps of the method of claim 6.

17. Method according to claim 7, comprising as a preceding step demultiplexing a received bitstream into a first bitstream which is provided for processing by said first coding scheme, a second bitstream which is provided for processing by said second coding scheme, and a third bitstream which is provided for selecting the currently required coding scheme.

18. Method according to claim 8, comprising as a preceding step demultiplexing a received bitstream into a first bitstream which is provided for processing by said first coding scheme, a second bitstream which is provided for processing by said second coding scheme, and a third bitstream which is provided for selecting the currently required coding scheme.

19. Method according to claim 7, wherein samples resulting in a processing by said first coding scheme and said second coding scheme are subjected to a Linear Prediction synthesis.

20. Method according to claim 8, wherein samples resulting in a processing by said first coding scheme and said second coding scheme are subjected to a Linear Prediction synthesis.