Spectral translation/ folding in the subband domain

• US 20040131203A1
• Filed: 01/06/2004
• Published: 07/08/2004
• Est. Priority Date: 05/23/2000
• Status: Active Grant

• Current Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Original Assignee: Coding Technologies Sweden AB
• Inventors: Ekstrand, Per, Liljeryd, Lars, Kjorling, Kristofer, Henn, Fredrik
• Granted Patent: US 7,483,758 B2
• Time in Patent Office: Days
• US Class Current: 381/98
• CPC Class Codes: G10L 19/0017 Lossless audio signal coding; Perfect reconstruction of coded audio signal by transmission of coding error G10L19/24 takes precedence
• CPC Class Codes: G10L 19/0204 using subband decomposition
• CPC Class Codes: G10L 19/0208 Subband vocoders
• CPC Class Codes: G10L 19/26 Pre-filtering or post-filtering
• CPC Class Codes: G10L 19/265 Pre-filtering, e.g. high frequency emphasis prior to encoding
• CPC Class Codes: G10L 21/038 using band spreading techniques

Abstract

The present invention relates to a new method and apparatus for improvement of High Frequency Reconstruction (HFR) techniques using frequency translation or folding or a combination thereof. The proposed invention is applicable to audio source coding systems, and offers significantly reduced computational complexity. This is accomplished by means of frequency translation or folding in the subband domain, preferably integrated with spectral envelope adjustment in the same domain. The concept of dissonance guard-band filtering is further presented. The proposed invention offers a low-complexity, intermediate quality HFR method useful in speech and natural audio coding applications.

Claims

1. Method for obtaining an envelope adjusted and frequency-translated signal by high-frequency spectral reconstruction of complex subband signals in channels within a reconstruction range using complex subband signals in source area channels derived from a lowband signal, using a digital filter bank having an analysis part (201) and a synthesis part (202), the reconstruction range including channel frequencies which are higher than frequencies in the source area channels, the method comprising the following steps:

filtering the lowband signal by means of the analysis part (201) to obtain the complex subband signals in the source area channels;

calculating a number of consecutive complex subband signals in channels within the reconstruction range using a number of frequency-translated consecutive complex subband signals in the source area channels and an envelope correction for obtaining a predetermined spectral envelope,

wherein a complex subband signal in a source area channel having an index i is frequency-translated to a complex subband signal in a reconstruction range channel having an index j, and wherein a complex subband signal in a source area channel having an index i+1 is frequency-translated to a complex subband signal in a reconstruction range channel having an index j+1, and

filtering the consecutive complex subband signals in channels within the reconstruction range by means of the synthesis part to obtain an envelope adjusted and frequency-translated signal.

2. Method according to claim 1, in which, in the step of calculating, the following equation is used:

^ν

M+k^(n)=eM+k^(n)ν

M−

S−

P+k⁽ⁿ⁾,

wherein M indicates a number of a channel of the synthesis part (202), the channel being a start channel of the reconstruction range,

wherein S indicates the number of source area channels, S being a integer greater than or equal to 1 and lower than or equal to M,

wherein P is an integer offset greater than or equal to 0 and lower than or equal to M−

S;

wherein v_i indicates a band pass signal v for a channel i of the synthesis part,

wherein e_i indicates an envelope correction for a channel i of the synthesis part to obtain the desired spectral envelope,

wherein n is a time index, and

wherein k is an integer index between zero and S−

1.

3. Method according to claim 2, wherein S and P are selected such that a sum of S and P is an even number.

4. Method according to one of the preceding claims, wherein the digital filterbank is obtained by cosine or sine modulation of a low pass prototype filter

5. Method according to one of claims 1 to 3, wherein the digital filterbank is obtained by complex-exponential-modulation of a low pass prototype filter.

6. Method according to claims 4 or 5, wherein the low pass prototype filter is designed so that a transition band of the channels of the digital filterbank overlaps a pass band of the neighbouring channels only.

7. Method according to one of the preceding claims, in which the synthesis part includes a dissonance guard band, the dissonance guard band being positioned between the source area channels and the reconstruction range channels.

8. Method according to claim 7, wherein, in the step of calculation, the following equation is used:

^ν

M+D+k^(n)=eM+D+k^(n)ν

M−

s−

P+k⁽ⁿ⁾,

wherein D is an integer representing a number of filterbank channels used as the dissonance guard band.

9. Method according to claim 1, wherein P, S, D are selected such that a sum of P, S and D is an even integer.

10. Method according to one of claims 7 to 9, in which one or several of the channels in the dissonance guard band are fed with zeros or gaussian noise, whereby dissonance related artefacts are attenuated.

11. Method according to one of claims 7 to 10, in which a bandwidth of the dissonance guard band is approximately one half Bark.

12. Method according to one of the preceding claims, in which the step of calculating implements a first iteration step, and in which the method further includes another step of calculating, implementing a second iteration step, wherein in the second iteration step, the source area channels include the reconstruction-arranged channels from the first iteration step.

18. Method according to claim 1, wherein P, S, D are selected such that a sum of P, S and D is an odd integer.

23. Method for decoding coded signals, the coded signals including a coded lowband audio signal, the method comprising the following steps:

separating (101) the coded lowband audio signal from the coded signals;

audio decoding (102) the coded lowband audio signal to obtain an audio decoded signal;

a method in accordance with claim 1 or claim 13, to obtain an envelope-adjusted and frequency-translated or frequency-folded signal using the audio decoded signal as the lowband signal,

wherein the envelope-adjusted and frequency-translated or frequency-coded signal is a high-frequency reconstructed version of the lowband audio signal.

13. Method for obtaining an envelope adjusted and frequency-folded signal by high-frequency spectral reconstruction of complex subband signals in channels within a reconstruction range using complex subband signals in source area channels derived from a lowband signal, using a digital filter bank having an analysis part (201) and a synthesis part (202), the reconstruction range including channel frequencies which are higher than frequencies in the source area channels, the method comprising the following steps:

filtering the lowband signal by means of the analysis part (201) to obtain the complex subband signals in the source area channels;

calculating a number of consecutive complex subband signals in channels within the reconstruction range using a number of frequency-translated consecutive conjugate complex subband signals in the source area channels and an envelope correction for obtaining a predetermined spectral envelope,

wherein a complex subband signal in a source area channel having an index i is frequency-folded to a complex subband signal in a reconstruction range channel having an index j, and wherein a complex subband signal in a source area channel having an index i+1 is frequency-folded to a complex subband signal in a reconstruction range channel having an index j−

1, and filtering the consecutive complex subband signals in channels within the reconstruction range by means of the synthesis part to obtain an envelope adjusted and frequency-translated signal.

14. Method according to claim 13, in which, in the step of calculating, the following equation is used:

^ν

M+k^(n)=eM+k<sup>(n)ν

*</sup>M−

P−

S+k⁽ⁿ⁾,

wherein M indicates a number of a channel of the synthesis part (202), the channel being a start channel of the reconstruction range,

wherein S indicates the number of source area channels, S being a integer greater than or equal to 1 and lower than or equal to M,

wherein P is an integer offset greater than or equal to 1−

S and lower than or equal to M−

2S+1;

wherein v_i indicates a band pass signal v for a channel i of the synthesis part,

wherein e_i indicates an envelope correction for a channel i of the synthesis part to obtain the desired spectral envelope,

wherein * indicates conjugate complex,

wherein n is a time index, and

wherein k is an integer index between zero and S−

1.

15. Method according to claim 14, wherein S and P are selected such that a sum of S and P is an odd integer number.

16. Method according to claim 13, in which the synthesis part includes a dissonance guard band, the dissonance guard band being positioned between the source area channels and the reconstruction range channels.

17. Method according to claim 16, wherein, in the step of calculation, the following equation is used:

^ν

M+D+k^(n)=eM+D+k<sup>(n)ν

*</sup>M−

P−

S−

k⁽ⁿ⁾,

wherein D is an integer representing a number of filterbank channels used as the dissonance guard band.

19. Apparatus for obtaining an envelope adjusted and frequency-translated signal by high-frequency spectral reconstruction of complex subband signals in channels within a reconstruction range using complex subband signals in source area channels derived from a lowband signal, using a digital filter bank having an analysis part (201) and a synthesis part (202), the reconstruction range including channel frequencies which are higher than frequencies in the source area channels, comprising:

means for filtering the lowband signal by means of the analysis part (201) to obtain the complex subband signals in the source area channels;

means for calculating a number of consecutive complex subband signals in channels within the reconstruction range using a number of frequency-translated consecutive complex subband signals in the source area channels and an envelope correction for obtaining a predetermined spectral envelope,

wherein a complex subband signal in a source area channel having an index i is frequency-translated to a complex subband signal in a reconstruction range channel having an index j, and wherein a complex subband signal in a source area channel having an index i+1 is frequency-translated to a complex subband signal in a reconstruction range channel having an index j+1, and

means for filtering the consecutive complex subband signals in channels within the reconstruction range by means of the synthesis part to obtain an envelope adjusted and frequency-translated signal.

21. Decoder for decoding coded signals, the coded signals including a coded lowband audio signal, comprising:

a separator (101) for separating the coded lowband audio signal from the coded signals;

an audio decoder (102) for audio decoding the coded lowband audio signal to obtain an audio decoded signal;

an apparatus in accordance with claim 19 or claim 20, to obtain an envelope-adjusted and frequency-translated or frequency-folded signal using the audio decoded signal as the lowband signal,

wherein the envelope-adjusted and frequency-translated or frequency-coded signal is a high-frequency reconstructed version of the lowband audio signal.

22. Decoder according to claim 21, in which the coded signals further include envelope data,

in which the separator (101) is further arranged to separate the envelope data from the coded signals,

wherein the decoder further includes an envelope decoder (103) for decoding the envelope data to obtain spectral envelope information,

wherein the spectral envelope information is fed to the apparatus for obtaining an envelope adjusted and frequency-translated or frequency-folded signal to be used as an envelope correction for obtaining the predetermined spectral envelope.

20. Apparatus for obtaining an envelope adjusted and frequency-folded signal by high-frequency spectral reconstruction of complex subband signals in channels within a reconstruction range using complex subband signals in source area channels derived from a lowband signal, using a digital filter bank having an analysis part (201) and a synthesis part (202), the reconstruction range including channel frequencies which are higher than frequencies in the source area channels, comprising:

means for filtering the lowband signal by means of the analysis part (201) to obtain the complex subband signals in the source area channels;

means for calculating a number of consecutive complex subband signals in channels within the reconstruction range using a number of frequency-translated consecutive conjugate complex subband signals in the source area channels and an envelope correction for obtaining a predetermined spectral envelope,

wherein a complex subband signal in a source area channel having an index i is frequency-folded to a complex subband signal in a reconstruction range channel having an index j, and wherein a complex subband signal in a source area channel having an index i+1 is frequency-folded to a complex subband signal in a reconstruction range channel having an index j−

1, and

means for filtering the consecutive complex subband signals in channels within the reconstruction range by means of the synthesis part to obtain an envelope adjusted and frequency-translated signal.