Spectrum Flatness Control for Bandwidth Extension

• US 20120016667A1
• Filed: 07/18/2011
• Published: 01/19/2012
• Est. Priority Date: 07/19/2010
• Status: Active Grant

• Current Assignee: Huawei Technologies Co., Ltd. (Huawei Investment & Holding Co., Ltd.)
• Original Assignee: Futurewei Technologies Incorporated (Huawei Investment & Holding Co., Ltd.)
• Inventors: Gao, Yang
• Granted Patent: US 9,047,875 B2
• Time in Patent Office: Days
• US Class Current: 704/203
• CPC Class Codes: G10L 19/002 Dynamic bit allocation for perceptual audio coders G10L19/032
• CPC Class Codes: G10L 19/022 Blocking, i.e. grouping of samples in time; Choice of analysis windows; Overlap factoring
• CPC Class Codes: G10L 19/24 Variable rate codecs, e.g. for generating different qualities using a scalable representation such as hierarchical encoding or layered encoding
• CPC Class Codes: G10L 19/26 Pre-filtering or post-filtering
• CPC Class Codes: G10L 21/038 using band spreading techniques
• CPC Class Codes: G10L 21/0388 Details of processing therefor
• CPC Class Codes: G10L 25/18 the extracted parameters being spectral information of each sub-band

Abstract

In accordance with an embodiment, a method of decoding an encoded audio bitstream at a decoder includes receiving the audio bitstream, decoding a low band bitstream of the audio bitstream to get low band coefficients in a frequency domain, and copying a plurality of the low band coefficients to a high frequency band location to generate high band coefficients. The method further includes processing the high band coefficients to form processed high band coefficients. Processing includes modifying an energy envelope of the high band coefficients by multiplying modification gains to flatten or smooth the high band coefficients, and applying a received spectral envelope decoded from the received audio bitstream to the high band coefficients. The low band coefficients and the processed high band coefficients are then inverse-transformed to the time domain to obtain a time domain output signal.

Claims

1. A method of decoding an encoded audio bitstream at a decoder, the method comprising:

receiving the audio bitstream, the audio bitstream comprising a low band bitstream;

decoding the low band bitstream to get low band coefficients in a frequency domain;

copying a plurality of the low band coefficients to a high frequency band location to generate high band coefficients;

processing the high band coefficients to form processed high band coefficients, processing comprising

modifying an energy envelope of the high band coefficients, modifying comprising multiplying modification gains to flatten or smooth the high band coefficients, and

applying a received spectral envelope to the high band coefficients, the received spectral envelope being decoded from the received audio bitstream; and

inverse-transforming the low band coefficients and the processed high band coefficients to a time domain to obtain a time domain output signal.

2. The method of claim 1, wherein:

the received bitstream comprises a high-band side bitstream; and

the method further comprises decoding the high-band side bitstream to get side information, and using Spectral Band Replication (SBR) techniques to generate the high band with the side information.

3. The method of claim 1, further comprising evaluating the modification gains, evaluation comprising analyzing and modifying the high band coefficients copied from the low band coefficients or analyzing and modifying an energy distribution of the low band coefficients to be copied to the high band location.

4. The method of claim 3, wherein the evaluating the modification gains comprises using a mean energy value obtained by averaging the energies of the high band coefficients.

5. The method of claim 3, wherein the evaluation the modification gains comprises evaluating the following equation:

Gain(k)=(C0+C1·

√

{square root over (Mean_—

HB/F_energy_—

dec[k])}), k=Start_—

HB, . . . ,End_—

HB-1,

where {Gain(k), k=Start_HB, . . . , End_HB-1} are the modification gains, F_energy_dec[k] is an energy distribution at each frequency location index k of a copied high band, Start_HB and End_HB define a high band range, C0 and C1 satisfying C0+C1=1 are pre-determined constants, and Mean_HB is a mean energy value obtained by averaging energies of the high band coefficients.

6. The method of claim 3, wherein the modification gains are switchable or variable according to a spectrum flatness classification received by the decoder from an encoder.

7. The method of claim 6, further comprising determining the classification is based on a plurality of spectrum sharpness parameters, each of the plurality of spectrum sharpness parameter being defined by dividing a mean energy by a maximum energy on a sub-band of an original high frequency band.

8. The method of claim 6, wherein the classification is based on a speech/music decision.

9. The method of claim 1, wherein decoding the low band bitstream comprises:

decoding the low band bitstream to get a low band signal; and

transforming the low band signal into the frequency domain to obtain the low band coefficients.

10. The method of claim 1, wherein modifying the energy envelope comprises flattening or smoothing the energy envelope.

11. A post-processing method of generating a decoded speech/audio signal at a decoder and improving spectrum flatness of a generated high frequency band, the method comprising:

generating high band coefficients from low band coefficients in a frequency domain using a BandWidth Extension (BWE) high band coefficient generation method;

flattening or smoothing an energy envelope of the high band coefficients by multiplying flattening or smoothing gains to the high band coefficients;

shaping and determining energies of the high band coefficients by using a BWE shaping and determining method; and

inverse-transforming the low band coefficients and the high band coefficients to a time domain to obtain a time domain output speech/audio signal.

12. The method of claim 11, further comprising evaluating the flattening or smoothing gains, evaluating comprising analyzing, examining, using and flattening or smoothing the high band coefficients or the low band coefficients to be copied to a high band location.

13. The method of claim 12, wherein evaluating the flattening or smoothing gains comprises using a mean energy value obtained by averaging energies of the high band coefficients.

14. The method of claim 12, wherein the flattening or smoothing gains are switchable or variable according to a spectrum flatness classification transmitted from an encoder to the decoder.

15. The method of claim 14, wherein the classification is based on a speech/music decision.

16. The method of claim 11, wherein:

the BWE high band coefficient generation method comprises a Spectral Band Replication (SBR) high band coefficient generation method; and

the BWE shaping and determining method comprises a SBR shaping and determining method.

17. A system for receiving an encoded audio signal, the system comprising:

a low-band block configured to transform a low band portion of the encoded audio signal into frequency domain low band coefficients at an output of the low-band block;

a high-band block coupled to the output of the low-band block, the high band block configured to generate high band coefficients at an output of the high band block by copying a plurality of the low band coefficients to a high frequency band locations;

an envelope shaping block coupled to the output of the high-band block, the envelope shaping block configured to produce shaped high band coefficients at an output of the envelope shaping block, wherein the envelope shaping block configured to

modify an energy envelope of the high band coefficients by multiplying modification gains to flatten or smooth the high band coefficients, and apply a received spectral envelope to the high band coefficients, the received spectral envelope being decoded from the encoded audio signal; and

an inverse transform block coupled to the output of envelope shaping block and to the output of the low band block, the inverse transform block configured to produce a time domain audio output signal.

18. The system of claim 17, further comprising a high-band side bitstream decoder block configured to produce the received spectral envelope from a high band side bitstream of the encoded audio signal.

19. The system of claim 17, wherein the low band block comprises:

a low band decoder block configured to decode a low band bitstream of the encoded audio signal into a decoded low band signal at an output of the low band decoder block; and

a time/frequency filter bank analyzer coupled to the output of the low band decoder block, the time/frequency filter bank analyzer configured to produce the frequency domain low. band coefficients from the decoded low band signal.

20. The system of claim 17, wherein:

the envelope shaping block is further coupled to the low band block; and

the envelope shaping block is further configured to evaluate the modification gains by analyzing, examining, using and modifying the high band coefficients or the low band coefficients to be copied to a high band location.

21. The system of claim 20, wherein the envelope shaping block uses a mean energy value obtained by averaging energies of the high band coefficients to evaluate the modification gains.

22. The system of claim 17, wherein the output audio signal is configured to be coupled to a loudspeaker.

23. A non-transitory computer readable medium has an executable program stored thereon, wherein the program instructs a processor to perform the steps of:

decoding an encoded audio signal to produce a decoded audio signal, wherein the encoded audio signal includes a coded representation of an input audio signal; and

postprocessing the decoded audio signal with a spectrum flatness control for spectrum bandwidth extension.

24. The non-transitory computer readable medium of claim 23, wherein the step of postprocessing the decoded audio signal further comprises:

flattening or smoothing an energy envelope of high band coefficients of the decoded audio signal by multiplying flattening or smoothing gains to the high band coefficients; and

shaping and determining energies of the high band coefficients by using a BWE shaping and determining method.