Primary-ambient decomposition of stereo audio signals using a complex similarity index

1. A method for processing a multichannel audio signal to derive primary and ambient components of the signal, comprising:

transforming at least a first and second channel of the audio signal to corresponding complex-valued time-frequency representations; and

determining the primary component and ambient components by comparing frequency subband content using a complex-valued similarity metric, wherein one of the primary and ambient components is determined to be the residual after the other is identified and extracted using the complex-valued similarity metric.

2. The method as recited in claim 1 wherein the multichannel audio signal is a stereo audio signal and wherein transforming at least a first and second channel of the audio signal comprises transforming left and right channels of the audio signal.

3. The method as recited in claim 1 wherein the sum of the primary and ambient components equals the original signal.

4. The method as recited in claim 1 wherein the complex-valued similarity index is determined for each transform component and wherein determining whether the component is primary or ambient is based on the magnitude and phase of the complex-valued similarity index.

5. The method as recited in claim 4 wherein transform components having a similarity index falling inside a predetermined region in the complex plane are deemed to be primary and the remainder of the signal is deemed to constitute ambient components.

6. The method as recited in claim 4 wherein the similarity index ψ LR is defined as

2

⁢

r

LR

r

LL

+

r

RR

where r LR represents the correlation of a first or left channel signal with a corresponding second ot right channel signal, r LL represents the autocorrelation of the first or left channel signal, and r RR represents the autocorrelation of the second or right channel signal.

7. The method as recited in claim 1 wherein the determination of primary and ambient components is based on whether the complex similarity index falls within a predetermined region in the complex plane.

8. The method as recited in claim 1 wherein the determination of primary and ambient components is based on determining a value for the primary component using a scaling factor applied to the channel vectors, said scaling factor being derived at least in part from the phase of the similarity index.

9. The method as recited in claim 1 wherein the determination of primary and ambient components is based on determining a value for the primary component using a scaling factor applied to the channel vectors, said scaling factor being derived at least in part from the magnitude of the similarity index.

10. The method as recited in claim 1 wherein the determination of primary and ambient components is based on determining a value for the ambient component using a scaling factor applied to the channel vectors, said scaling factor being derived at least in part from the phase of the similarity index.

11. The method as recited in claim 1 wherein the determination of primary and ambient components is based on determining a value for the ambient component using a scaling factor applied to the channel vectors, said scaling factor being derived at least in part from the magnitude of the similarity index.

12. The method as recited in claim 1 wherein the complex similarity index is a function of the correlation between the vectors for corresponding channels.

13. The method as recited in claim 2 further comprising taking the derived ambient components to synthesize surround-channel signals for stereo-to-multichannel upmix and further comprising using the derived primary components to generate a center-channel signal for stereo-to-multichannel upmix.

14. The method as recited in claim 1 further comprising taking the derived ambient and primary components and performing separate spatial audio coding techniques on the separated components.

15. The method as recited in claim 1 wherein the determination of primary components is configured to extract vocal content and wherein extracting vocal content comprises determining the center-panned components of the original signal.

16. The method as recited in claim 1 further comprising deriving an enhanced primary component as a result of projecting the original signal onto the derived primary signal and determining the ambient component as the projection residual.

17. The method as recited in claim 1 further comprising leaking a small amount of the original signal into the extracted primary and ambience components to reduce artifacts.

18. The method as recited in claim 1 further comprising taking the derived (extracted) ambience components, and applying allpass filtering to them to further decorrelate the extracted ambience.

19. The method as recited in claim 1 further comprising taking the derived (extracted) ambience components, determining the inverse of the spectrum of the estimated ambience and applying the inverse of the ambience spectrum as a weight to the extracted primary components.

20. A method for processing a stereo audio stereo signal to derive primary and ambient components of the signal, comprising:

transforming left and right channels of the audio signal to corresponding frequency-domain subband vectors;

determining the similarity between the channel vectors using a complex-valued similarity index applied to the vectors representing the transformed audio signal; and

determining the primary and ambient components based on the value of the complex similarity index.