Audio upmixer operable in prediction or non-prediction mode

1. An apparatus for outputting a stereo audio signal having a left channel and a right channel, the apparatus comprising:

a demultiplexer configured to receive an audio bitstream and extract therefrom a prediction coefficient for each of a plurality of frequency bands;

a decoder configured to generate a downmix signal and a residual signal from the encoded audio bitstream; and

an upmixer configured to operate in either a prediction mode or a non-prediction mode based on a parameter encoded in the audio bitstream, and to output the left channel and the right channel as the stereo audio signal,

wherein, when the upmixer operates in the prediction mode, the residual signal represents a difference between a side signal and a predicted version of the side signal, and the upmixer generates the left channel and the right channel from a combination of the downmix signal, the residual signal, and the prediction coefficients for each of the plurality of frequency bands, and

wherein, when the upmixer operates in the non-prediction mode, the residual signal represents the side signal, the upmixer generates the left channel based on a sum of the downmix signal and the residual signal passed through from the decoder, and the upmixer generates the right channel based on a difference of the downmix signal and the residual signal passed through from the decoder.

2. The apparatus of claim 1 wherein the parameter is the prediction coefficient.

3. The apparatus of claim 2 wherein the upmixer operates in the non-prediction mode for a time frame when a value of the prediction coefficient for all of the plurality of frequency bands equals zero or is smaller than a predetermined tolerance and the upmixer operates in the prediction mode for the time frame for all other values of the prediction coefficient.

4. The apparatus of claim 1 wherein the prediction coefficient reduces or minimizes an energy of the residual signal.

5. The apparatus of claim 1 further comprising a noise shaper configured to shape a noise associated with the downmix signal, wherein the noise shaper is arranged upstream of the upmixer.

6. The apparatus of claim 5 wherein the noise shaper is a temporal noise shaper configured to shape the noise over time.

7. The apparatus of claim 1 wherein, when the upmixer operates in the prediction mode, the upmixer generates the left channel and the right channel using a filter having three taps.

8. The apparatus of claim 1 wherein the downmix signal comprises a mid signal formed by a linear combination of an original left channel and an original right channel.

9. The apparatus of claim 1 wherein the prediction coefficient is a real valued coefficient.

10. The apparatus of claim 1 wherein the prediction coefficient is a complex valued coefficient.

11. The apparatus of claim 1 wherein the upmixer combines the side signal with the downmix signal by adding a version of the downmix signal with a version of the side signal to generate the left channel and by subtracting a version of the side signal from a version of the downmix signal to generate the right channel.

12. The apparatus of claim 1 wherein the prediction coefficient is coded in the audio bitstream and the demultiplexer is further configured to decode the prediction coefficient.

13. The apparatus of claim 1 wherein the upmixer is further configured to add the residual signal to the side signal when operating in the prediction mode.

14. A method for outputting a stereo audio signal having a left channel and a right channel, the method comprising:

receiving an audio bitstream and extract therefrom a prediction coefficient for each of a plurality of frequency bands;

generating in a decoder a downmix signal and a residual signal from the encoded audio bitstream;

upmixing in either a prediction mode or a non-prediction mode based on a parameter encoded in the audio bitstream; and

outputting the left channel and the right channel as the stereo audio signal,

wherein, when the upmixing operates in the prediction mode, the residual signal represents a difference between a side signal and a predicted version of the side signal, and the upmixing generates the left channel and the right channel from a combination of the downmix signal, the residual signal, and the prediction coefficients for each of the plurality of frequency bands, and

wherein, when the upmixing operates in the non-prediction mode, the residual signal represents the side signal, and the upmixing generates the left channel based on a sum of the downmix signal and the residual signal passed through from the decoder, and generates the right channel based on a difference of the downmix signal and the residual signal passed through from the decoder.

15. A non-transitory computer-readable medium containing instructions that when executed by a processor perform the method of claim 14 .

16. The apparatus of claim 1 wherein the prediction coefficient is coded in the audio bitstream using a plurality of parts, and the demultiplexer is further configured to decode the prediction coefficient by accessing the plurality of parts.

17. The apparatus of claim 1 wherein a value of the prediction coefficient for a current time frame is determined from a value of the prediction coefficient from a previous time frame.

18. The apparatus of claim 1 wherein the upmixer, when operating in the non-prediction mode, compensates for a delay introduced by the upmixer when operating in the prediction mode.