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 decode therefrom at least one prediction coefficient, wherein the audio bitstream is segmented into frames and values of the at least one prediction coefficient may change for each of the frames;

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

an upmixer configured to operate in either a prediction mode or a non-prediction mode based on at least one 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 at least one prediction coefficient, wherein the at least one prediction coefficient is used as one or more weights in a weighted summing operation that generates the left channel and the right channel, 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, and the upmixer generates the right channel based on a difference of the downmix signal and the residual signal.

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

3. The apparatus of claim 2 wherein the upmixer operates in the non-prediction mode for a time frame when the value of all of the at least one prediction coefficient 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 at least one prediction coefficient.

4. The apparatus of claim 1 wherein the at least one 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 at least one prediction coefficient is a real valued coefficient.

10. The apparatus of claim 1 wherein the at least one 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 the version of the side signal from the version of the downmix signal to generate the right channel.

12. 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.

13. The apparatus of claim 1 wherein the downmix signal is partitioned into frequency bands, the at least one prediction coeeficient comprises a prediction coeffieint for each of the frequency bands, the demultiplexer is configured to receive the audio bitstream and decode therefrom each of the prediction coefficients, and the upmixer, when operating in the prediction mode, generates the left channel and the right channel from a combination of the downmix signal, the residual signal, and each of the prediction coefficients.

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 decoding therefrom at least one prediction coefficient, wherein the audio bitstream is segmented into frames and values of the at least one prediction coefficient may change for each of the frames;

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

upmixing in either a prediction mode or a non-prediction mode based on at least one 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 at least one prediction coefficient, wherein the at least one prediction coefficient is used as one or more weights in a weighted summing operation that generates the left channel and the right channel, 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.

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