Methods and devices for encoding and/or decoding immersive audio signals

1. A method for encoding a multi-channel input Ambisonics signal

wherein the method comprises:

determining a plurality of downmix channel signals from the multi-channel input Ambisonics signal;

performing an energy compaction of the plurality of downmix channel signals to provide a plurality of compacted channel signals;

determining audio reconstruction metadata based on the plurality of compacted channel signals and based on the multi-channel input Ambisonics signal; wherein the audio reconstruction metadata enables a recipient device to upmix the plurality of compacted channel signals to an approximation of the multi-channel input Ambisonics signal; and

encoding the plurality of compacted channel signals and the audio reconstruction metadata.

2. The method of claim 1 , wherein the energy compaction is performed such that an energy of a compacted channel signal is lower than an energy of a corresponding downmix channel signal.

3. The method of claim 1 , wherein performing an energy compaction comprises

predicting a first downmix channel signal from a second downmix channel signal, to provide a first predicted channel signal; and

subtracting the first predicted channel signal from the first downmix channel signal to provide a first compacted channel signal.

4. The method of claim 3 , wherein

predicting the first downmix channel signal from the second downmix channel signal comprises determining a scaling factor for scaling the second downmix channel signal; and

the first predicted channel signal corresponds to the second downmix channel signal scaled according to the scaling factor.

5. The method of claim 4 , wherein the scaling factor is determined such that at least one of (1) or (2) below is true:

(1) an energy of the first compacted channel signal is reduced compared to an energy of the first downmix channel signal;

(2) an energy of the first compacted channel signal is minimized.

6. The method of claim 3 , wherein performing an energy compaction comprises

determining several compacted channel signals based on a prediction from the second downmix channel signal; and

applying one of: a Karhonen-Loeve-Transform, a Principle Components Analysis transform, or a Singular Value Decomposition transform, to the several compacted channel signals.

7. The method of claim 1 , wherein at least one of (1) or (2) below is true:

(1) the plurality of downmix channel signals is a first order ambisonics signal, in a B-format or in an A-format;

(2) the plurality of compacted channel signals is represented in a format of a first order ambisonics signal, in a B-format or in an A-format.

8. The method of claim 7 , wherein performing an energy compaction comprises

predicting an X channel signal, a Y channel signal and a Z channel signal from a W channel signal of the plurality of downmix channel signals, to provide a predicted X channel signal, a predicted Y channel signal and a predicted Z channel signal;

subtracting the predicted X channel signal from the X channel signal to determine a X′ channel signal;

subtracting the predicted Y channel signal from the Y channel signal to determine a Y′ channel signal;

subtracting the predicted Z channel signal from the Z channel signal to determine a Z′ channel signal; and

determining the plurality of compacted channel signals based on the W channel signal, the X′ channel signal, the Y′ channel signal and the Z′ channel signal.

9. The method of claim 8 , wherein performing an energy compaction comprises

applying one of: a Karhonen-Loeve-Transform, a Principle Components Analysis transform, a Singular Value Decomposition transform, to the X′ channel signal, the Y′ channel signal and the Z′ channel signal to provide a X″ channel signal, a Y″ channel signal and a Z″ channel signal; and

determining the plurality of compacted channel signals based on the W channel signal, the X″ channel signal, the Y″ channel signal and the Z″ channel signal.

10. The method of claim 1 , wherein performing an energy compaction comprises applying one of: a Karhonen-Loeve-Transform, a Principle Components Analysis transform, a Singular Value Decomposition transform, to at least some of the plurality of downmix channel signals.

11. The method of claim 1 , wherein the joint coding audio reconstruction metadata, comprises at least one of:

upmix data, an upmix matrix, enabling the upmix of the plurality of compacted channel signals to an approximation of the multi-channel input Ambisonics signal comprising a same number of channels as the multi-channel input Ambisonics signal; or

decorrelation data enabling the reconstruction of a covariance of the multi-channel input Ambisonics signal.

12. The method of claim 1 , wherein the audio reconstruction metadata is determined for a plurality of different subbands of the multi-channel input Ambisonics signal.

13. The method of claim 1 , wherein encoding the plurality of compacted channel signals comprises performing waveform encoding of each one of the plurality of compacted channel signals, using a mono encoder for each compacted channel signal.

14. The method of claim 1 , wherein the audio reconstruction metadata is encoded using an entropy encoder.

15. The method of claim 1 , wherein

the multi-channel input Ambisonics signal comprises one or more object signals of one or more audio objects; and

the method comprises encoding, using an entropy encoder, object metadata for the one or more audio objects.

16. The method of claim 1 , wherein

the multi-channel input Ambisonics signal comprises a soundfield representation, referred to as SR, signal, a Lth order ambisonics signal, with L≥1, and one or more object signals of one or more audio objects; and

the plurality of downmix channel signals is determined by downmixing the multi-channel input Ambisonics signal to an SR signal, a Kth order ambisonics signal, with L≥K.

17. The method of claim 16 , wherein

determining the plurality of downmix channel signals comprises mixing the one or more object signals of one or more audio objects to the SR signal of the multi-channel input Ambisonics signal in dependence of object metadata of the one or more audio objects; and

the object metadata of an audio object is indicative of a spatial position of the audio object.

18. The method of claim 1 , wherein

the method comprises determining that the multi-channel input Ambisonics signal is to be encoded using a second mode; and

in the second mode, the audio reconstruction metadata is determined based on the plurality of compacted channel signals and based on the plurality of downmix channel signals, such that the audio reconstruction metadata allows reconstructing the plurality of downmix channel signals from the plurality of compacted channel signals.

19. The method of claim 18 , wherein

determining the audio reconstruction metadata based on the plurality of compacted channel signals and based on the multi-channel input Ambisonics signal corresponds to a first mode;

the multi-channel input Ambisonics signal comprises a sequence of frames; and

the method comprises determining for each frame of the sequence of frames whether to use the first mode or the second mode.

20. The method of claim 18 , wherein the method comprises

generating a bitstream based on coded audio data derived by encoding the plurality of compacted channel signals and based on coded metadata derived by encoding the audio reconstruction metadata; and

inserting an indication into the bitstream, which indicates whether the second mode has been used.

21. An encoding apparatus for encoding a multi-channel input Ambisonics signal wherein the encoding apparatus is configured to

determine a plurality of downmix channel signals from the multi-channel input Ambisonics signal;

perform an energy compaction of the plurality of downmix channel signals to provide a plurality of compacted channel signals;

determine audio reconstruction metadata based on the plurality of compacted channel signals and based on the multi-channel input Ambisonics signal; wherein the audio reconstruction metadata enables a recipient device to upmix the plurality of compacted channel signals to an approximation of the multi-channel input Ambisonics signal; and

encode the plurality of compacted channel signals and the audio reconstruction metadata.