Determination of spatial audio parameter encoding and associated decoding

An apparatus comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to:

• obtain encoded values for a sub-band of a frame of an audio signal, the values comprising a plurality of azimuth indexes, a plurality of elevation indexes an energy ratio index and a spread and/or surround coherence index;
• decode the plurality of azimuth indexes to give a plurality of azimuth values;
• determine a codebook for decoding the spread and/or surround coherence index for the sub-band by the apparatus being configured to determine whether a variance of the plurality of azimuth values for the sub-band is more than or equal to a threshold value, and
• select the codebook based on the energy ratio index and whether the variance of the plurality of azimuth values for the sub-band is more than or equal to the threshold value;
• inverse discrete cosine transform the spread and/or surround coherence index to generate a vector, the vector comprising the plurality of spread and/or surround coherence values for the sub-band;
• and
• parse the vector to generate the plurality of spread and/or surround coherence values for the sub-band for use in reproduction of the audio signal by a multi-channel loudspeaker or headphones.

A method comprising:

• receiving values for a sub-band of a frame of an audio signal, the values comprising a plurality of azimuth values, a plurality of elevation values, a plurality of energy ratio values and a plurality of spread and/or surround coherence values;
• determining a codebook for encoding the plurality of spread and/or surround coherence values for the sub-band by:
• obtaining an index representing an average value of the plurality of energy ratio values for a sub-band,
• determining whether the variance of the plurality of azimuth values for the sub-band is more than or equal to a threshold value, and
• selecting the codebook based on the index and whether the variance of the azimuth values for the sub-band is more than or equal to the threshold value;
• discrete cosine transforming at least one vector, the at least one vector comprising the plurality of spread and/or surround coherence values for the sub-band;
• encoding a number of components of the discrete cosine transformed vector based on the determined codebook;
• and
• storing and/or transmitting the encoded number of components of the discrete cosine transformed vector for use in reproduction of the audio signal by a multi-channel loudspeaker or headphones.

A method comprising:

• obtaining encoded values for a sub-band of a frame of an audio signal, the values comprising a plurality of azimuth indexes, a plurality of elevation indexes, an energy ratio index, and a spread and/or surround coherence index;
• decoding the plurality of azimuth indexes to give a plurality of azimuth values;
• determining a codebook for decoding the spread and/or surround coherence index for the sub-band by:
• determining whether a variance of the plurality of azimuth values for the sub-band is more than or equal to a threshold value, and
• selecting the codebook based on the energy ratio index and whether the variance of the plurality of azimuth values for the sub-band is more than or equal to a threshold value;
• inverse discrete cosine transforming the spread and/or surround coherence index to generate vector, the vector comprising the plurality of spread and/or surround coherence values for the sub-band;
• and
• parsing the vector to generate the plurality of spread and/or surround coherence values for the sub-band for use in reproduction of the audio signal by a multi-channel loudspeaker or headphones.