Spatial audio parameter decoding

An apparatus for decoding a spatial audio signal direction index to a direction value, the direction index representing a point in a spherical grid generated by covering a sphere with smaller spheres, wherein the centres of the smaller spheres define points of the spherical grid the points arranged substantially equidistant from each other on circles of constant elevation, wherein the apparatus comprising at least one processor and at least one memory including computer program code configured to, with the at least one processor, cause the apparatus to at least to:

• obtain a spatial audio signal direction index value;
• estimate, by application of a defined polynomial comprising the spatial audio signal direction index value, a grid circle index value;
• determine from the grid circle index value a low direction index value and a high direction index value;
• and
• determine an elevation index value and an azimuth index value based on the grid circle index value, the low direction index value, the high direction index value and the spatial audio signal direction index value.

A method for an apparatus for decoding a spatial audio signal direction index to a direction value, the direction index representing a point in a spherical grid generated by covering a sphere with smaller spheres, wherein the centres of the smaller spheres define points of the spherical grid the points arranged substantially equidistant from each other on circles of constant elevation, the method comprising:

• obtaining a spatial audio signal direction index value;
• estimating, by application of a defined polynomial comprising the spatial audio signal direction index value, a grid circle index value;
• determining from the grid circle index value a low direction index value and a high direction index value;
• and
• determining an elevation index value and an azimuth index value based on the grid circle index value, the low direction index value, the high direction index value and the spatial audio signal direction index value.