Layered coding and data structure for compressed higher-order ambisonics sound or sound field representations

An apparatus for decoding a compressed Higher Order Ambisonics (HOA) representation of a sound or sound field, the apparatus comprising:

• a receiver configured to receive a bit stream comprising the compressed HOA representation, wherein the bit stream comprises a plurality of hierarchical layers that comprise a base layer and one or more hierarchical enhancement layers,
• a decoder configured to:
• determine a highest usable layer among the plurality of hierarchical layers for decoding;
• extract a HOA extension payload assigned to the highest usable layer, wherein the HOA extension payload includes side information for parametrically enhancing a reconstructed HOA representation corresponding to the highest usable layer, wherein the reconstructed HOA representation corresponding to the highest usable layer is based on transport signals assigned to the highest usable layer and any layers lower than the highest usable layer;
• decode the compressed HOA representation corresponding to the highest usable layer based on layer information, wherein the layer information indicates an active enhancement layer, and wherein the active enhancement layer can be used to determine a number of active directional signals in a current frame of the active enhancement layer;
• and
• parametrically enhance the decoded HOA representation using the side so information included in the HOA extension payload assigned to the highest usable layer.