Audio Encoding and Decoding

An audio encoder ( 109 ) has a hierarchical encoding structure and generates a data stream comprising one or more audio channels as well as parametric audio encoding data. The encoder ( 109 ) comprises an encoding structure processor ( 305 ) which inserts decoder tree structure data into the data stream. The decoder tree structure data comprises at least one data value indicative of a channel split characteristic for an audio channel at a hierarchical layer of the hierarchical decoder structure and may specifically specify the decoder tree structures to be applied by a decoder A decoder ( 115 ) comprises a receiver ( 401 ) which receives the data stream and a decoder structure processor ( 405 ) for generating the hierarchical decoder structure in response to the decoder tree structure data. A decode processor ( 403 ) then generates output audio channels from the data stream using the hierarchical decoder structure.