An audio processing unit comprising: a buffer configured to store at least one block of an encoded audio bitstream; a bitstream payload deformatter coupled to the buffer and configured to demultiplex at least a portion of the at least one block of the encoded audio bitstream; and a decoding subsystem coupled to the bitstream payload deformatter and configured to decode at least a portion of the at least one block of the encoded audio bitstream, wherein the at least one block of the encoded audio bitstream includes: a fill element with an identifier indicating a start of the fill element and fill data after the identifier, characterised in that the fill data includes: at least one flag identifying whether enhanced spectral band replication, eSBR, processing is to be performed on audio content of the at least one block of the encoded audio bitstream, and enhanced spectral band replication metadata which does not include one or more parameters used for both spectral patching and harmonic transposition, wherein the enhanced spectral band replication metadata is metadata configured to enable at least one eSBR tool which is described in the Moving Picture Experts Group, MPEG, Unified Speech and Audio Coding, USAC, standard and which is not described in the ISO/IEC 14496-3:2009 MPEG standard; the audio processing unit further comprising: an enhanced spectral band replication processing subsystem configured to perform enhanced spectral band replication processing using or in response to the at least one flag, wherein the enhanced spectral band replication includes harmonic transposition, wherein the audio processing unit is an audio decoder, and the identifier is a three bit unsigned integer transmitted most significant bit first and having a value of 0x6. a buffer configured to store at least one block of an encoded audio bitstream; a bitstream payload deformatter coupled to the buffer and configured to demultiplex at least a portion of the at least one block of the encoded audio bitstream; and a decoding subsystem coupled to the bitstream payload deformatter and configured to decode at least a portion of the at least one block of the encoded audio bitstream, wherein the at least one block of the encoded audio bitstream includes: a fill element with an identifier indicating a start of the fill element and fill data after the identifier, characterised in that the fill data includes: at least one flag identifying whether enhanced spectral band replication, eSBR, processing is to be performed on audio content of the at least one block of the encoded audio bitstream, and enhanced spectral band replication metadata which does not include one or more parameters used for both spectral patching and harmonic transposition, wherein the enhanced spectral band replication metadata is metadata configured to enable at least one eSBR tool which is described in the Moving Picture Experts Group, MPEG, Unified Speech and Audio Coding, USAC, standard and which is not described in the ISO/IEC 14496-3:2009 MPEG standard; the audio processing unit further comprising: an enhanced spectral band replication processing subsystem configured to perform enhanced spectral band replication processing using or in response to the at least one flag, wherein the enhanced spectral band replication includes harmonic transposition, wherein the audio processing unit is an audio decoder, and the identifier is a three bit unsigned integer transmitted most significant bit first and having a value of 0x6. a fill element with an identifier indicating a start of the fill element and fill data after the identifier, characterised in that the fill data includes: at least one flag identifying whether enhanced spectral band replication, eSBR, processing is to be performed on audio content of the at least one block of the encoded audio bitstream, and enhanced spectral band replication metadata which does not include one or more parameters used for both spectral patching and harmonic transposition, wherein the enhanced spectral band replication metadata is metadata configured to enable at least one eSBR tool which is described in the Moving Picture Experts Group, MPEG, Unified Speech and Audio Coding, USAC, standard and which is not described in the ISO/IEC 14496-3:2009 MPEG standard; the audio processing unit further comprising: an enhanced spectral band replication processing subsystem configured to perform enhanced spectral band replication processing using or in response to the at least one flag, wherein the enhanced spectral band replication includes harmonic transposition, wherein the audio processing unit is an audio decoder, and the identifier is a three bit unsigned integer transmitted most significant bit first and having a value of 0x6.