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, wherein the fill data includes at least one flag identifying whether a base form of spectral band replication or an enhanced form of spectral band replication is to be performed on audio content of the at least one block of the encoded audio bitstream, wherein the base form of spectral band replication includes spectral patching, the enhanced form of spectral band replication includes harmonic transposition, one value of the flag indicates that said enhanced form of spectral band replication should be performed on the audio content, and another value of the flag indicates that said base form of spectral band replication but not said harmonic transposition should be performed on the audio content; and wherein the fill data further includes enhanced spectral band replication metadata, and the enhanced spectral band replication metadata does not include one or more parameters used for both spectral patching and 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, wherein the fill data includes at least one flag identifying whether a base form of spectral band replication or an enhanced form of spectral band replication is to be performed on audio content of the at least one block of the encoded audio bitstream, wherein the base form of spectral band replication includes spectral patching, the enhanced form of spectral band replication includes harmonic transposition, one value of the flag indicates that said enhanced form of spectral band replication should be performed on the audio content, and another value of the flag indicates that said base form of spectral band replication but not said harmonic transposition should be performed on the audio content; and wherein the fill data further includes enhanced spectral band replication metadata, and the enhanced spectral band replication metadata does not include one or more parameters used for both spectral patching and 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, wherein the fill data includes at least one flag identifying whether a base form of spectral band replication or an enhanced form of spectral band replication is to be performed on audio content of the at least one block of the encoded audio bitstream, wherein the base form of spectral band replication includes spectral patching, the enhanced form of spectral band replication includes harmonic transposition, one value of the flag indicates that said enhanced form of spectral band replication should be performed on the audio content, and another value of the flag indicates that said base form of spectral band replication but not said harmonic transposition should be performed on the audio content; and wherein the fill data further includes enhanced spectral band replication metadata, and the enhanced spectral band replication metadata does not include one or more parameters used for both spectral patching and 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.