A method of encoding, and decoding, a human-inaudible acoustic signal 240 embedded within audio content, suitable for use within the processor of an encoding and broadcasting system or a decoding and receiving system 200, 202. A binary message is encoded into a sequence 1000, 2000 of symbols selected from a group of four or eight symbols, each symbol encoding two or three binary bits with an associated frequency. The human-inaudible acoustic signal may have a variable length, with the length of the signal encoded in a header 1002-1008, 2002-2009. The encoding uses both forward error correction and cyclic redundancy coding to increase the robustness of the transmission. Furthermore, the binary bits encoded by the symbols are selected such that a Hamming distance of a single bit transformation exists between symbols associated with adjacent frequencies, so that a frequency error creates only a single bit error.