Encoding and decoding an audio watermark using key sequences comprising of more than two frequency components

A system for decoding a message comprising multiple bits of data from an audio watermark signal in audible frequency range, whereby all the bits of the message are located in a frame of the audio watermark signal, the system comprising a processing device to:

• receive the audio watermark signal, wherein the audio watermark signal comprises a message encoded in a frame of the audio watermark signal, wherein the message comprises a plurality of message bits, each encoded in different, more than two frequency components of the frame;
• retrieve from a storage device a plurality of key sequences, wherein each key sequence corresponds to a different message bit in the message, wherein each key sequence comprises more than two frequency components that represent frequencies of the frame of the audio watermark signal at which a message bit of the message corresponding to the key sequence is encoded within the frame, and wherein each key sequence has an equal number of positive components and negative components, each having substantially the same amplitude;
• read a frame from the audio watermark signal, wherein the frame comprises all the message bits of the message to be decoded;
• transform the frame of the audio watermark signal containing the message from time domain to frequency domain;
• convert the frame of the audio watermark signal to decibels;
• apply the plurality of key sequences to the frame of the audio watermark signal to recover a plurality of recovered signals from the frame that represent all the message bits of data of the message;
• wherein applying the plurality of key sequences to the frame of the audio watermark signal to recover the message comprises:
• for each of the key sequences, multiplying each of the frequency components of the respective key sequence by the corresponding frame frequency components of the frame of the audio watermark signal, resulting in a recovered signal that represents a single bit of data of the message that corresponds with the key sequence, the recovered signal comprising a plurality of frequency components each having an amplitude;
• for each of the recovered signals, average the amplitudes of the plurality of frequency components;
• for each of the recovered signals, determine, based on the average of the amplitudes of the plurality of frequency components of the recovered signal, the bit of the message corresponding to the key sequence.