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A method and apparatus encodes and decodes machine readable signals in audio signals for producing humanly perceived audio transmissions. The encoding device includes circuitry for identifying portions of the audio signal having relatively low energy in a given band and relatively high energy in a b

A method and apparatus encodes and decodes machine readable signals in audio signals for producing humanly perceived audio transmissions. The encoding device includes circuitry for identifying portions of the audio signal having relatively low energy in a given band and relatively high energy in a band proximally below the given band. The machine readable signals are then inserted into the identified portions of the audio signal. According to another embodiment, the machine readable signals are encoded as a spread-spectrum signal which is added to the original audio signal. The spread-spectrum signal is scaled using a unique technique referred to as Common Mode Scaling.

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[ What is claimed as new and desired to be secured by Letters Patent of the United States is:] [1.] An encoding device for encoding digital data onto audio frequency signals, said device comprising:means for generating, in a given band of the audio frequency, a pulse-width-modulated signal encoded w

[ What is claimed as new and desired to be secured by Letters Patent of the United States is:] [1.] An encoding device for encoding digital data onto audio frequency signals, said device comprising:means for generating, in a given band of the audio frequency, a pulse-width-modulated signal encoded with digital information;means for identifying temporal portions of the audio frequency signal having low energy in given band and relatively high energy in a band proximally below the given band; andmeans for summing said-pulse-width modulated signal and the audio frequency signal to produce an encoded audio frequency signal containing the encoded digital information in the low energy temporal portion such that human perception of the encoded audio frequency signal is substantially identical to that of the original audio frequency signal, whereinsaid encoding device encodes digital information in both first and second audio frequency channel signals of a stereo audio frequency signal, wherein said generating means generates a second pulse-width-modulated signal encoded with digital information inversely corresponding to said first pulse-width-modulated signal and said summing means adds the fist pulse-width-modulated signal to the first audio frequency channel signal and the second pulse-width-modulated signal to the second audio frequency channel signal.