This paper describes a method of encoding a very low data rate (<1 bit/s) digital signal on to linear PCM audio. Key attributes of the encoding include: it is inaudible to professional standards, does not become audible when subjected to postprocessing of the audio signal, and is inexpensive to implement. A key feature is the unauthorized application of any form of processing to the audio signal completely destroys the data signal. The presence of the data signal may be used as a reliable indicator that a signal is genuine and unadulterated. The encoder includes a new biased quantizer which permits the addition of the data signal with a change of less than 0.025 dB in the noise floor of the audio signal. The development process makes use of fast prototyping techniques to allow frequent evaluation of the evolving encoding by listening tests.
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