Apparatus and method for including codes in audio signals

An apparatus and method for including a code having at least one code frequency component in an audio signal are provided. The ability of an audio signal frequency component to mask a code frequency component to human hearing when the audio signal is reproduced as sound during a first time interval and the code frequency component is reproduced as sound during a second time interval offset from the first interval is evaluated. An amplitude is assigned to the code frequency component based on the masking evaluation and is included within a portion of the audio signal within the second time interval.