1. An apparatus for including a code having at least one code frequency component with an audio signal having a plurality of audio signal frequency components, comprising: first masking evaluation means for evaluating an ability of a first set of the plurality of audio signal frequency components to mask the at least one code frequency component to human hearing to produce a first masking evaluation; second masking evaluation means for evaluating an ability of a second set of the plurality of audio signal frequency components different from the first set thereof to mask the at least one code frequency component to human hearing to produce a second masking evaluation; amplitude assigning means for assigning an amplitude to the at least one code frequency component based on a selected one of the first and second masking evaluations; and code inclusion means for including the at least one code frequency component with the audio signal.