Abstract: Apparatus and methods for including a code having at least one code frequency component in an audio signal are provided. The abilities of various frequency components in the audio signal to mask the code frequency component to human hearing are evaluated and based on these evaluations an amplitude is assigned to the code frequency component. Methods and apparatus for detecting a code in an encoded audio signal are also provided. A code frequency component in the encoded audio signal is detected based on an expected code amplitude or on a noise amplitude within a range of audio frequencies including the frequency of the code component.

Abstract: A method and system for automatically qualifying a signal reproduction device for installation of monitoring equipment. In a household in which viewing patterns are to be monitored, for example, a relatively inexpensive and simple set usage device is installed in proximity to a television set which is expected to receive little or no use. Times when the set is in use are sensed, and data concerning those times are stored and transmitted to a central location. At the central location, the times represented by the transmitted data are compared to a minimum usage threshold, and a more sophisticated monitoring device, able to detect the channel tuning state of the T.V. set, is installed in proximity to the T.V. set only if the usage of the T.V. set exceeds the minimum usage threshold. The number of relatively expensive and complex tuning-monitoring devices required to be installed is thereby reduced.