An autofocus mechanism for a spectroscopic system determines a time varying optical property of a volume of interest. In accordance with a specific embodiment, a circuit and method are designed, among other things, to effectively minimize power consumption by memory cells and to increase speed and reliability of sense amplification. Television programming, which may be furnished by any number of sources, is accompanied by a signal that indicates the availability of other programming, such as interactive television service related to the program being viewed, as well as information to be used in requesting such service.