1. An apparatus for recovering an identification message encoded into an analog sound signal forming at least a part of a program, the identification message being encoded by modulating the energy of a component of a digitized sound signal in at least one band of audible frequencies thereof, the analog sound signal being reproduced by a loudspeaker, comprising: an enclosure carryable on a person, microphone means within the enclosure for converting the analog sound signal reproduced by said loudspeaker into an electrical sound signal, means within the enclosure for detecting modulations in the energy of the electrical sound signal in said at least one band of audible frequencies, means within the enclosure for retrieving the identification message from the detected modulations, carry detection means for determining whether said apparatus is being carried by a person, and means within the enclosure for storing the identification message and for storing an indication of such determination whether said apparatus is being carried by a person.