This invention involves a system of communicating information using audio signals. In one embodiment, the information is encoded in touch tones on the sound track of a television game show. The information relates to the action of the show, and is broadcast in-band with the audio portion of the show....http://www.google.com.tw/patents/US5213337?utm_source=gb-gplus-share專利 US5213337 - System for communication using a broadcast audio signal
