Secure two channel sca broadcasting system

Abstract

A secure information broadcasting system including a central broadcasting station for encoding data in binary form utilizing a keyword and transmitting on one frequency a FM-SCA signal phase shift modulated by the encoded data. The encoded data is also supplied to one or more remote broadcasting stations, typically via telephone lines, where it is transmitted on a second frequency as a FM-SCA signal phase shift modulated by the encoded data. Each remote station transmits a relatively low power signal in a predetermined local area, such as an office building, while the central station transmits a relatively high power signal in a general area including one or more of the local areas. The general area signal is transmitted with a time delay relative to the remote stations. Each of a plurality of portable receiving terminals has an identification number stored therein. When a subscriber desires to display the broadcast information on his terminal, he enters a unique password which is combined with the identification number to generate the keyword. The keyword is utilized to decode the received binary data. The terminal alternately tunes in the two frequencies and will lock on the first frequency signal when the terminal is within the general area but outside the local areas. The terminal will lock on the signal first received in the local area and a reset switch is provided to force the terminal to lock on the second frequency signal when the first frequency signal is present and is first selected in a local area.