Method for selectively interrogating decoding devices which are located at remote stations. In one embodiment, a number of transponders which make up a final group within a larger group of transponders, all attached to a common line running from a central station, are selected and simultaneously caused to retransmit modified interrogation signals in accordance with certain informational characteristics and the identity of each transponder retransmitting. The final group of transponders, which include respective decoding devices, chosen to be interrogated is selected by first sending a number n of tones of a larger group of tones m down a branch line to partially activate some of the decoding devices. These tones are followed by a smaller number of tones k chosen from the group n to which only the decoding devices in the final group respond. The system is particularly designed for use in a subscriber television network and specific circuitry for accomplishing the logical and electrical functions required are described.
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