Two-way digital communication system and method without carrier lock-on or ingress interference

摘要

A communication system that provides two-way digital communication between a central site and a plurality of user terminals via standard cable television networks and facilities free from interference from "carrier lock-on" and "ingress" by transmitting an FSK signal over one frequency channel from the central site to the user sites and transmitting two spaced FSK subcarrier signals, on two other frequency channels, from the user terminals to the central site, and where the receiver at the central site has two channels matched in frequency to the subcarrier frequencies and having a decoder that will not pass received information unless it has been received for a prescribed duration at a prescribed level.