| 摘要 |
A local network (10), forming part of a cable TV system, includes subscriber outlets (21), from which signals can be transmitted in the return direction to a head end (11) of the system within a predetermined carrier frequency band. The network (10) is provided with a noise reduction unit (23) comprising an input (24), connected to the subscriber outlets (21), an output (27), connected to the head end (11), a plurality of band-pass filters (28-31), connected to said input (24) and having mutually different pass bands lying within said carrier frequency band, and a corresponding plurality of detectors (32-35), which are arranged each to compare the energy level of the output signal from one of said filters (28-31) with at least one predetermined reference level and to deliver logic signals depending upon the result of said comparison to a logic control unit (36), which is arranged to determined whether these signals satisfy certain conditions stored in the control unit (36) and to deliver a control signal, depending hereupon, to a controllable blocking circuit (26), which is connected in a separate line (25), serving to transmit signals from said input (24) to said output (27), and which is arranged to permit signals to pass therethrough only when it receives a control signal from the control unit (36), indicating that the logic signals received by the control unit (36) satisfy the conditions stored in said unit. |
