| 摘要 |
1507104 CATV systems HOCHIKI CORP 13 Feb 1976 [13 Feb 1975] 05790/76 Heading H4R In a CATV system in which, in addition to the normal television programme signals, a control signal and locally produced information signals in the television signal frequency band are distributed during an emergency, a control unit 14, Fig. 2, is provided which includes a filter 22 to separate the control signal from the local information signals and programme signals, a frequency converter 25 to change the information signals to the intermediate frequency of the television receiver, and a change over switch 26, operated by the control signal to break the connection between the tuner 28 and if stage 27 of the receiver 7 and connect the output of the frequency converter 25 to the if stage 27 during an emergency. As described, in the distribution system, Fig. 1(a)(not shown), signals from a TV aerial (1) are amplified and mixed with locally generated programme signals together with control signals and alarm information signals during an emergency. All receivers may be tuned to receive the locally generated programme or alarm signals in addition to the normal broadcast signals but in an emergency all the receivers equipped with a control unit (14) are automatically switched to receive the alarm information signals. In the system of Fig. 1(b)(not shown) all receivers have control units, while in the system of Fig. 1(a)(not shown) only receivers used by custodians, or those responsible for blocks of subscribers, are provided with control units. The control units may include a second switch operated by receipt of the control signal to switch the receiver on if it should be switched off at the normal control switch, Fig. 3 (not shown). |
