MODULATOR ARRANGEMENT FOR CATV SYSTEMS

1432445 Alarm systems HOCHIKI CORP 2 April 1973 [1 April 1972] 15633/73 Heading G4H [Also in Division H4] In a CATV system provision is made for return or "up" signals from subscribers to the central station, e.g. for alarm &c. The signals originate at the subscribers at frequencies different from the general "up" frequency of the system and are converted in modulator units having signal splitters which by-pass the broadcast TV, or "down", signals around the modulator circuit so that only the "up" signals pass through the modulators. As described the "down" TV signals are in the band 70 to 300 MHz and the "up" alarm signals in the band 20 to 60 MHz, but generated by the subscribers equipment, 14, at frequencies in the audio band, or alternatively in the frequency band,. up to 60 MHz. The alarm signals are returned from the subscribers equipment to the central station 6, 7, 8, via the distribution system which includes modulator units 10 to change the audio of r.f. alarm signal to the normal "up" frequency band 20 to 60 MHz. Each modulator unit includes a modulator 17 and signal splitters 18, 19. The splitters have filters 24, which by-pass the TV signals in the band 70 to 300 MHz, filters 23, which select the audio or radio frequency alarm signals for application to the modulator, and filters 22, which pass "up" signals in the band 20 to 60 MHz at the output of the modulator. If modulator units are required to be connected in cascade then the pass band of the bypass filters 24 is extended to include the "up" frequency range or further by-pass filters provided, to pass up signals from downstream modulator units, and the filter at the output side of modulator 17 is modified to eliminate the particular frequencies generated by downstream modulator units. Similar circuits Figs. 7, 8 and 9 (not shown) may be used in systems where some of the alarm frequencies from the subscribers are already in the normal "up" frequency band in addition to frequencies below 10 MHz, and the alarm signals in the up frequency band are transposed to different frequencies in the same band.