Patent GB-1444258-A (GB)
Title: Telephone systems
IPC: H04J1/02
Filed: 1973-07-10
Abstract: 1444258 Party line systems CARRIER TELEPHONE CORP OF AMERICA Inc 10 July 1973 [14 July 1972 (2)] 32915/73 Heading H4K In a frequency multiplexed carrier communication system in which a central station communicates with a number of remote stations via SSB channels of the same width and in which communication from the central station to each remote station is on a respective different central station carrier frequency while transmission from each remote station to the central station is on a respective different remote station carrier frequency and all the central station carrier frequencies and all the remote station carrier frequencies are in respective non- overlapping bands, a single stable oscillator is used per channel, located at one of the central and remote stations, to provide the carrier frequency for one direction of transmission and a frequency converter is provided to derive the carrier frequency for the other direction of transmission from the stable oscillator. Preferably one of the frequencies is an integral multiple of the other so the frequency converter is a multiplier or divider. As shown in Fig. 2 transmission from the central station to the remote stations takes place in the band 64 to 136 kHz and from the remote station to the central station in the band 8 to 56 kHz. Signals in the band 0-4 kHz being modulated in a modulator 13 at the central station, the upper sideband selected by filter 17, and a portion of carrier added via amplifier 19 before transmission over the multiplex circuit so that the carrier for the demodulator 29 at the respective remote station can be obtained from the incoming signal with the use of a filter 31. Each remote station uses a carrier frequency for transmission to the central station which is an integral sub multiple of the carrier used for incoming signals to that station so that the carrier is derived from the output of filter 31 by a divider 39, the incoming and outgoing carrier frequencies for the different stations being shown in Fig. 2. Signals from the remote station are modulated on the divided carrier in modulator 37, and the upper sideband is selected in filter 45. At the central station the respective sideband is selected by filter 21 for demodulation, in 23, by a carrier derived via a similar divider 25, as was used at the remote station, to restore the signal to the speech band. In a modification of the system, Figs. 5 and 6 (not shown), the modulators are each direct single sideband modulators using quadrature related signals and quadrature related carriers in respective modulators whose outputs are combined to provide single sideband signals without filtering. Carrier transmission for synchronizing the opposite station is provided by unbalancing one of the modulators to provide a carrier leak which is used at the receiving end to synchronize a phase locked, or other synchronized, oscillator. Expansion and compression of the audio is provided and ringing is accomplished from the central station by tone sending and from the remote station by switching the carrier. In a further alternative embodiment. Figs. 3 and 4 (not shown) the signals incoming to a station are translated to an intermediate frequency, e.g. in the 455 to 459 kHz band, before translation to the transmission or audio frequency band, so that most of the filters in all the stations can be common.