Patent GB-1310411-A (GB)
Title: Methods and apparatus for communicating between a main station and secondary stations
IPC: H04B11/00
Filed: 1970-03-26
Abstract: 1310411 Selectively calling a remote station from a central station INSTITUT FRANCAIS DU PETROLE DES CARBURANTS ET LUBRIFIANTS 26 March 1970 [3 April 1969] 14906/70 Heading G4H Apparatus for transmitting messages between a central station and a remote station such as an underwater oil well head comprises means for transmitting from one of the stations messages consisting of a series of signals containing coded information, the information being contained in a carrier wave amplitude modulated by at least two sub-carriers each having a frequency selected from two values and means for determining at the other station the frequency of each sub carrier, which frequency is representative of the information. The transmitting means comprises a hydrophone which transmits the messages by acoustic waves such as ultra or infra sonie waves but the message may be transmitted by electromagnetic waves or along an electric cable. The central station (not shown) calls a remote station, Figs. 2A, 2B, by transmitting a message consisting of a call signal in the form of the pure carrier wave and an address signal in the form of a series of four amplitude modulated signals. Each of the address signals is amplitude modulated by four different frequencies F 1   # to F 4   8 the maximum and minimum values of the frequencies F 3 , F 4  being used to binary encode the address whilst the maximum and minimum values of the frequencies F 1 , F 2  binary encode the order of the signals in the series constituting the address. The hydrophone 2 at the remote station receives the message which is passed through a demodulator 6 and filters 7-10 each tuned to one of the frequencies F 1 -F 4 . Detectors 14-17 which detect the absence of the frequencies F 1 -F 4  in the call signal feed signals to an " AND " gate in a device 11 to enable a memory 26. Each discriminator 18-21 provides an output at one of its terminals if the frequency has its minimum value (i.e. F 1 -#) and at the other terminal if the frequency has its maximum value (i.e. F 1  + 8) so that the received address is fed to the memory 26. Devices 22, 106 ensurecorrect operation of the discriminators by blocking the member 26 via a gate 25 if the outputs of the discriminators are disturbed by parasitic signals. The received address is then fed from the memory to a decoder 28 so that if the received and actual address of the station do not match a signal is fed to a delay in the device 11 which returns the station to a rest condition. If the addresses match, a signal is fed to a device 30 connected to a control 31 which enables a device 100 to execute subsequent instructions such as the operation of a valve when an instruction is sent from the central station. A device 34 controlling the transmission of signals from the remote station is also enabled. The remote station then transmits via a transmitter 35 a " ready to operate " message comprising the address (fed from the decoder 28 to an address coder 33) and information relating to the valve positions and pressure at the well head, this pressure being measured by a voltage-frequency transducer 32 and transmitted as a carrier wave amplitude modulated by a subcarrier, the frequency of which varies linearly with pressure. The message is transmitted from the hydrophone 2 to the central station which, on receiving the message, is able to order the remote station to actuate a selected valve by sending instructions via the memory 26 to the control 31. A capacitive timing device 24 automatically returns the remote station to a rest condition at a predetermined time after the end of the received message. At the central station, the address of the called remote station, the valve position and the pressure values are displayed.