Patent GB-1361937-A (GB)
Title: Keysending signalling systems
IPC: H04Q1/442
Filed: 1972-08-03
Abstract: 1361937 Substation circuits; signalling STANDARD TELEPHONES & CABLES Ltd 3 Aug 1972 36230/72 Heading H4K A signalling system for transmitting the number of a required subscriber from a subset to an exchange includes in the subset a signal generator producing a continuous sinusoidal tone in the frequency range beyond the limit of human audibility i.e. above 15 kHz. and which transmits said tone to the exchange when the subscriber goes off hook, the subset further including a keysender for deriving binarycoded signals which are used for modulating said tone in bi-phase mode; the exchange includes a receiver for deriving the binary signals from said modulated tone and for converting the binary-coded signals into a form suitable for utilization by the exchange. The Figures show a block diagram of such a system, Fig. 1 showing the subset equipment and Fig. 3 the receiver at the exchange. Most of the equipment employed is known. The subset includes a 25 kHz. oscillator for producing a continuous tone at both an "inphase" and "anti-phase" output. The oscillator may be a quartz crystal suitably divided down, a timing fork, or another electro-mechanical type of resonator. Which of the two oscillator outputs is connected to the line is controlled by an electronic switching unit ESU which in turn is operated by a serial four-bit binary signal representing a digit of a required number, the four-bit binary number being produced in known manner as a result of the depression of a corresponding number key. In operation, upon the calling subscriber going off-hook, a continuous 25 kHz tone is transmitted to the exchange to act as an off-hook signal. This is equivalent to a binary 0. This is followed by a binary 1 start bit produced by instantaneously changing the phase of the tone by 180 degrees, i.e. by operating the switching unit ESU. Thereafter the phase of the tone is changed only when there is a change from binary 1 to binary 0 or vice versa, the signal reverting to or remaining at binary 0 after the last binary bit of the digit has been transmitted. Extra bits may be added for checking purposes at the exchange receiver. The receiver uses a phase-locked loop receiver of known form. These signals received are divided into two paths and fed- (a) to a loop circuit 31 where they are used to lock a local oscillator at the basic frequency 25 kHz, to the increasing signal, and (b) to a phase detector 32, which detects the varying phase against a reference signal frequency from the phase-locked local oscillator so as to generate the original 4-bit binary signal. The signals are fed to a check circuit and shift register where the binary signal is checked and reconverted to decimal form. Such an arrangement may also be used for data transmission between subscribers once a connection has been completed. To this end the subscriber may be provided with a "send data" button which switches the oscillator tone into the voice band range. This new tone can then be modulated in biphase mode to transmit data between the subscribers in a similar manner to the way in which dialling information was transmitted to the exchange. The transmitter in the subset may be constructed using MOS technology and large scale integrated circuit techniques. The microphone can be cut out of circuit during signalling and signalling speeds of the order of 200 Bauds can be used.