Patent GB-948929-A (GB)
Title: Improvements in or relating to communication systems
IPC: H04Q11/02
Filed: 1960-06-14
Abstract: 948,929. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. June 14, 1960 [June 16, 1959], No. 20838/60. Heading H4K. A satellite is connected with a main exchange by go and return frequency multiplex systems, and when a line connected to the satellite is found to be calling, a carrier frequency is allotted thereto to connect it with the main exchange. Transmission arrangements.-The system is designed to transmit not only speech but also picture signals. Accordingly, each channel comprises a band of 0-1.04 mc/s. for picture signals provided by equipment STV at the substation and a band of 1.04-1.05 mc/s. is provided for speech signals. At the concentrator the audiofrequency signals are provided by equipment STE and modulate at SM1 a carrier of 1.045 mc/s., the double side band signal, together with the carrier being emitted in the above-mentioned band by the modulator SM1. The carrier generator TG is only switched on when the handset is off-hook and may be interrupted by the operation of the dial D for transmitting a wanted subscriber's number. One of ten selectable carrier frequencies 4.0, 7.0 . . . 31.0 mc/s. is modulated at OPM by the composite speech-plus-vision signal and mixed with other such signals in the mixer 3 whence it is passed to the line 1 via the 2.6-32.5 mc/s. band-pass filter 5. At the exchange each channel is filtered and detected to produce the composite signal which is then fed to terminals such as 1A of a switching network 39. A filter such as SCF1 also accepts signals from 0-1.5 mc/s. which are fed to a scanner 47 for the reception of off-hook and dial signals as described below. Signals going towards the subscriber such as those emitted from terminal 1B of the switching network 39 are modulated or fixed carriers and transmitted to the line 2. At the concentrator the signals are fed to the demodulators IPM. Each demodulator is fed with an appropriate channel frequency and the demodulated signal is fed to the vision reproducing equipment STV and to a detector 28 which demodulates the sound for the speech equipment STE. Provision is also made for the cyclic application of a scanning frequency of 34 mc/s. to the modulators OPM of all idle subscribers. When such a subscriber goes offhook, this frequency is modulated by the 1.045 mc/s. signal from TG and diverted to a detector 9 which causes the 1.045 mc/s. signal to be transmitted to the main exchange where it is accepted by filter SCF and fed after detection to the scanner 47. The channel frequencies are generated both at the exchange and the concentrator by harmonic generators C29, L29 each fed (in the latter case over the cable 2) from a 1 mc/s. master oscillator 29. Switching arrangements.-These are controlled by a common control circuit 41 at the exchange in conjunction with a permanent store of instructions 43 and a temporary store 45 which holds indications of the busy, idle &c. conditions of lines and channels as described in Specification 904,164. Control information from circuit 41, destined for the concentrator, is transmitted thereto by means of combinations of audio-frequency tones sent from a tone generator TG1 via gates G1 . . . G7 controlled by the circuit 41, over the line 2. At the concentrator these tones are accepted by filters FSD1 . . . FSD7 whose outputs operate monostable circuits 53, whose outputs are translated at 55 into a series of control conditions on individual lines. The control information consists of two binary bits to indicate " scan," " mark " or " disconnect." The " scan " signal is sent in conjunction with the subscriber's line identity to indicate that scanning frequency of 34 mc/s. is to be applied to the subscriber's modulators. The " mark " signal is sent in conjunction with both the subscriber's line identity and the carrier channel identity to indicate that the subscriber's modulator and demodulator are to be supplied with the particular channel frequency until further notice. The " disconnect " signal is sent in conjunction with the channel frequency identity to indicate that the provision of this channel frequency to its presently associated subscriber is to cease. It is to be noted that connection and disconnection of a subscriber and a channel is effected solely by the provision or withdrawal of the carrier frequency from the associated modulator and demodulator. The temporary store 45 supplies the control circuit with the identities of all lines which are idle (as indicated by the most recent test) and the control circuit feeds these identities cyclically together with a " scan " signal to the concentrator which thereupon, as described above, feeds the 34 mc/s. carrier to the modulators of these subscribers in turn, each subscriber being scanned once every 100 msec. If a subscriber has meantime gone off-hook, this frequency is modulated by 1.045 mc/s. and a signal of this frequency is transmitted over line 1 and detector SCD to scanner 47 which is simultaneously addressed to receive the output therefrom. When such an output is received the common control circuit 41 registers the off-hook condition in the temporary store 45. A free channel is now allotted to the call, the control circuit causing the appropriate audio-frequency code to be sent to the concentrator to cause the carrier frequency in question, say 4 mc/s., to be applied to the subscriber's modulator and demodulator. At the same time the switching network 39 is actuated to connect the dial tone source 85 to terminal 1B whence it is sent over the channel to the subscriber. The off-hook condition of the subscriber is now detectable from the detector SCD1 and the rate of scanning of this detector is now increased to ensure proper reception of the dial pulses. The digits are now received and dial tone disconnected as in Specification 904,164. When reception is complete, reference is made to the temporary store 45 to determine whether the called subscriber is free, this information being already present due to the continuous scanning operations previously described. If the line is busy the control circuit connects busy tone from source 87 to terminal 1B for transmission to the caller. If the line is free the control circuit takes into use a free channel, say that connected to terminals 2A, 2B of the network 39 and corresponding to a 7 mc/s. carrier and the concentrator is signalled to apply that carrier frequency to the called subscriber's modulator and demodulator. Connection is effected between terminals RCB, RCA of a ringing tone generator 89 and terminals 1B, 2B of the switching network so that appropriate tones are sent to the calling and called subscribers. When the called subscriber answers the detector SCD2 output is received by the scanner 47 so causing the control circuit 41 to disconnect the ringing connections and to establish connections between terminals 1A, 2A and 1B, 2B respectively to complete the connection between the subscribers. Other types of connection.-More than one concentrator may be connected to the exchange, in which case the switching network contains additional A and B terminals leading thereto. The connections are, however, set up in a manner similar to that described above. Connections may also be set up to a further exchange or to an operator over two-way trunks which are connected to both sides of the switching network 39 via a hybrid network HN. This involves the use of a trunk scanner 97 which provides indications of free trunks so that one may be selected by the control circuits for forwarding the call. The control circuit is provided with means 99 for forwarding the wanted subscriber's identity. Similarly, incoming calls are detected by the trunk scanner 97 and set up to the wanted subscriber in accordance with signals received over the trunk. Channel carrier selector, Fig. 4.-This equipment connects appropriate channel carriers on horizontal leads H1 . . . H10, or the scan frequency on lead N 1  to verticals V 1  . . . V 30  leading to the respective subscriber's modulators and demodulators. When such a connection is to be made, say between the carrier on H 1  and the lead V 1 , lead M 1  (indicative of the carrier) is pulsed negatively and lead ML1 (indicative of the subscriber) is pulsed positively. The resulting change in anode-cathode voltage on cold cathode tube 73 fires the tube, the tube current passing through the normally conducting transistor 77. Diode 61, which normally provides a conductive shunt to prevent the carrier reaching V 1 , is now backbiased to permit the carrier to reach V 1 . Disconnection is effected by applying a positive pulse to the base of transistor 77 (one of these being supplied for each carrier) to render it non-conducting so extinguishing the tube 73. Similar arrangements are provided for the provision of scanning frequency, the lead SM being negatively pulsed instead of M 1 . Disconnection, however, is effected automatically after a short interval in this case by a pulse applied to the base of the transistor a prescribed time after the firing of the tube in question. Specifications 704,108, 775,832, 789,660, 790,180 and 899,761 also are referred to.