Patent GB-1444603-A (GB)
Title: Time division communication systems
IPC: H04M3/56
Filed: 1973-07-27
Abstract: 1444603 Automatic exchange systems WESTERN ELECTRIC CO Inc 27 July 1973 [31 July 1972] 35898/73 Heading H4K  A TDM conferencing system comprises an incoming and an outgoing highway that are coupled by a summing circuit which in any time slot adds all the signals appearing on the outgoing highway and applies the sum signal to the incoming highway, and in each station of the system means is provided for subtracting that station's own contribution to the sum signal from the signal applied thereto from the incoming highway. In Fig. 1 the stations 1 are connected via unbalanced lines 137 to respective station line circuits 101 each of which is connectable to an outgoing and an incoming highway 160 and 164 respectively by means of gates 105, 109 respectively. The gates are enabled during a time slot allocated to a call by means of a pulse pattern circulating in a local shift register 150 (Fig. 10, not shown) and inserted therein by a central controller 170. Assuming the two stations shown are in communication, then at all times during a frame, except during this communication time slot, their incoming stores 110 contain a signal sample inserted therein from the incoming highway 164 during the last occurring time slot. This sample is applied to the station 1 via line 133 and is also applied via a symmetrical amplifier 112 to both sides of an outgoing signal store 130 so as to ensure that the latter only contains a signal sample emanating from the local station. During the allocated time slot the signals A 1  and A 1 <SP>-</SP> occur so as to respectively open and close the various gates. The outgoing stores 130 are thus connected via amplifiers 159 to the outgoing highway 160 and also to one side of their respective incoming stores 110 so as to ensure that the incoming signals from highway 164 stored thereon are devoid of any signal components due solely to their own stations. The impedances 103 serve to isolate each station from the signals on the outgoing highway produced by the other communicating stations, and also serve as matching impedances to the summing circuit 162. In practical embodiments (Figs. 2 and 3, not shown) the amplifiers, the gates and possibly the impedances are formed by IGFETS produced by integrated circuit techniques, while the stores are provided by capacitors. In Fig. 4 the line circuit is modified in order to compensate for aliasing and aperture effects. The former arises when sampling occurs at less than twice the highest frequency to be transmitted such that out-of-band high frequency signal components appear. These are eliminated by a low pass filter formed by resistor 424 and outgoing storage capacitor 430. The aperture effect arises from the sample and hold nature of the transmission. The latter results in a transfer function whereby infinite loss occurs at multiples of the sampling frequency. This causes losses up to the sampling frequency within the desired passband. In order to compensate, it is necessary to subtract a portion of the preceding signal sample in the incoming storage capacitor 410 from any newly inserted signal sample. The sample on 410 is applied for most of a time frame to a compensating capacitor 436 via gate 434. (This is in addition to the application of the sample to the station line 133 and symmetrically to outgoing storage capacitor 430 via amplifier 440, 445.) During the allocated time slot, capacitor 436 is isolated from store 410 by the opening of gate 434, for a brief interval following the slot, capacitor 436 is connected to capacitor 410 via switch 432 such that a redistribution of charge occurs. This redistribution acts as a filter to shape the frequency characteristic of the signal in store 410 such as to compensate the aperture effect. Lightning protection is afforded by oppositely poled diodes 496, 494. In Fig. 7 and Figs. 5 and 6 (not shown) provision is made for stations having balanced lines. Each line wire in this case supplies an outgoing signal to a respective outgoing storage capacitor 730a and 730b. The incoming storage capacitor supplies signals of opposite phase to the two outgoing capacitors such that when the latter are connected in series to the outgoing highway on terminal 543 during the allocated time slot, only the signal due to the local station is actually transferred thereto. An incidental effect of the series connection is to eliminate signal components caused by longitudinal currents on the line wires. It is arranged that D.C. voltages are either not stored on the capacitors 730 during active connections or are not transmitted, as by use of a capacitor 737, since such voltages, particular in a conference call, degrade the communication. However loss of these voltages causes difficulty in supervising the on- and off-hook state. This is overcome, by means of an additional switch 731 which turns on in response to an open-loop, i.e. on-hook, condition thereby to transmit a large negative going transient voltage through capacitor 737 during a special time slot devoted to station supervision. The conference aspects are specifically claimed in divided Specification 1,444,604.