Patent GB-932649-A (GB)
Title: Time division multiplex system
IPC: H04Q11/04
Filed: 1959-09-04
Abstract: 932,649. Automatic exchange systems. AUTOMATIC ELECTRIC LABORATORIES Inc. Sept. 4, 1959 [Oct. 2, 1958], No. 30323/59. Addition to 840,921. Class 40 (4). A two-way pulse transmission system comprising signal storage means 214, 234, Fig. 2, between which signals are to be exchanged, comprises two oppositely poled constant current sources 218, 238 which charge the condensers until the difference in the voltages they present to a highway TL1, TL2 reaches a prescribed value whereupon gates 217, 237 conduct to terminate the charging of the condensers, and means are provided responsive to a flow of current in the highway to turn off the constant current sources. The device 105 which is responsive to the current flow may also include means for inserting a voltage source in series with the highway which voltage may be fixed and merely for the purpose of facilitating the use of convenient bias potentials at the ends of the highway, or alternatively may vary linearly throughout the pulse period to provide amplification of the transferred signals. The invention is applied to an automatic exchange system in which connection is established between line circuits 101, 102 via an audio link circuit 103, using one pulse position for the connection between circuits 101, 103 and another pulse position for connection between circuits 102, 103. Considering the simplest case where device 105 inserts no voltage and with the bias voltages being EBW = - 15V and EBE = - 5V if no modulation is present on condensers 214, 234 the condensers are charged during the pulse interval by sources 218, 238 until they each achieve a potential of - 10V after which rectifiers 217, 237 conduct and further charging ceases. This conduction is arranged to occur in this case at the mid-point of the pulse interval. Assuming now the condenser 214 is charged positively, then the point of conduction occurs earlier and condenser 234 has less time to be depressed towards the - 10V value before current from the source 238 is diverted along the highway. Thus condenser 234 becomes charged more positively in response to the positive charge on condenser 214. Likewise if condenser 214 is charged negatively then conduction occurs later and condenser 234 becomes more negative. If a linearly increasing control voltage is inserted in the highway, with charging conditions again arranged so that for no modulation, highway current occurs at the midpoint of the interval, then if condenser 14 receives positive modulation, so that without the control voltage conduction occurs earlier than the mid-point interval, then with the linearly increasing control voltage conduction occurs earlier still thus resulting in an enhanced positive change on the condenser 234. Similarly for negative modulation the control voltage results in an even later conductive condition on the highway, again giving rise to amplification. Detailed mathematical formulµ are given for the circuit in its various forms. Detailed embodiment, Figs. 6, 7, 8. The subscriber's subset A is connected via a matching transistor 621 and low-pass filter 623, 624 to the condenser 614. For setting up a call the control circuit 106 selects a free link circuit 803 which has two pulse positions DP1, DP2 permanently associated therewith, the first for the incoming side and the second for the outgoing side. Each pulse position occupies 1.5ÁS and is separated from its neighbour by a 0.5ÁS guard interval during which a pulse is emitted on lead CP. This pulse is operative via transistor amplifiers 540, 541 and transformer 543 to render the rectifier network 547 conducting to clamp the highway TL to a voltage of - 10V between pulse positions to prevent cross talk between adjacent channels. This pulse also puts the bi-stable circuit 772, 771 into a position where negative potential is applied to conductor TC. The control circuit emits a pulse in position DP1 on tens and units leads PU, PT of the calling subscriber's line, thereby operating the AND gate 644 to render transistor 618 conducting to start charging the condenser 614. Simultaneously, the pulse is emitted on lead ST1 and lead DP1 is energized to operate AND gate 806 to cause transistor 838 to conduct and charge condenser 834. The clock pulse also turns on transistor 751 to charge condenser 754 which then discharges during the succeeding 1.5ÁS pulse period to produce a saw-tooth voltage which is amplified by transistor 752 and inserted into the highway by transformer 750. When eventually current flows in the highway it induces in transformer 759 a voltage which when it exceeds a small threshold voltage (for guarding against noise) imposed by silicon diode 761, turns off the normally conducting transistor 762 to change the bi-stable circuit 771, 772 to its other state in which it removes the negative potential from lead TC thereby disabling the AND gates 644, 806 to cut-off transistors 618, 838, thereby cutting off the current supplied to condensers 614, 834. In the audio link circuit the charge on condenser 834 is transferred to condenser 844. Connection between the latter and condenser 614 in the called subscriber's circuit is established in a similar manner during pulse period DP 2 . In an alternative embodiment, Figs. 3, 4 (not shown), the varying voltage is obtained by charging a condenser in series with the highway which further contains a resistor, the onset of current in the highway developing a voltage across it to operate a transistor which interrupts the charging current. In a further embodiment, Fig. 5 (not shown), the sawtooth voltage is developed across the primary of a transformer whose secondary, which lies in the highway, is split by a condenser. The latter is charged via a rectifier during the flyback portion of the saw-tooth to provide a standing voltage upon which the saw-tooth is superposed. Specifications 841,836 and 892,435 are referred to.