Patent GB-1387168-A (GB)
Title: Tdm telecommunications systems
IPC: H04Q11/04
Filed: 1972-10-06
Abstract: 1387168 Muliplex pulse code signalling SIEMENS AG 6 Oct 1972 [6 Oct 1971] 46105/72 Heading H4L In a system for controlling the clock pulse frequency in a P.C.M. t.d.m. exchange interconnected with other exchanges, incoming pulse trains on lines I to L derived from the remote exchanges are frequency divided at ZI to ZL respectively and the resulting outputs are supplied to corresponding phase discriminators KI to KL which receive the output of the clock oscillator O frequency-divided at ZO as their second input. The frequency-dividers are arranged in known manner so that the two inputs to each phase discriminator are in antiphase and the discriminator outputs are combined (RI to RL) and supplied via a low pass filter TP and combining stage US to control the frequency of oscillator O. Frequency-divider ZO is an mstage counter having outputs r, m/2 and m-r, the intervals m/2 to r and m/2 to m-r representing a maximum frequency difference (plus or minus) between the osicllators O of the exchanges. A bistable RB is connected to the outputs r and m-r and if these limits should be exceeded an AND gate RU is enabled. Frequency-divider ZL supplies the other input to gate RU via OR gate RO and if the counter ZL reaches its last step before the counter ZO has reached step r or after it has reached step m-r, a bi-stable RP supplies a signal to reset counter ZL and hold it in this position until bi-stable RP is reset. Bi-stable RP is reset by a signal from output m/2 of counter ZO and counter ZL now commences with a time delay of 180 degrees relative to ZO. This operation may be triggered by an external signal via OR gate RO and it is stated that the other counters ZI &c. may be included in the operation. The incoming pulses are also frequency-divided at IZ to LZ and supplied to phase-discriminators IK to LK together with the output of oscillator O frequency-divided at OZ. The operating range of these discriminators corresponds to the regulating range of the previous circuit plus the maximum transit time fluctuation on a t.d.m. line. This synchronizing circuit also includes a circuit SR similar to the circuit RS. The outputs of discriminators IK to LK, are combined at IR to LR and supplied via a low pass filter to a comparator V which can be a threshold circuit acting as a reference value. The output of the comparator is supplied direct and via an inverter to control a binary signal generator BG which is triggered only when there is an output from circuit RS. The generator BG, via circuit US, superimposes on the regulating signal from the first syncronizing circuit an additional signal in response to a signal from RS. This additional signal, in the case of a positive difference between the exchange pulse train phase and the average line pulse train phase, selects a frequency regulating range below the frequency f 0  of the oscillator O with no input signals, and in the case of a negative phase difference, selects a frequency range above the frequency f 0 , the oscillator O being controlled within the selected range by the output from TP. Instead of analogue phase comparison circuits, the comparison may be carried out in digital form.