Patent GB-1222685-A (GB)
Title: Digital communication systems
Inventor: MILLER MICHAEL ROBERT
IPC: H04J3/06
Filed: 1968-05-31
Abstract: 1,222,685. Multiplex pulse code signalling. POST OFFICE. 29 May, 1969 [31 May, 1968], No. 26270/68. Heading H4L. In a digital communication system including at least three interconnected transmission centres, each centre having a local oscillator for timing the digits at that centre, one of the centres being designated as the reference centre and each of the other centres including comparator means for comparing the digit timing of that centre with the digit timing as received from the other centres and means for combining the outputs of the comparator means to produce a control signal to bring the local timing oscillator into synchronism with that of the reference centre, means are provided for producing an alarm signal whenever the condition exists that the phase difference between the local oscillator of any one of the other centres and that of the reference centre exceeds a threshold value of not more than 90 degrees for a predetermined time and in response to the alarm signal the control signal of the local oscillator of each of the other centres becomes dependent on the output of one only of the comparator means of the particular centre until the condition is removed. By this means operation of the system in an out-of-phase mode in which there is a relative phase shift of 360 degrees or a multiple thereof between centres is avoided. The various alarm and control signals may be transmitted in digital form in selected time slots or may be transmitted in analogue form over A.C. paths in in known manner. General description, Fig. 1.-The system includes main or primary transmission centres MA, M1, M2 of which MA is the reference centre, which may be interconnected as shown. Each main centre is parent to a number of directly connected district centres D11, D12, DA1, DA2, D21 &c. which in turn are parent to directly connected group centres G11, G21, G22 &c., to which local transmission centres L, operating on a slave basis, are connected. Direct connections may exist between main centres, district centres and group centres which each include a local timing oscillator and synchronizing apparatus, so that a number of closed loops are formed which could cause the system to operate in an out-of-phase mode as referred to above. Phase error detecting apparatus. Fig. 2.-Considering the main centre M1, a phase comparator PM1 at this centre compares the arrival times of incoming digits from the reference centre MA with the digit timing determined by its own local oscillator to provide a signal 9 M indicating the phase error of its oscillator relative to the reference oscillator. A phase comparator PD1 at the district centre D12 compares incoming digits from M1 with its own locally generated digits to provide a phase error signal #D-#M. The centre D12 also includes phase comparators such as PD2 for comparing incoming digits from its satellite group centres G21 &c. to produce phase difference signal #D-#GThe reference centre MA is connected directly to its district centre DA1 which has a phase comparator PDA operating similarly to PD1 to produce phase error signal #DA and a phase comparator PDB corresponding to PD2 producing error signal #DA-#GA. Alarm system.-The phase error signal #M generated at the main centre M1 is supplied to phase comparators CMP1, CMN1, Fig. 3, which receive second inputs representing +90 and -90 degree phase errors respectively. When the output of comparator CMP1 is positive, i.e. #M is greater than 90 degrees an alarm ALPI is operated and similarly an alarm ALN1 is operated when the output of CMN1 indicates that #M is less than -90 degrees. The district centre D12 receives from M1 the error signal #M which is combined at ADD with the error signal #D-#M from comparator PD1 to provide an error signal #D referred to the phase of the reference oscillator at MA. Signal #D is supplied to phase comparators CMP2, CMN2 which also receive inputs corresponding to phase errors of +90 and -90 degree phase errors respectively, the alarm ALP2 being operated when CMP2 provides an output showing an error exceeding 90 degrees and alarm ALN2 being operated when the output of CMN2 indicates a phase error less than -90 degrees. The output of CMP2 and CMN2 are also supplied to phase comparators CMP3 and CMN3 respectively each of which receives the signal #D-#G from the comparator PD2, Fig. 2. When the output of the comparators CMP3, CMN3 is positive or negative respectively the alarm ALP2 or ALN2 is operated. When any of the alarm generators is operated a signal is transmitted back to the reference centre MA. The alarm system for centre DA1 operates in a similar manner but is somewhat simpler, Fig. 4 (not shown). When the centre MA receives an alarm signal from any centre in the system it sends a cut off signal to each centre which disconnects the connections providing control of its oscillator with the exception of that from its parent centre thus breaking all the closed loops in the system and allowing the the system to revert to a stable in-phase mode. To avoid the transmission of an alarm signal in response to transient out-of-phase conditions each centre D may include a timer which inhibits transmission of the alarm signal until it has persisted for a predetermined time. When centre M1 receives a cut-off signal it is controlled by the master centre MA only, D12 on receipt of a cut-off signal is controlled by centre MA through centre M1, G21 is controlled by MA through M1 and D12 and similarly for the other centres. System details.-The plurality of incoming error signals at a centre are suitably weighted in inverse proportion to the number of inputs and combined to provide the control signal for the local oscillator, Fig. 5 (not shown), a relay responding to the cut-off signal from the master centre to switch out all but the required one of the inputs and adjusting the weighting to that required for a single input. If the link to the parent centre is not functioning when the cut-off signal is received an alternative parent centre is selected automatically, Fig. 6 (not shown), the alternative centres being arranged in a predetermined order of priority for this purpose. Modified system.-This arrangement Fig. 7 (not shown) operates on the basis that if a centre is connected to the master centre over n links (i.e. via n-1 centres) and that centre is operating in an out-of-phase mode there must be over at least one of those links a phase error of at least 90 degrees/n. In this case the alarm signal is produced if the phase difference between the oscillators of any two adjacent centres of the chain exceeds 90 degrees/n.