Patent GB-1489286-A (GB)
Title: Digital data transmission
IPC: H04M11/06
Filed: 1975-04-23
Abstract: 1489286 Inserting signalling information into multiplex frames WESTERN ELECTRIC CO Inc 23 April 1975 [24 April 1974] 16917/75 Heading H4M In a digital transmission system whereby a two state signalling information (e.g. an on hook/off hook signal in a telephone system) may be simultaneously transmitted with multistate signalling information in a recurring single digit space which recurs in selected frames of a t.d.m. system (e.g. a multi-channel telephone system), the two state information is converted into a unidigit binary representation and the multistate information is converted into a multi-digit binary representation, e.g. a coded 5 bit address, a modulo 2 sum of the unidigit representation with each digit of the multi-digit representation is developed, and each digit of the modulo 2 sum is transmitted in a respective occurrence of the recurring digit space. Telephone message signals on 24 input channels 101 are sampled in turn (at 103) and coded at 104 normally into 8 bit format. The 8 parallel bits from the coder representing each sample are passed to parallel-serial converter 106, where a framing bit is added at 107 and thence to transmission channel 108, such that a frame contains 24 time slots of 8 bits plus 1 framing bit, i.e. 193 bits. Every 6th frame, as determined by the clock 112, the samples are coded into 7 bit code and the eighth bit in each time slot is made available for signalling information, which is inserted into the bit position via gate 105. The signalling information is assembled in an interface 109, one for each channel. Multidigit information providing address information in 2 out of 5 code, is applied on lines 110, while unidigit information, indicating on hook/off hook condition is applied on line 111. Before bit by bit modulo 2 addition of these two signals is carried out, prior to inserting the modulo 2 sums into the signalling positions in the multiplex transmissions, constraints are placed on the multistate and two state information to prevent possible decoding inaccuracies, and transitions in the two state information are inhibited during transmission of a series of multistate words and vice versa. Multistate information in parallel words, preceded and terminated by start and stop words, is applied via lines 110 to a parallel/ serial converter 117 which feeds serial data to the line 126 in response to clock pulses from the clock 112 applied via the gate 125. The gate is controlled from a flip-flop 124 in response to message available signals from the P/S converter, but these may be inhibited by a signal from active lead 122 indicating that a transition is occurring on the line 111. In this case, clock pulses do not pass to the P/S converter and the message words are held in a buffer in the converter. Transitions in the two state signalling information on line 111 may be similarly delayed when the message words are passing to the line 126, since active lead 122 is energized in response to this, and the flip-flop 130 fails to respond to the transition. The outputs from the lines 126 and 131 (output of flip-flop 130) are applied to an exclusive OR gate 132 forming a modulo 2 sum on a bit by bit basis which is gated into the appropriate position in the t.d.m. frame by a gate 133. The active lead 122 is energized when the count from a count to 7 counter 138 is less than 7. This counter is reset in response to a change at the output of flip-flop 130, detected by differentiator 135, and thus, following a transition, the active lead stays energized for 7 clock pulses before becoming de-energized. Similarly, the active lead is energized in response to the appearance of a START word on the line 126 from the converter 117. However since the signalling information on line 126 is coded 2 out of 5, the largest string of zeros which can reach the counter reset is 6, and thus the active lead remains energized as long as multistate information is present. At the receiver Fig. 1B after the received t.d.m. signals have been converted to parallel format at 145, signalling bits from bit position 8 in every sixth frame, are clocked through a gate 155 to an exclusive OR gate 156 and a clocked S-R flip-flop 157. The state of this can only be changed when a counter 159 reaches 5. During quiescent periods the output of this counter is zero. When a transition occurs in the received signalling data a 1<SP>1</SP> appears at the output of the exclusive OR gate 156, and this is applied to the counter 159 via the gate 160. Once a change has occurred, each next successive digit differs from the present condition stored in the flip-flop 157, and thus the exclusive OR gate delivers a succession of '1's which are counted, finally allowing the flip-flop 157 to change state. The output from 156 provides the signalling information and that from the flip-flop 157 provides the two state information. The five count of the counter 159 is designed to allow for the situation where the maximum number of 1's occur together in the received signal (4) so that this in itself will not enable 157 to be reset.