Patent GB-1102020-A (GB)
Title: Improvements relating to digital telephone systems
Inventor: HEATON PETER
IPC: H04M3/56
Filed: 1965-08-26
Abstract: 1,102,020. Conference circuits. FERRANTI Ltd., P. HEATON, and N. WEST. 24 Aug., 1966 [26 Aug., 1965], No. 36649/65. Heading H4K. In a conference circuit for a telephone system in which the speech is digitally coded a detector responds to a code signal representing speech signals above a predetermined level on one input channel to inhibit signal inputs on the other channels, and feed the detected signal into the other channels. If the detected signals cease for a predetermined time the inhibition is removed from the other channels so that another subscriber may capture the circuit. One subscriber may be nominated chairman and be permitted to break in on any other subscriber while the remaining subscribers are prevented by the inhibition applied to their respective channels. In the embodiments the speech is coded in either delta modulation or six digit p.c.m. Figs. la and 1b show a conference circuit for subscribers 1 to 5 whose speech signals are coded by delta modulation. A speech signal on any one of the input lines 1 to 5 is detected by the respective code detector D1 to D5, each of which comprises simply an integrator of predetermined charge and discharge time constants, to operate the respective trigger circuit T1 to T5 to its O state for the duration of the speech signal input. Operation of a trigger circuit to its O state causes the closing of the gates 12 to 15 on the inputs of all the other code detectors other than that of the conference chairman D1 and that one of the other detectors which has detected the signal input, and whose input gate control circuit is therefore inhibited by the closing of the respective one of the gates G2 to G5 by the trigger circuit T2 to T5 associated therewith. Closing of the gates 12 to 15 prevents any subscriber, except the chairman, from breaking in. Fig. lb shows the interconnections of the speech paths, by which the signal from the subscribers A to E are combined in a buffer B6 and routed through gates G7 to G11 back to the subscriber's lines 1<SP>1</SP> to 5<SP>1</SP>. Connections U to Y from the trigger circuits T1 to T5 to the gates G7 to G11 ensure that the gate associated with the subscriber who has captured the conference circuit is closed to prevent feedback and an excessive sidetone level. Conference circuit for a t.d.m. exchange using p.c.m. of the speech signals, Fig. 4, shows a part of an exchange, including two incoming truck links L1 and L2, each link carrying eight channels, each channel coding speech in groups of six digits. It is assumed that subscribers in channels SP1 and SP7 in link L1 and SP1 and SP7 in link L2 wish to engage in a conference, they are therefore connected through the cross office 40 to channels SP3, SP4, SP5 and SP8 of link L<SP>1</SP>1, these channels being reserved for conferences, and with the subscriber on SP3 being nominated chairman. The channels in link L<SP>1</SP>1 are connected to a traffic store S5 and to the conference switch 4 which is shown in detail in Fig. 5a to 5d. Incoming signals on L<SP>1</SP>1 are fed via a shift register 50 to a six-input coincidence gate 52 together with clock pulses CP and a pulse D5 which occurs toward the end of the time slot of each channel so that gate 52 will open only if the first four-digit position in a channel are occupied by 1's. If the gate 52 is opened, indicating that the respective channel is carrying a large amplitude signal, the signal is passed by the appropriate gate 53 to 54, operated by the channel pulses SP3, SP4, SP5 and SP8, to set a respective bi-stable, 155, 157, 158, 159 to its " 1 " state. The bi-stable is reset one every 5 milliseconds by a pulse on R2 and its " 0 " state is coupled to a respective one of the two input " and " gates 60 to 63 which receives a pulse on R1 just before the pulse on R2. A signal is therefore passed through the respective gate to reset the respective counter 64 to 67 only if the bi-stable has not been operated during the time interval between the resetting of R2 and the resetting of R1, i.e. a time of nearly 5 millisecs. Counters 64 to 67 are fed with counting pulses at 50 p.p.s. and if the counter reaches a count of 11, (binary), without being reset, indicating that a large amplitude signal has persisted by 20 milliseconds and therefore a high probability that the signal is speech, the respective gate 69 to 72 is operated to turn the respective bistable 73 to 76 to its " 1 " state. Gates 77 to 80 convert the state of bi-stables 73 to 76 to a signal in the appropriate channel time, which signal on the respective one of lines 3<SP>1</SP>, 4<SP>1</SP>, 5<SP>1</SP> or 8<SP>1</SP> is fed through a buffer 81 to the gate 54 to reset a counter 92 and a bi-stable 156 which, if the counter is not reset, inhibits the application of further inputs through the gates 55 to 57 preventing a subscriber on one of these inputs from breaking in to the circuit until the speech signal on the input of gate 54 has ceased for a predetermined minimum period, e.g. 20 milliseconds. Operation of one of the bi-stables 73 to 76 causes, via the buffers 82 and 86, the setting of the remaining bi-stables to their " 0" condition and the outputs from the gates 77 to 80 are also applied via gates 88 to 91 to the computer 42 in Fig. 4 to establish the talking connection from the subscriber whose speech has been detected to the remaining subscribers via the cross office from the traffic store S5 to the traffic stores S1 to S4 and hence all the subscribers engaged in the conversation except that subscriber actually talking. The omission of the link from bi-stable 156 to the gate 53 provides the subscriber on channel SP3 with breaking in facilities as chairman.