Patent GB-845174-A (GB)
Title: Improvements in or relating to signal transmission systems
IPC: H04J3/17
Filed: 1958-09-24
Abstract: 845,174. Telephone transmission systems. WESTERN ELECTRIC CO. Inc. Sept. 24, 1958 [Sept. 26, 1957], No. 30503/58. Class 40 (4). In a system in which a plurality n of talkers utilizes a smaller number c of transmission channels, and connections are effected at the transmitting end via a time division multiplex highway to which the transmission channels are connected in a regular repetitive cycle, a register is provided for queueing the identities of talkers as they become active, and feeding them into idle time slots of a memory circulating in synchronism with the said cycle, these identities then being used to gate the talkers on to the multiplex highway at the appropriate instants. The circulating memory 309 comprises c time slots each of which is associated with a transmission channel. Each time slot has three portions, the middle portion accommodating a seven-digit binary number indicative of a talker assigned to the channel, a left-hand portion indicative of the state of the channel, and a right-hand portion used for timing the transmission of a connection-establishing signal. The left-hand portion contains the combinations 00, 01, 10, 11 according as the channel is idle, in the process of establishing a connection, in the process of disestablishing a connection, or in a thorough-connected condition capable of transmitting speech. The memory 309 and the distributer 403 circulate at 8000 revs/sec, in synchronism. Commutators 206, 209 each have n segments one for each talker and these are driven in synchronism with a binary counter 302 which emits seven digit binary numbers identifying the positions of the commutators, which positions are in turn respectively associated with the n talkers. Connection of newly active talker.-Each talker is provided with a speech detector such as 201 which emits an output either on lead 204 or lead 205 according as the talker is active or not. This output feeds a corresponding segment on the commutator and when this is scanned, switch 304 is opened to pass the identity of the corresponding talker to the queueing register 305. This identity is passed through successive stages of the register, stepping always taking place provided the succeeding stage is empty. If all stages become occupied the enabling input to switch 304 is disconnected at 308. When 00 (corresponding to an idle transmitter channel) is read out of the left-hand portion of the memory 309, translator circuit 310 energizes lead 311 to open switches 315 to effect read out of the talker identity into this time slot at the next instant of the cycle. Simultaneously 00 is replaced by 01 in the left-hand portion of the memory, the 01 being fed in from the permanent source 321. When the 01 is received by translator 310 a resulting output on lead 312 opens switch 322 to pass the talker identity to source 323 which with each revolution of the memory produces a pulse modulated with a frequency code corresponding to the talker identity. This is fed via the multiplex highway 107 to distributer 403 which passes it to the appropriate channel filter FTL1 ... FTLc for demodulation and transmission over the appropriate channel. The duration (10 msec.) of this signal is controlled as follows. A 0.8 kc/s. output from source 325 continuously steps a three-digit binary counter 326 whose output is fed to the righthand portion of memory 309 at the same time as the insertion of the talker identity. At the same time this number is fed to an eight-stage stepping register 327 from which it emerges 10 msec. later. This output is compared with that of the right-hand portion of the memory and when coincidence occurs, comparator 328 emits an output to cause the write-and-erase circuit 330 to substitute 11 for 01 in the lefthand portion of the memory. Removal of 01 terminates the transmitted frequency code. Detection of 11 by the translator 310 opens switch 316 to feed the talker identity to translator 317 which pulses one of n leads associated with the talkers, in this case lead 318. Gates 104 are opened thereby, the upper one gating the talker's speech on to the multiplex highway 107. Connection is completed from the latter over distributer 403 to the corresponding transmission channel. Disconnection of inactive talker.-Talkers normally retain their channels whether they are active or not if there is no other demand for them. Such talkers, and those about to obtain connection, are distinguished by having their control switches such as 210 set to produce an output signal. These signals are fed to a counter 216 which produces an output to close switch 217 if it receives (c-1) or more such output signals. When the switch such as 104 of the next inactive subscriber is closed, the output on lead 205 from his speech detector is fed via switch 217 firstly to reset his control circuit 210, and secondly to cause the write-and-erase circuit 330 to replace the 11 in the memory 309 by 10, which produces an output on lead 313 from translator 310. Generator 405 produces sevendigit binary numbers identifying the transmission channels in synchronism with distributer 403. The output on lead 313 operates switch 406 (provided register 407 is empty) to feed the appropriate channel identity to register 407. At the same time the pulse on 313 starts a 10 msec. timer 408 (consisting of a counter which counts up to 80c) and which holds switch 410 open for this period to transmit the stored channel identity to the source 411 which transmits a corresponding frequency code over disconnect channel 413. This may be a channel, e.g. in the case of a frequency multiplex system on the outer edge of a fiequency band which is inadequate for good speech transmission. At the end of the 10 msecs. the timer emits a signal on lead 414 which clears the register 407 and operates erase circuit 331 which erases all the digits from the appropriate time slot and leaves 00 in the left-hand portion. Receiver.-Distribution of the incoming speech to the appropriate listener channels is again effected over a pulse multiplex highway 135 which receives the transmitted speech via commutator 418 and distributes it to the listener channels via gates such as 136 opened at appropriate instants. Each transmission channel such as 145 is connected to a voice-frequency receiver 501 which on receiving the talker (listener) identity converts it to a seven-digit binary code and sets the register 507 therewith. A further output from the receiver sets control circuit 505 to disconnect the input to the receiver at 506. The register outputs are connected to similarly placed segments on seven commutators 514 . . . 516 rotating in synchronism with commutator 418. Thus as the outputs from register 507 are scanned a pulsed binary indication of the listener's identity is fed to translator 517 which produces a pulse output on a corresponding one of n leads (say 518). This pulse closes gate 136, in synchronism with the connection by commutator 418 to the appropriate transmission channel, to complete the connection over the pulse multiplex highway 135. On receipt of a disconnect signal over channel 417, the receiver 438 converts the frequency coded talker (listener) identity into a seven-digit binary code and at the same time starts a 10 msec. timer 420 which passes the received code to a register 423 for a prescribed period and then clears it. During this period the code is translated at 424 into an output on one of c leads, say 425, which clears the register 507 and resets switch 505. Specification 781,622 is referred to.