Patent GB-844558-A (GB)
Title: Improvements in or relating to time division multiplex transmission systems
IPC: H04J3/16
Filed: 1958-09-24
Abstract: 844,558. Pulse code modulation systems. WESTERN ELECTRIC CO. Inc. Sept. 24, 1958 [Sept. 26, 1957], No. 30502/58. Class 40(5). In a pulse code time division multiplex system a wide range permutation code pulse group of a given number of digits or a narrow range-group of fewer digits is transmitted respectively according to whether the sampled energy level of the signals from talkers 0, 1, 2, Fig. 10, to be transmitted, are above or below a predetermined energy level measured by detector samplers 317 one of each of such groups being transmitted as a single group for two talkers. The system is an elastic time assignment arrangement i.e. one in which the sampling rate of the channels varies with the number of active channels i.e. channels over which the talker is actually speaking. Speech samples taken by samplers 317 are coded in a seven digit pulse code modulator 326, Fig. 11, and written in positions corresponding to the talkers on the first seven columns of a storage tube 341, Fig. 14, for subsequent read out to the transmission circuit 399, arrangements being made to rewrite into the store the read out code which is erased from the store by the reading process. Delay networks 366 produce from the simultaneous pulses on the read out lines the sequential pulse group required for transmission. Activity data, i.e. whether the channel is active, busy, or idle is stored in the 8th or 9th columns of tube 341 and a hunting arrangement comprising counters 307, 343, 344 is pulsed by the output of oscillator 301 to search the store for active and busy lines and to read in to the store a fresh sample at every fifth cycle, the sequence being 4 active hunt steps-record - 4 active hunt steps-record-4 busy hunt stepsrecord-4 busy hunt steps-record. The transmitted signal comprsises groups of ten pulses, 7 from a fully active speaker, 2 from a busy line not fully active to produce a background during speech intervals, and 1 of the two activity data pulses, the complete activity data being contained in two successive pulse groups. Active and busy groups are transmitted as they are found but if a search cycle is unsuccessful the code in the last store examined during that cycle is transmitted and read into the distant store. The activity data is read in through busy level switch 355 and active entry switch 342 and is transmitted simultaneously with the reading, the active read in circuit including a hangover memory tube wherein the active signal is made to persist for a period after activity ceases. The beam of tube 341 is positioned by analogue decoder 346 in accordance with counter 307. Arrangements are also included to prevent the busy hunt stopping at an active talker by rendering busy stop-gate 367. The receiver Figs. 5, 6, 7 which comprises similar equipment for storing samples and controlling scanning, includes a shift register 262 to convert the received sequence of pulses to a simultaneous code on a number of output lines, and means for recovering by shock excitation the pulse timings for control purposes. Synchronization and framing pulses are transmitted by selection counter 378 triggering a multivibrator 379 which in turn triggers the framing divider 380, the condition of switch 380 being gated to the receiver by framing switch 381. Counter position identification signals are sent in a 10 digit group at 2 millisecond intervals and additional supervisory and checking data may be sent. The telephone audio circuits include a received/transmitted speech differential arrangement for controlling the activity detector to prevent its response to cross talk. Specifications 550,814 and 679,725 are referred to.