Patent GB-892399-A (GB)
Title: Improvements in or relating to telecommunication systems
Inventor: WRIGHT ESMOND PHILIP GOODWIN
IPC: H04M11/06
Filed: 1960-09-23
Abstract: 892,399. Multiplex pulse code signalling. STANDARD TELEPHONES & CABLES Ltd. Sept. 23, 1960, No. 32785/60. Class 40(5). A telecommunication terminal apparatus for transmitting over a plurality of channels intelligence signals originating in a group of sources designated as having high-priority and in a source or group of sources designated as having low-priority includes means for allocating a high-priority source to a channel outlet and is arranged so that when the intelligence content, represented by a predetermined criterion, offered by the high-priority source for transmission at the outlet falls below a critical level, the outlet is made available for connection to a low-priority source until such time as the intelligence offered for transmission by the allocated high-priority source again exceeds the critical level. In the embodiment described the high-priority sources are the subscriber's lines in a telephone service, the voice frequency signals being amplitude sampled and coded for transmission. The code group corresponding to amplitudes below the lowest sampling level is then used to indicate the availability of the channel for the transmission of low-priority signals which may be, for example, digitally coded data. As shown, in a time division multiplex system three switching centres A, B, C connected by lines 10, 11 are provided with synchronized cycling distributers 12a to 12d connecting the line terminations to the terminal apparatus of each channel in turn. The audio-frequency signals from a transmitting subscriber's apparatus 13 are supplied via exchange switching circuits represented by selector 14 to a coder 15 where they are sampled and converted into a pulse code, the duration of each pulse group being such that it can be transmitted via the line 10 within the time interval appropriate to a single channel and the group repetition frequency being the same as the cycling frequency of distributers 12a, 12b. Further switching circuits 16 select the route A to B and a selector 17 selects a vacant channel on the line 10, the signals being passed to the appropriate connection on distributer 12a via a gate 18. At B the pulse groups are passed without decoding to the line 11 via a channel selector 19. Signals received at C are routed by local switching circuits 20 to the decoder 21 and the decoded signals are passed via further switching circuits represented by selector 22 to the receiving subscriber's apparatus 23. This conventional arrangement is modified in that data for transmission from apparatus 30 over line 10 is passed via local switching circuits 31 to a main terminal store 32 and is routed via a switch 33 to a line store 34. The output of the store 34 through a cycling distributer 35 synchronized with distributers 12a, 12b is offered for transmission over each channel successively, a gate 36 determining whether or not the data offered is accepted for transmission. At B the data signals are routed by selective operation of gates 37, 38 via the cycling distributer 39 and the assembled data is stored at 40. Data to be transmitted to a subscriber at B is fed out over line 41. Data for transmission over line 11 is passed to line store 42, transmitted as described previously to centre C, assembled in store 43, and extracted and passed over the local line to the receiving apparatus 44. The first digit of each code group is used to distinguish between data and speech signals, data groups always having "0" for the first digit and all outputs from the coder 15 having "1". If the voice frequency amplitude is lower than the lowest sampling level the coder output will be a pulse group representing zero level, e.g. 100 which is detected at 45 to close gate 18 to the coder output signals and open gate 36 to allow data signals to pass to line 10. A detector unit 45 is provided for each channel. A further detector unit 46 at B and C opens gate 38 in response to all speech code groups and opens gate 37 in response to data signals.