Patent GB-680702-A (GB)
Title: Mixed modulation in pulsed intercommunication systems
Inventor: DELORAINE EDMOND MAURICE
IPC: H04Q11/04
Filed: 1947-12-07
Abstract: 680,702. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Dec. 3, 1948 [Dec. 7, 1947], No. 26922/47. Class 40 (iv). In a telecommunication exchange system in which communication channels forming a group are allotted time positions in a recurring cycle of such time positions, the aggregate of a number of such groups is concentrated in a main artery in a further single recurring cycle by retiming and utilization of a higher pulse repetition frequency for the latter cycle. During this transfer the method of modulation is altered to suit better the transmission characteristics of the new transmission channel, e.g. to obtain a maximum number of channels for a given bandwidth or to minimize cross-talk between adjacent channels. Thus, in the groups, intelligence may be conveyed by amplitude, width or time (displacement) modulation of pulses while in the main artery the intelligence may be conveyed by pulse code modulation or similar types of modulation in which the intelligence is converted into a " stepped " waveform. In one such method waveform samples are converted to the nearest or next lowest of 32 discrete values which are transmitted in a 5- element 2-condition code. In another method, a 3-condition code is used to indicate whether, during the intervals between sampling, the amplitude has increased or decreased by more than a prescribed amount or not. Or, in a similar method a series of fixed amplitude positive or negative pulses are transmitted for integration at the distant end, the said series being also integrated at the near end to determine whether the integrated total is less or greater than the wave sample, a positive or negative pulse being transmitted accordingly. In parts of the system where a stepped waveform may be of advantage, e.g. for elimination of low level noise, conversion to such a form may be made without immediate conversion into code form. Where intelligence of a less precise nature has to be conveyed a less accurate transmission system can be used, e.g. a 2- condition code having less than five elements, and utilizing coarser " steps ". In order to avoid distortion at low levels due to the use of the step system, the waveform may be compressed prior to entering the system and expanded after leaving it. This process may be applied to the audio channels themselves or may be incorporated in the equipment for converting the form into a stepped form and vice versa, the size of the steps being then non- uniform. The change in method of transmission may be used for the purpose of setting up calls over interexchange junctions in an exchange system comprising a number of exchanges in close proximity to each other. The invention is described in detail with reference to a system in which 10 sub-centres A ... J, Fig. 1 (each serving ten lines) feed lines 13 ... 22 respectively, and these in turn are concentrated in one artery 36, the signals in lines 13... 22 taking the form of time modulated pulses and the signals in the artery 36 being carried by a 5-element 2-condition code. A time cycle of 100 ÁS is used, so that each channel occupies 10 ÁS in the time cycle of its appropriate group and 1 ÁS in the time cycle of the artery 36. Each line 13 ... 22 feeds associated equipment 24 ... 25 for retiming and coding the signal and transmitting it to the main artery. The arrivals of the signals at their coder-retimer units are staggered in 1 ÁS steps by suitable delay means so that during a 10 ÁS period all the channel periods appropriate to channels A1 ... J1 commence in succession. Each coder-retimer contains two coder circuits which operate alternately, one receiving an incoming signal and coding it during a period of 10 ÁS, one-tenth of which is used for retransmission of a coded signal from the other (the remaining nine-tenths being used for retransmission of signals from the other coder-retimers). Each coder-retimer contains a frequency divider 28 producing 10 ÁS-spaced pulses from 1 ÁS-spaced pulses incoming from bus-bar 42, the outputs of the various dividers being staggered by 1 ÁS for each group. The output of 28 is passed to a scale-of-two circuit 29 which passes 10 ÁS pulses alternately to gates 26, 27 which thereby admit alternate channel signals. The circuit 29 also feeds a pulse shaper 37 which produces 1 ÁS reference pulses which are fed together with the incoming time modulated pulses to the differentiator 38, the time difference between the resulting pulses being recorded in the coder 32. The output from the pulse shaper then opens the output gate 32 for the succeeding period of 1 ÁS during which successive pulses of <SP>1</SP>/ 5  ÁS duration are fed or not from bus-bars 43 ... 47 to the artery 36 according to the setting of the coder. A pulse derived by differentiating the trailing edge of the output from the pulse shaper 37 zeroizes the coder in preparation for the next signal. Fig. 6 (not shown) shows a one-part coder retimer which may be used where the interval used for the whole of the time modulated signal is less than 8 ÁS in which case the coded signal is transmitted half-way between that signal and the next. In equipment for redistribution of the signals and reconversion from pulse code to time modulation, Fig. 4, a 1 ÁS pulse generator 57 synchronized by master pulses received over the artery 36 feeds frequency dividers 60 whose 1 ÁS output pulses for the various decoderretimers are staggered by 1 ÁS. The decoders are set according to the code received through gates 61 ... 65 opened successively by 1/ 5  ÁS pulses from bus-bars 43 ... 47, the gates being enabled at every tenth 1 ÁS period by the outputs from the dividers 60. During the remaining nine ÁS the decoder retimers translate the code into a time modulated pulse for transmission to the group sub-centres A ... J. Specifications 535,860, 627,462, 639,827, [Group XL (c)], 644,571, 665,819 and 710,071 are referred to.