Patent GB-697782-A (GB)
Title: Improvements in or relating to systems for the transmission of telegraphic signals
Inventor: CARTER ROBERT OWEN
IPC: H04L5/26
Filed: 1949-09-14
Abstract: 697,782. Electric selective signalling. POSTMASTER-GENERAL. Oct. 4, 1951 [Sept. 14, 1949], No. 23732/49. Class 40 (i) [Also in Groups XL(b) and XL(c)] High speed telegraph signals, i.e., signals which can have either of two magnitudes but no other, are transmitted by the conjoint use of a plurality of low speed channels each of which alone has a band width insufficient to accommodate the high speed signals. The transmitting terminal includes apparatus for determining the signalling conditions to be applied to the low speed channels so that a change in the condition of the high speed signal produces a change in the signalling condition of one only of the channels, changes in the signalling conditions of any one of the channels occurring at a speed within the normal operating speed of the channel, and the receiving terminal includes apparatus for determining the transmitted signal in accordance with the conditions of all the channels. Considering, for example, the case of two D.C. channels A, B, the signalling proceeds as follows for a succession of mark (M) and space (S) signals on the high speed channels: High speed channel M, A channel +, B channel +; high speed channel S, A channel +, B channel -; high speed channel M, A channel -, B channel -; high speed channel S, A channel -, B channel +; and so on. The changes on channels A and B occur at half speed of the changes on the high speed channel and may therefore be within the channel capabilities. In general, systems according to the invention may operate at speeds up to n times the operating speed of one channel where n is the number of channels. The low speed channels may employ D.C., voice-frequency or radio signalling. Transmitting terminal. Fig. 1 illustrates a system employing two low speed channels. The high speed signals are received in double-current form at Tl and are applied directly to the suppressors of valves V2 and V4, through a phase reversing stage V5 to the suppressors of valves V1 and V3, and through condensers Cl, C2 to a scale-of-two counter V6, V7, which responds to negative-going signals and hence triggers only on the transitions between space (+) and mark (-). The counter controls the bias on the control grids of valves V1-V4. Assuming mark (-) on terminal Tl and the counter to be set with V6 conducting, only valve V1 of the group V1-V4 conducts. On the transition to space (+) the counter remains unchanged and V2 conducts, VI cutting off. On the next transition to mark the counter reverses and V3 conducts, V2 cutting off. On the next transition to space, V4 conducts and V3 cuts off. The operation then repeats, valves V1-V4 conducting in turn for successive changes in the high speed signal. The anodes.of valves V1-V4 are connected through rectifiers to control relays AR, BR applying signals to the two low speed channels as shown in Fig. 2. The two halves of each winding are connected in series aiding so that a current flowing upwards in either half operates the relay to space and a current flowing downwards to mark. When valve V1 conducts, current flows downwards in the lower halves of both relays and space condition appears on both channels. In a similar manner, with valves V2, V3 and V4 conducting the appropriate combination of conditions is set up on the two channels. In an alternative arrangement, Fig. 3 (not shown), valves V1-V4 are connected to the channel relays in a bridge circuit. This arrangement is suitable where relays AR and BR take the form of static relays as described for example in Specification 448,927, [Group XL]. The thermionic valves illustrated may be replaced by hot or cold cathode gas filled valves, or electro-magnetic relays. An alternative transmitting terminal may employ a scale-of-four counter responsive to every change in the high speed signals to determine the low speed channel conditions. Receiving terminal, Fig. 4a. The normal receiving relays in channels A and B deliver doublecurrent signals to the input terminals T2, T3. The relay for regenerating the high speed signals has two windings CR1, CR2. When the signals applied to the input terminals are of the same polarity no current flows through winding CR2, but current flows through either the top or bottom half of winding CR1 as determined by rectifiers W9 and W10. In each case the relay operates to mark When the signals are of opposite polarity no current flows through winding CR1 since the junction of resistors 7 and 8 is at earth potential, but a current flows through either the top or bottom half of winding CR2 as determined by rectifiers W11 and W12 to operate the relay to space. An alternative circuit, Fig. 4b (not shown), is arranged to reduce the slowing down of operation produced by the shunting effect of the winding which, at any particular instant, is not receiving current. Unless the transmission times of the two channels are equal a marking bias is produced in the regenerated high speed signal and delay means in the form of networks or chains of trigger stages are inserted in the channels to equalise the delays. Regenerative repeaters may be included at the receiving end of each channel, the part of the regenerators controlling the instants of selection being common to the two channels. Signalling by alternating currents. The low speed channels may employ voice frequency signalling with tone and no tone to represent the two conditions. In this application, a circuit, Fig. 5, is incorporated in the transmitting terminal to prevent the system being left during a long idle period with the condition of no tone on both channels. The circuit alters the condition of both channels from no tone to tone, but since both these conditions represent mark there is no effect on the high speed signals. Where the receiving terminal employs automatic gain control this arrangement prevents breaks in transmission comparable with the time-constant of the control system from rendering the control system ineffective, whereby the first signal or signals occurring after the break are distorted. The circuit includes a chain of trigger stages V8, V9; V12, V13; V14, V15, shown incompletely, connected together by delay means in the form of pulse trigger stages V10, V11. 1. The input to the chain Q1, Q2 is connected to P1, P3 in the circuit of figure 1 so as to receive the high speed signals, and the output Q3, Q4 is connected to P2, P4, the connections between P1, P2 and P3, P4 being broken. The delays due to pulse triggers V10, V11 are such that a condition applied to one trigger stage is applied to the next after an interval less than the duration of a unit element on the high speed channel, and the number of triggers in the chain is such that the time delay between the application of a condition to the first stage and its transference to the last is not less than the duration at a unit element on the low speed channels. An auxiliary control circuit V17-V20 is employed to detect the condition of no tone on both channels for which purpose the control grid and suppressor of V17 are connected respectively to the control grid and suppressor of. the particular valve V3 in the group VI-V4 which conducts during this condition. Valve 17 accordingly becomes conducting with valve V3 and when this occurs a negative pulse is applied to valve V18 which, together with valve V19, forms a pulse trigger having a delay period at least equal to the duration of a unit element on the low speed channel. At the end of this delay period a positive pulse is applied from the screen of valve V19 to the suppressor of valve V16. Valve V16 is normally non-conducting and also receives its grid potential from a network R12-R17 connected between the negative supply and the anodes of the valves in the trigger chain which are non- conducting when the triggers are in the mark condition. Only when all the triggers are in this condition is the bias on V16 grid raised so that the valve conducts on the positive pulse from V19. The resulting negative anode pulse is applied to the suppressor of valve V6 (Fig. 1) causing the scale-of-two counter V6, V7 to reverse and allow valve VI to conduct in place of V3. This produces the condition of tone on both low speed channels. The change occurs only after the no tone condition has persisted for a time equal to the delay imposed by circuit V18, V19 and then only provided all the triggers are at mark. This latter condition ensures that no further change of condition due to the high speed signals will be applied to the low speed channels until a time at least equal to the total trigger chain delay has elapsed so preventing even momentary increases in signalling speed. Valve V20 resets trigger circuit V18, V19 if valve V17 ceases to conduit before the end of the delay period. The pulse trigger stages V10, V11 in the trigger chain may be replaced by delay lines.