Patent GB-596658-A (GB)
Title: Improvements in or relating to pulse modulation communication systems
IPC: H04J3/00
Filed: 1945-03-01
Abstract: 596,658. Multiplex pulse signalling. STANDARD TELEPHONES & CABLES, Ltd., CHATTERJEA, P. K., SCULLY, C. T., AMBROSE, D. M., and BENEY, J. K. March 1, 1945, No. 5185. [Class 40 (v)] A time-phase modulated pulse communication system, which may comprise a plurality of intercalated channels, employs a train of synchronizing signals, each consisting. of a group of three or more pulses of the same form and amplitude as the modulated pulses. Generating channel pulses. In one embodiment, a sawtooth wave generator 1, Fig. 1, channel pulse generator 2 and synchronizing signal generator 3 are provided for one channel and channel pulse generators 4, 5, 6 identical with 2 for the other channels. The saw-tooth generator comprises condenser 11 charged through resistance 13 and discharged by gas-filled valve 10 at a periodicity selected by the cathode biassing potentiometer 14. The cathode is also connected to earth through condenser 16, resonant circuit 17 and resistance 18. The rising positive saw-tooth voltage from the anode of valve 10, fed through condenser 19 and potential divider 20, 21 to the grid of gas-filled valve 23 triggers the latter at a time determined primarily by the setting of its cathode biassing potentiometer 25 and also modulated by the channel signal applied to its grid from terminal 35. The valve is rendered non-conducting again on occurrence of the flyback stroke of the saw-tooth voltage. The connection from the tuned circuit 17 through condenser 31 superposes on the flyback voltage a very short damped train of oscillations which ensures the extinction and/or firing of the valve 23 for extreme adjustments of the cathode bias as described in Specification 596,638. The negative rectangular pulses produced at the anode of valve 23 are -applied to a differentiating circuit 32, 33 and the resulting short negative pulses corresponding to the leading edges of the applied pulses are fed through diode 34 to the output potential divider 7, to form the time-phase modulated signal pulses. Generating synchronizing signals. The synchronizing signal generator comprises a hard pentode valve 40 having its anode connected to the control grid through a condenser 41. The anode, screen grid and control grid are connected to H.T. + through resistances 42, 43, 44 respectively, the latter being variable. The suppressor grid has an adjustable negative biassing arrangement 47 ... 50 and the cathode circuit includes a resonant circuit 46. The anode current is normally cut off by suppressor grid bias, but control grid current flows. On the occurrence of the saw-tooth flyback, a very short positive pulse derived from the cathode resistance 18 of valve 10 is applied to the suppressor grid of valve 40, anode current being suddenly established to cause a fall in anode potential. The latter charges condenser 41 and biasses the control grid to anode-current cut-off. The resonant circuit 46 is excited by the change in anode current and produces a few complete oscillations until the discharge of condenser 41 again allows cathode current to flow and damp the tuned circuit. The number of oscillations executed can be controlled by the setting of variable resistance 44 and in the system described is made three, which, after clipping by diode 53, pass to the output circuit 7 to form the synchronizing signal pulses. The channel and synchronizing pulses, before transmission are passed to a pentode amplifier and gas-filled triode pulse shaper, Fig. 2 (not shown). Calling. Each channel pulse generator is provided with contacts 80, Fig. 1, for suppressing the channel pulses for calling purposes, by the application of a prohibitive negative bias to the grid of valve 23. Pulse demodulator and channel selector. At the receiver, channel pulses 76 ... 79, Fig. 7, and synchronizing pulse groups 74, 76 are applied to a gasfilled triode shaper 89, Fig. 4, and thence to the first valve 98 of a synchronizing signal selector circuit. This comprises three valves 98, 99, 100 coupled by circuits 101, 102, tuned to the recurrence frequency of the three pulses forming each synchronizing group. Each pulse produces two damped cycles of oscillation which reinforce each other at onecycle intervals so that when the correct three - pulse signal is received, valve 98 passes on to valve 99 after clipping in the grid circuit, two pulses, while valve 99 passes on to valve 100, in a similar way, a single pulse. This pulse is inverted by the inductance 115 to become a positive synchronizing pulse, 137, 138, Fig. 7, at the point 118. Gas-filled triodes 119, 120 are connected as saw-tooth generators with a common charging capacity formed by condensers 121,122,123, and produce waves K, L, Fig. 7, respectively synchronized by the pulses from point 118. The combined rectangular stepped waveform M., Fig. 7, appears across condenser 123 and resistance 128. Duration modulated signal pulses N, Fig. 7, are formed from the phase modulated received channel pulses by charging condenser 146 through diode 147 at each flyback of the saw-tooth generator 119 and discharging it through diode 153 on the occurrence of a channel pulse in the load circuit of shaper valve 89. The last small flyback stroke of the synchronizing cycle from generator 119 is reinforced by applying the synchronizing pulse at point 118 to condenser 146 through diode 150. The stepped wave M in resistance 128 and the duration-modulated pulses N in resistance 162 are passed through cathode-follower valves 163, 164 respectively to a mixer resistance 167 where they combine to form wave P, Fig. 7, which is a stepped waveform in which the step durations are modulated, the four channels still being unseparated. An amplifier and inverter tetrode 171 passes this inverted waveform Q, Fig. 8, to the channel separator diodes 177, 178, 185 ... 188, Fig. 5. The oppositely directed pair 177, 178 are biassed by potentiometer 182 so that 177 passes the upper three steps Sand 178 passes the lower two steps R. The other pairs of diodes, in a similar manner, separate the individual steps to pass the outputs U, T, W, V to the demodulators 208, 209, 120, 211 respectively. Each demodulator comprises a low-pass filter 212, having a cut-off frequency above the highest frequency of importance in the modulating signals, and an output valve 214 and transformer 215. Reception of calling signal. The outputs from the separator diodes 185 ... 188 are also applied through a condenser 223, for example, to the grid of a gas-filled triode 222 whose anode is connected to a source 227 of A.C. voltage. When channel pulses are being received, the time-constant of grid circuit 224, 225 is long enough to prevent the firing of valve 222. When the channel pulses are suppressed for calling, the resulting shortening of the rectangular pulse such as V applied to the grid, causes a positive peak of sufficient amplitude to fire valve 222 and operate a ringing relay 226. A second embodiment of the invention is described which also uses a triple-pulse synchronizing signal, but in the circuits of which no gas-filled valves are used. Generating channel pulses. A single sawtooth generator 233, Fig. 9, is associated with individual channel pulse generators 234 ... 237. The saw-tooth generator comprises condenser 247 charged through resistance 243 and discharged through the left-hand portion of the double triode valve 239 which is connected as a flip-flop multivibrator triggered off by the rise in anode potential of the left half as the condenser 247 charges to a critical potential. At the right-hand anode, and left-hand grid, a short rectangular positive-voltage pulse occurs during the flyback period which is fed over condenser 298 to the synchronizing signal generator, Fig. 10. The saw-tooth voltage is fed to the grid of the left half of a double triode valve 248, the grid bias voltages of which are stabilized by a neon tube 254, and which is connected as a multivibrator with two conditions of stability as described in Specification 596,654. The rising saw-tooth voltage upon reaching a critical value, triggers the circuit so that the left half becomes conducting and the flyback voltage restores it to the other condition in which the right half is conducting. The resulting positive rectangular pulses at the right-hand anode are differentiated in the circuit 270, 271 and the short leading-edge positive pulse is passed through diode 272 to the output circuit 273. When the valve 248 is adjusted so that the left half conducts very soon after the flyback stroke, in order that the latter may be of sufficient amplitude to cut off the valve, it is reinforced by a negative pulse from an auxiliary amplifier 287 fed from the right-hand anode of valve 239 through a differentiating circuit 292, 293. For a four channel system it is generally only necessary to provide valve 287 in one of the channels. Modulation of the channel pulses, in phase, is produced by the application of signal voltages to the left-hand grid of valve 248 from transformer 277. Calling. Each channel pulse generator is provided with contacts 251, Fig. 9, for suppressing the channel pulses for calling purposes, by the application of a large positive bias to the right-hand grid of valve 248. Generating synchronizing signals. A pentode oscillator 302, Fig. 10, is unblocked by the positive pulse from condenser 298, Fig. 9, by way of terminals 297, 301, for a time corresponding to the generation of three complete oscillations. The negative tips of these waves are passed on by the biassed diode 309 to the load resistance 317 of a pentode 312, to the control grid of which are applied the channel pulses from terminal 300. The mixed pulses are applied to the suppressor grid of a pentode 322 which produces in its anode circuit positive pulses of constant amplitude and shape. These are inverted by valve 336 and passed to a power tetrode output valve 335. Pulse demodulator and cha