Patent GB-630292-A (GB)
Title: Improvements in or relating to signalling systems
IPC: H03M1/00
Filed: 1947-05-02
Abstract: 630,292. Transmission systems. WESTERN ELECTRIC CO., Inc. May 2, 1947, No. 11873. Convention date, May 10, 1945. Addition to 630,094. [Class 40 (iv)] [Also in Group XL (c)] In a signalling system, the amplitude of a complex signal wave, preferably after compression, is sampled at predetermined intervals and each sampled amplitude is compared at a number of instants with an electrical quantity varying exponentially with time, pulses being transmitted when, at a comparison, the sample amplitude exceeds said quantity and the sample being reduced simultaneously by an amount depending on the value of the exponentially varying quantity at the time the pulse is transmitted. At the receiver, each series of pulses representing the sampled signal amplitudes is associated with a locally generated electrical quantity varying exponentially with time and an electrical quantity produced which comprises the amplitude of the exponentially varying quantity at the time of receipt of each pulse of the series, the signal wave being reconstituted from the latter succession of quantities. The whole system is controlled by a master oscillator 410, Fig. 4; with a local oscillator 425 controlled thereby at the transmitter, and a receiver local oscillator is controlled over the coaxial line 480. Control pulse generator, Fig. 4.-Synchronizing pulses of width adjustable by variable condenser 412 are generated by multivibrator 411 controlled by oscillator 425. The pulses are shaped by the limiting amplifier 414 ... 416 and the output valve 417 supplies positive pulses 421 at its cathode and negative pulses 423 at its anode, which are slightly delayed by circuit 422. These pulses correspond respectively to pulses 604, 605, Fig. 6, a pair being produced for each sample 602 of the complex signal wave 601. The recurrence frequency may be 8000 c/s. when telephony is being transmitted. Code element timing circuit, Fig. 3.-The negative pulses 605 are fed to valve 310 and thence to valve 311 to cause the cathode oscillatory circuit 312, 313 to ring to produce the sinusoidal wave-form 606, Fig. 6. This is fed through cathode follower valve 319 to limiting amplifier 320 where the squared waveform 607 is produced and fed to output valve 321. Exponential circuit, Fig. 3.-The delayed synchronizing pulse 605 after inversion in valve 310 is also fed to valve 323, the cathode circuit of which comprises a condenser 325 shunted by resistances 326, 324. The condenser is thereby charged and subsequently discharges exponentially, the time constant of the circuit being chosen to ensure that the condenser potential is halved during each interval between successive waves of wave-form 607. The output valve 331 produces positive and negative exponential wave outputs 609, 608 from its anode and cathode respectively. Compression and sampling circuit, Fig. 3.- The source 370 of signal wave is connected through terminal equipment 371 and transformer 360 to a non-linear element 361 which introduces volume compression, the amount of which can be varied by potentiometer 362 or which may be removed by operation of key 350. The compressed signal wave is fed to the lefthand triode of valve 366, the right-hand triode being fed with the negative synchronizing pulses 605. When a synchronizing pulse is applied, the common anode of valve 366 goes positive by an amount determined by the instantaneous amplitude of the signal wave, and the resultant pulse of anode current in the following tube 367 charges the cathode condenser 376 to a corresponding potential. This potential is repeated by the cathode follower 381, and is the signal sample to be coded. Comparing and coding circuit, Figs. 3 and 4.- The signal sample from valve 381 is applied to valve 384 and the exponential voltage 609 from valve 331 to valve 389. Valves 384, 389 have a common anode resistance 387. If the current flowing in 387 is the sum of current 621, Fig. 6, in valve 384 and 609 in valve 389, during the first coding period the total current will be 623, which rises above a threshold value 630, which causes valve 451 to cut off. The output of valve 451 is fed to limiting amplifiers 452, 454, the anode of the latter being coupled to the suppressor grid of valve 378 and to the control grid of valve 455. When synchronizing pulse 605 is applied to the system a current 621 flows in the valve 384. Immediately thereafter the output 607 of valve 321 changes from a negative to a positive'value 629. The resultant positive pulse applied to the grid of 378 will not cause it to conduct as its suppressor grid is negative due to current 622 being below the threshold  value 630. Condenser 376 which is connected to the anode of valve 378 therefore loses no charge. The timing wave 607 is also applied through a pulse-shaping circuit 468 to the suppressor grid of valve 455. The pulse-shaper includes a differentiating circuit 471, 472, a cathode follower valve 470, an output condenser 474 and network 473 which controls the output pulse length. When the first shaped pulse is applied to valve 455, its control grid is negative so that no pulse is passed to the output valve 460 and transmission line 490. Thereafter the current 622 in resistance 387 increases until at point 623 the output potential 612 of valve 454 rises to a positive value. When the timing wave 607 again changes from negative to positive, positive pulses are applied to valves 378 and 455. The control grid of 455 is now positive so that a pulse is transmitted through valve 460 to the line 490. At the same time the screen of valve 378 has had its potential reduced to half maximum value from valve 331 so that the charge on condenser 376 is reduced to one half. The combined currents of valves 384, 389 then follow curve 611, the output 612 of valve 454 drops to zero and the next coding cycle recommences. During each of the five periods during which pulses 615 ... 619 may be transmitted the signal sample is compared with the exponential voltage and either a pulse is transmitted and the sample voltage reduced or no pulse is transmitted and the sample voltage unchanged. Decoding and receiving circuits, Fig. 5.-At the receiving station a local oscillator 510 controlled over line 480 from the master oscillator is connected to synchronize a control pulse generator 511, 514, ... 517 similar to the control pulse generator at the transmitter. The negative pulse 705, Fig. 7, delayed at 522 and inverted in valve 545, opens up valve 546 to charge condenser 547, which then discharges exponentially through resistance 560, curve 701. This voltage is supplied to decoding circuits through cathode follower valve 551 and switches 548, 549. The received pulses from line 490 are amplified by valve 530 and applied to valve 571 whose screen is fed with the exponential voltage from switch 549. The delayed inverted pulse 705 is also applied to valve 570 to charge condenser 574 to potential 721. Every time a received pulse is applied to valve 571, condenser 574 is discharged by an amount depending on the value of the exponential voltage applied to the screen of the valve, giving a discharge curve 721 ... 724 in the example being considered. The condenser 574 is connected also to the grid of the cathode follower valve 572 whose cathode is connected to the screen of valve 573, and the control grid of 573 is fed with the positive control pulse 704. The magnitude of the resulting output pulse from valve 573 is thus. determined by the final potential 724 of condenser 574 which depends on the code combination received. The complex wave 751 is finally regenerated by low-pass filters 575, 578 and ampifier 576 and passes to receiving instrument 580 through terminal equipment 579. An alternative receiving circuit comprises valve 533 to the control grid of which are fed the received pulses and whose screen is controlled by the exponential voltage from switch 548. The magnitude of the output pulses, e.g. 732, 734, 736 is thus determined by the exponential voltage at the time. These pulses regenerate the complex wave 751 after passing low-pass filters 535, 538 and amplifiers 536, 537 and feed it to the terminal equipment 539 and receiver 540. Monitoring.-One of the receiving equipments described is used as a monitoring set for use by an operator. The monitoring equipment 499 at the transmitter, Fig.4, is similar to the receiving equipment 570 ... 580, Fig.5.