Patent GB-655473-A (GB)
Title: Improvements in or relating to pulse code signalling systems
IPC: H04B14/04
Filed: 1948-03-12
Abstract: 655,473. Pulse code modulation systems and circuits; pulse generating circuits. WESTERN ELECTRIC CO., Inc. March 12, 1948, No. 7582. Convention date, March 13, 1947. [Classes 40 (v) and 40 (vi)] In a pulse signalling system, a complex wave is sampled at successive instants of time and pulses are produced in a code group having a predetermined number of pulse positions in respect of each sampling of the complex wave transmission being effected by a pulse train, each pulse of which occupies one or other of two instants of time depending upon the presence or absence of a pulse in the code group. Timing pulse generator.-Short positive synchronizing pulses from a synchronizing pulse generator 420 (Fig. 4) are transmitted through a delay line 421 and grounded grid amplifier 422 to the first valve 310 (Fig. 3) of the timing pulse generator. The valve 310 is biassed so as to be non-conducting except on receipt of a synchronizing pulse when the condenser 311 and inductance 312 are set in oscillation. Pulses 819 (Fig. 8) represent the delayed synchronizing pulses applied to the grid of the valve 310 and graph 815 represents the oscillatory discharge of the condenser. The oscillatory wave is applied to valves 314 and 315 employed as limiting and clipping valves which serve to produce a square wave 816 (Fig. 8), which is passed to a valve 318 through condenser 317. The condenser 317 and resistance 319 differentiate the square wave, whilst the valve 318 is biassed so that only positive pulses applied to the grid are produced in the output. Coupling circuit 320, 321 and valve 322 further improve and shorten the pulses, as shown at 817 (Fig. 8). The output of the valve 322 is applied to the control grids of two output valves 325 and 326 which produce positive amplified pulses for application to the exponential step wave generator and coding circuits. Exponential step wave generator.-A valve 356 is normally biassed so that it is non- conducting except on the application of a delayed synchronizing pulse from the amplifier 422. During the application of such a pulse valve 356 will conduct, thereby allowing a condenser 355 connected in the cathode circuit of a valve 354 to discharge. At the same time the synchronizing pulse is applied to the grid of a valve 350 to cause it to conduct, thereby discharging a condenser 351. After the cessation of the synchronizing pulse the condenser 351 then starts to charge up through a resistance 361 exponentially as shown in curve 820 (Fig. 8). The condenser 351 is connected to the control grid of the valve 354 which is biassed so that at this time no current is passed, and the potential of the condenser 355 remains at a constant low valve 821 (Fig. 8). The timing pulses from the valve 325 are applied to the grid of a valve 352 which is such that it will conduct only on the application of a timing pulse. When the first timing pulse arrives therefore, the valve 352 passes a high current and produces a relatively high voltage drop across a resistance 353 in series with the condenser 351. This voltage is added to the instantaneous voltage of the condenser 351 and the sum of these voltages is sufficient to cause the valve 354 to conduct so that the voltage of condenser 355 rises also to the sum of the two voltages. Upon the termination of the timing pulse the valve 352 ceases to conduct so that condenser 351 returns to its previous potential, and the valve 354 will also cease to conduct. The potential of the condenser 355 will remain at its new valve 824 (Fig. 8) until the arrival of the next timing pulse, while the condenser 351 will again follow the exponential curve 820 (Fig. 8). Thus the application of each timing pulse causes the charge on the condenser 355 to be increased by an amount equal to the increased potential of the condenser 351 due to the exponential charging circuit at the time of each pulse application. The waveform derived from condenser 355 for application to the grid of a valve 357 is therefore an exponential step wave. After the application of all the timing pulses, in this case five, to the grid of the valve 352, the next synchronizing pulse will be applied to the grids of valves 350 and 356 causing them to discharge the condensers 351 and 355 so that the cycle may be repeated. The exponential step wave output is taken from the anode of the valve 357 in a decreasing sense 830, 831 (Fig. 8) and from the cathode of valve 357 through a further valve 358 in an increasing sense for application to the coding circuits. Sampling.-Speech currents or other complex waveforms from a microphone 410 (Fig. 4) are transmitted through terminal equipment 411 to the control grid of a valve 430, which is such that it will only conduct on the application of a synchronizing pulse from the amplifier 422 to the suppressor grid. The output of the valve 430 thus consists of pulses representing the amplitude of the speech wave at the time of application of the synchronizing pulses. These pulses are applied through a transformer 431 to the grid of a valve 432, which is such that it will only conduct upon reception of one of these pulses. The step output of the valve 358 is applied to the cathode circuit of the valve 432, but is, at the time of application of a speech pulse to the grid of valve 432, at a minimum value so that a condenser 433 in the cathode circuit of the valve 432 receives a charge which is a function of the amplitude of the speech wave at this time. Graph 860 (Fig. 8) represents the potential of the upper terminal of condenser 433 and section 861 (Fig. 8) the potential of the upper terminal immediately after a particular speech pulse has been received by valve 432. Coding.-The potential of the upper terminal of condenser 433 remains constant at level 861 until the step waveform advances to the next step 824 (Fig. 8) when the potential of the condenser 433 is caused to increase abruptly positively, the potential of the step waveform being added to the potential on the condenser due to the speech pulse. The sum of these potentials is applied to the control grid of valve 437. Valves 437 and 438 operate as limiting and clipping valves inasmuch as a positive or negative potential of constant amplitude is applied to valve 440 according as the potential on the grid of valve 437 due to potential on the condenser 433 is above or below a predetermined level 850 (Fig. 8). This potential is repeated through valves 440, 441, 442 and 510 (Fig. 5). The output of valve 510 is coupled through a differentiating network 511, 512 to the grid of a valve 513 which is biassed so as to repeat only positive pulses. The negative output of the valve 513 is coupled to the grids of valves 514 and 515 which repeat the differentiated pulses. Assuming, therefore, that the potential of condenser 433 due to the speech pulse and first step of the step wave exceeded the valve 850, a negative pulse 871 (Fig. 8), is derived from the output of valve 514 which is applied through a delay device 520 to the grid of valves 443 and 530. The delay interval may be of the order of threeeighths of a timing pulse interval. After amplification in the valve 443 the delayed pulse 881 (Fig. 8), is applied to the control grid of a valve 435 in a positive sense. Valve 435 will only conduct during the application of one of these positive pulses. Returning to the condenser 433, the potential of which was assumed to be above the level 850 (Fig. 8), e.g. at a level. 840, at the time of the consequent delayed positive pulse on the control grid of valve 435, the screen grid of this valve, being fed from the valve 357, will be at a potential represented by the first step 831 (Fig. 8) of the inverted step waveform. This value is such that sufficient current flows through the valve 435 to discharge condenser 433 by an amount equal to half the total maximum charge that may be stored on the condenser in response to the amplitude of the complex or speech wave at the sampling instant of time. Thus the pulse transmitted due to the charge of the condenser 433 being above the level 850 represents half the total possible magnitude of the sample. The potential of the upper terminal of condenser 433 and therefore that applied to the grid of valve 437 is thereby reduced below the level 850, and remains at this reduced value until the second step of the step waveform is applied from valve 358. In the example assumed, the second step is insufficient to cause the condenser 433 potential to rise above the level 850 so that no pulse will be transmitted and none will arrive back on the control grid of valve 435 to discharge the condenser 433. The potential of the upper terminal of the condenser 433 therefore remains at this new higher level slightly below the level 850. The third step of the step waveform, however, causes the potential (843, Fig. 8) to rise again above the level 850 so that a pulse 882 (Fig. 8) will be applied to the control of valve 435. The screen of this valve now has a potential corresponding to the third step of the step waveform applied to it, so that sufficient current flows to remove a charge represented by one-eighth of the maximum possible charge on condenser 433 due to the speech waveform. As shown in Fig. 8, the fourth step of the step waveform again produces a pulse 873 and therefore a delayed pulse 883 to discharge the condenser 433 by a sixteenth of the maximum charge whilst the total potential on the condenser 433 due to the fifth step of the stepped waveform is insufficient to cause a pulse to be generated. After the fifth step a further sample is obtained and the above cycle repeated. In the embodiment described therefore each sample is represented by the presence or absence of any combination of five pulses, and for the particular sample taken three pulses are derived as shown at 870 (Fig. 8) for application to grid of valve 515. Translating to a time-modulated code.-The derived pulses are translated into five