Patent GB-637848-A (GB)
Title: Improvements in or relating to signalling systems for the transmission of complex waveforms by means of code groups of pulses
IPC: H03M1/00
Filed: 1948-01-16
Abstract: 637,848. Pulse code modulation systems and circuits. WESTERN ELECTRIC CO., Inc. Jan. 16, 1948, No. 1395. Convention date, Feb. 21, 1946. [Classes 40 (v) and 40 (vi)] [Also in Group XXXIX] The amplitude of a complex signal wave, such as speech, is sampled at predetermined intervals, means being provided capable of producing, for each sample, a series of pulses representing successively greater fractions of a maximum signal wave amplitude and also means for controlling the transmission of those pulses of each series which together represent the sample amplitude. For restoring the original signal wave at a receiver, the energy stored in a circuit is changed by a predetermined amount on the receipt of each pulse of a series and returns to the normal amount at such a rate that, during the time interval between successive possible pulses of the series, it returns to a predetermined fraction of the said change. The remaining energy is then converted into a pulse of amplitude representing the sample from which the pulse series was derived. As described, the sampling process is initiated prior to or during the transmission of a pulse series representing a previously determined sample amplitude. Transmitter. In the embodiment described, the output of a control oscillator 331, Fig. 3, is fed to a frequency changer 332, clipper 333, differentiator 334 and clipper 335 to provide a train of positive pulses which are applied to delay line 346 from tappings on which suitably timed control pulses are taken. These pulses 700 ... 707, Fig. 7, all arise from one applied pulse, the next input pulse giving rise to a new series, the first of which, 800, is shown. Pulse 707, on lead 338, initiates a sampling operation by rendering valve 315 conducting, valve 316 conducting and diodes 313, and subsequently 312, conducting. Thereupon, condenser 307 is charged to a potential depending on the amplitude of the positive wave from microphone 310 applied to the cathode of diode 312. When the pulse ceases, condenser 307 retains this charge, the diode 313 reverting to its non- conducting state. The charge 120 on condenser 307 is then measured by discharging the condenser in equal steps until its potential drops below a critical value 733, the steps being counted on a binary scale and the result stored ready for subsequent transmission as a pulse series. To this end, a multivibrator 329, 330 which may be synchronized, through switch 305, by the control oscillator 331, applies wave 715 to one of the grids 327 of valve 325. To another grid, 328, is applied a positive potential from lead 323 as long as condenser 307 is charged above the level 733. During this period, condenser 308 is charged negatively on every positive excursion of the wave 715, through the left-hand section of valve 326, until the righthand section conducts to discharge condenser 308 to neutralize a small portion of the positive charge on condenser 307. Thus for each multivibrator cycle the charge on condenser 307 is reduced by a small amount, to give the step potential wave 720 ... 723, until the critical potential 733 is intersected and the lead 323 goes negative to cut off valve 325. The lead 323 is the output of a series of three flip-flop pairs of valves 317 ...322 connected as shown, the input grid of 317 being connected to condenser 307. When valves 317, 320, 321 are non-conducting, valves 318, 319, 322 are conducting and vice versa and the arrangement changes over at the critical potential 733. The potential at the lead 323 is shown as 716 before the sampling pulse 707 occurs, rising abruptly at 731 on the occurrence of this pulse and falling abruptly at 732 to cut off valve 325 as described. The counting, storing and pulse transmitting circuits are shown in Fig. 4. The step wave is differentiated and amplified in circuit 413 ... 415 and the resultant negative pulses 725 applied to the suppressor grids of the pair of valves 435, 436 forming the first stage 410 of a five-stage scale-of-two counter of which three stages 410, 411, 412 are shown. These pairs of valves are connected so that only one of the pair can be conducting at any one time and a negative pulse on both suppressor grids renders the conducting valve non-conducting and vice versa. The output wave 726 from the anode of valve 435 operates the next counter stage 411 giving an output wave 727 of half the frequency. The output waves 728 ...730 of the remaining stages are each of half the frequency of the preceding stage and when the counting is complete, the final condition of the stages represents the number of pulses 725 applied to the input. In the example shown, nineteen pulses cause the three stages shown to assume the condition when the lower valves are non- conducting, and the other two stages have their lower valves conducting. The anodes of these valves are connected to the grids of valves 426, 429, 432, &c., respectively. When synchronizing pulse 800 occurring in lead 340 renders valve 416 conducting, those of valves 426, 429, 432, &c. which have negative grids do not conduct and the associated diodes 427, 430, 433, &c. conduct to discharge the relevant condensers 423, 424, 425, &c. The condition of the counter stages is now stored on these condensers. In order to transmit a series of pulses corresponding to those of condensers 423, 424, 425, &c. which remain charged, the five successive pulses 701 ... 705 derived from the delay line 346, Fig. 3, are applied to the grids of triodes 428, 431, 434, &c. respectively, and repeated as negative pulses to the outgoing transmission line 450 in those cases where the storage condensers have been discharged, the condensers becoming recharged thereby. Where the condenser is already charged the pulse is not repeated to the line, so that the series of transmitted pulses represents the condition to which the counting stages have been set and hence the amplitude of the sample of the signal wave originally stored on condenser 307. Receiver.-At the receiver, the first pulses from the transmitter delay line, over channel 340, Fig. 5, or the signal channel 450 control the multivibrator 518 which produces positive pulses shaped by amplifier 519 and fed to a delay line 520. From tappings on this line, gating pulses are fed through isolating diodes 521, 522, 523, &c. to a control grid of valve 511. The signal code pulses from channel 450, after shaping in circuit 510, are applied to another control grid of valve 511. Whenever both grids receive a pulse simultaneously the valve conducts to remove a predetermined quantity of charge from condenser 512 which is in a circuit having a time constant such that its potential changes by one half its initial value during the interval between two successive pulses from the delay line 520. Curves 911 ... 915 and 921 ... 925, Fig. 9, show the discharge and recharge curves respectively of condenser 512 corresponding to the receipt of a single pulse only of the series of five possible pulses 901 ... 905 respectively. The curve 931, 941, 932, 942, 935, 945 is the potential variation due to the receipt of pulses 901, 902 and 905. The potential to which condenser 512 attains after the last pulse time is past controls the amplitude of the pulse repeated by valve 514 when sampling pulse 908 from the delay line 520 is applied to its control grid. These pulses, the amplitude of which corresponds to that of the samples of the signal wave applied to the codes at the transmitter, are passed through low-pass filter 515 to reconstruct the signal wave for the terminal equipment 516 and reproducer 517.