Patent GB-660672-A (GB)
Title: Pulse code modulation system
IPC: H03M1/00
Filed: 1949-02-04
Abstract: 660,672. Pulse code modulation circuits; frequency modulation. STANDARD TELEPHONES & CABLES, Ltd. Feb. 4, 1949 [March 16, 1948], No. 3136/49. Class 40 (v). To produce pulse code groups from amplitude modulated pulses, a first primary frequency is modulated by the pulses, and a second primary frequency is modulated by a " weighing " signal which varies as an exponential function of time, the difference between the two primary frequencies determining the presence or absence of pulses in a binary code group, corresponding to the original pulse amplitude. As described, a six-unit code is used enabling 64 amplitude levels to be discriminated. The incoming amplitude modulated pulses, modulate the frequency F1 of oscillator 1, Fig. 1, while the weighing signal from generator 3 modulates the frequency F2 of oscillator 2 through modulators 4, 13. The oscillator outputs pass to mixer 7 and limiter 8 which feeds frequency discriminator 9. When F1>F2, the discriminator output is positive and when F2>F1, it is negative. The following amplifier 11 and limiter 12 do not pass negative inputs and give a constant output for variable positive inputs, the amplifier 11 being gated by groups of six coding pulses from generator 6. Thus the coded pulse group appears at the output of limiter 12. The generator 6 is controlled by a master pulse generator 5, operating at group recurrence frequency, which also controls the weighing signal generator 3, to produce exponentially decaying pulses 41, Fig. 3, c, at group recurrence frequency. The modulator 4 is gated on by the coding pulses from generator 6 and controlled by the amplitude of the weighing signal at these times to modulate oscillator 2 through a circuit having a discharge time constant of the order of the code pulse recurrence period. The modulator 13 is gated on by the coded output pulses from circuit 11, 12 applied through a delay circuit 14 which delays them for the duration of one pulse and controlled by the amplitude of the weighing signal at these times to modulate the oscillator 2. Only the first occurring pulse is effective, however, owing to the long discharge time constant circuit through which the modulating voltage is applied. At the end of the pulse group this time constant circuit is discharged by a pulse from generator 5 passed through delay network 15. The unmodulated frequency difference between oscillators 1, 2 is maintained constant by applying the output of the discriminator 9, to control the frequency F1 of oscillator 1, through the amplifier 16 and lowpass filter 17, the amplifier being gated on in the intervals between coding by pulses from generator 5. The oscillator 2 comprises a positive grid valve 18, Fig. 2, of the type described in U.S.A. Specification 1,987,989, the frequency of which is varied by variations of the potential of the helicoidal grid 20 and/or the outer retarding electrode 22. The modulator 4 controls the potential of grid 20 and comprises valve 35, the anode circuit 36, 37, of which is of short time constant. The modulator 13 controls the potential of electrode 22 and comprises valve 27 the anode circuit 28, 29 of which is of long time constant. The grid circuits of valves 35, 27 are gated respectively by coding pulses across resistance 39 and coded pulses across resistance 34, both being supplied with modulating voltage from the weighing signal generator. A discharge device 30, operated by the master pulse generator, discharges condenser 29 at the conclusion of a coding period. As an example of the coding operation, a pulse 40, Fig. 3, a, of amplitude 50 units is applied to oscillator 1, the resulting changes in frequency of oscillator 2 being shown in Fig. 3, e, and the resulting coded pulses in Fig. 3, d. During the first coding pulse, modulator 4 is operated to change F2 by 32 weighing units. As F1 has been varied by 50 units, F1>F2 and a code pulse is produced at the output of circuits 11, 12. This code pulse is applied through delay circuit 14 to the modulator 13, whereby subsequently F2 is maintained at the value it had during the first code pulse owing to the long time constant circuit 28, 29. The next frequency comparison therefore takes place when F2 is shifted a further 16 units by modulator 4. F1 is again greater than F2 by 2 units so a further code pulse is produced, and modulator 13 changes the shift in F2 from 32 to 48 units ready for the next comparison at which F1 is 50 units up and F2 is 48+8=54 units up, no code pulse being produced, and so on.