GB592414A - Pulse modulation system - Google Patents

Abstract

592,414. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. June 2, 1944, No. 10683. Convention date, April 1, 1941. [Class 40 (v)] Transmission is effected by means of a series of pulses which may be considered as the combination of two series of alternate pulses in which pulses 1, 3, 5, &c. are time modulated in one direction, and the alternate pulses 2, 4, 6, &c. are time modulated in the opposite direction. The two series of pulses have the same periodicity, but are relatively displaced so that in no case does a pulse of one series equally divide the interval between successive pulses of the other series as this would merely double the pulse frequency. It is shown mathematically that such a combined series of pulses includes odd harmonics of the pulse frequency which are amplitude modulated in accordance with the time modulation of the pulses, and there is no phase modulation of the odd harmonics. Reception thus merely requires amplitude detection of an odd harmonic. Fig. 4 shows a circuit for producing the pulses and Fig. 5 shows the waveforms at various points. A sinusoidal input of pulse frequency applied at 23, 24 produces an output from limiter V1 as in Fig. 5 (a) which is applied by potentiometer 25 to a push-pull pair V2, V3 to produce an output as in Fig. 5 (b). Potentiometer 25 is set to give an asymmetric input so that the lobes of curve, Fig. 5 (b) are uneven, resulting in relative displacement of the alternate series of pulses. In addition, pulses of the form shown in Fig. 5 (c) are applied to grids 26, 27 of V2, V3 to give the waveform, Fig. 5 (d), which is applied to a multivibrator pair V4, V5 to give an output, Fig. 5 (e). Valves V4, V5 are arranged so that a pulse of a certain magnitude will cause one tube to conduct and then a fall below a predetermined magnitude (28, Fig. 5 (e)) causes the other tube to conduct. Also each valve has in its output circuit a time-constant decay circuit for maintaining the potential output obtained by each valve. Thus, the output of the multivibrator will be proportional to the magnitude of the applied pulse, e.g. 29, Fig. 5 (e), proportional to 27, Fig. 5 (d), and the time taken to decay to the initial valve 28, Fig. 5 (e), is proportional to the magnitude of the pulse. Thus, although the leading edges 29, 31, &c. of successive pulses are strictly periodic, the trailing edges are spaced in accordance with the magnitude of the pulses. As the pulses 27, 30, Fig. 5 (d) are mounted on the uneven lobes of curve, Fig. 5 (b), the trailing edges, Fig. 5 (e), form two series, alternate ones of which are equally spaced, but the members of one series do not equally divide the members of the other series. The waveform, Fig. 5 (e), is then applied to a differentiating circuit R2, C2 and over-biassed valve V6 to give the waveform, Fig. 5 (.f). The upper peaks 32, 33, 34, &c. corresponding to the trailing edges give the desired two series of pulses, while the lower peaks 35, 36, 37 are strictly periodic and are suppressed to leave the unmodulated pulses, Fig. 5 (g), for use in transmission. If now a speech signal is applied through transformer 38 to appropriate grids of valves V2, V3, the waveform, Fig. 5 (d), is amplitude modulated as shown in Fig. 5 (i), causing alternate peaks to be increased and decreased. This, due to the multivibrator action causes time modulation in opposite senses of the two series of alternate pulses, Fig. 5 (l), which are then modulated on to a carrier for transmission. In the receiver, Fig. 3, the detected pulses from 47 are fed to a band-pass filter 53 which selects an odd harmonic of the pulse frequency for amplitude detection at 54, amplification at 55 and reproduction at 56. A sharply selective circuit 48 tuned to twice the pulse frequency controls a square wave generator 49 which produces a blocking signal which is fed back to block the H.F. stages except when pulses are due to arrive. During the blocking periods, a jamming signal may be radiated. Also during the interval between blocking periods an abnormally high anode voltage may be momentarily applied to the audio amplifier to increase the gain.