GB593139A - Improvements in or relating to demodulating arrangements in time modulation electricpulse communication systems - Google Patents

Abstract

593,139. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. Sept. 8, 1944, No. 17229. Convention date, Oct. 7, 1942. [Class 40 (v)] The demodulator in a timemodulated pulse - transmis - sion system comprises means for producing a train of waves in response to the initiation of each time period defined by the pulses, modifying the amplitude of the waves according to the length of the period, terminating a wave train before the initiation of the next period and demodulating the modified waves. The arrangement is independent of the pulserecurrence frequency and may be used with various time-modulated-pulse systems. The received signals are amplified at 10, Fig. 1, the pulses are recovered and re-shaped at 11, and from there passed to a shock-excited tuned oscillatory circuit 12 which is provided with means such as the inherent damping of the circuit for terminating the produced wave prior to the reception of a succeeding impulse group. The waves produced in the tuned circuit 12 are then detected and amplified at 13 and fed to the headphones 14 or other reproducer. An auxiliary amplitude modulation detector 15 caters for any amplitude modulation of the pulses that may be present. Fig. 2A indicates the output of the re-shaper circuit 12 in the case where the pulse formations are pairs of pulses 20, 21-22, 23, &c., the pulse width D and pulse formation recurrence period R being constant and the spacing P between the two impulses of each pair of pulses varied in accordance with the modulation envelope representing the transmitted intelligence. The periodicity, of the shock-excited tuned circuit 12 is arranged to be equal to 2D, or an odd submultiple thereof, so that the leading edge of impulse 20 causes the circuit 12 to produce the wave 30, Fig. 2B, and the trailing edge acts in phase on the generated wave to produce a stronger oscillation. The pulse 21 produces a second wave by exciting the circuit 12 which modifies the generated wave 30, e.g. killing it as shown at 32 in one extreme modulation position and enhancing it as shown at 42, Fig. 2C, at the other extreme position. The amplitude of the resultant wave train is thus varied in accordance with the spacing P and when successively produced wave trains are detected at 13 the modulation envelope of the time-modulated-pulses is reproduced. To prevent any interaction between successively received pulse formations, the characteristics of the tuned circuit are arranged to damp out the wave train before receipt of the next pulse formation, e.g. as indicated at 33, 43, Fig. 2. The tuned circuit 12 may comprise a triode, Fig. 3 (not shown), across the output circuit of which the tuned circuit is connected. In a modification, Fig. 4, the pulses 20, 21.... are applied over a delay line 70 to a triode 60 having a substantially undamped tuned circuit 61, 62. Connected across the tuned circuit is a variable impedance tube 71, the grid of which is connected directly to the re-shaper 11. This tube conducts only when its grid is energized by incoming pulses when it rapidly damps out any oscillations in the tuned circuit 61, 62, so that a wave train generated in the tuned circuit 61, 62 persists until the next pulse formation arrives and energizes tube 71 direct. A further wave train is then initiated by shock excitation of the tuned circuit after the incoming pulse has passed through the delay line 70. The delay line may alternatively be fitted in the circuit supplying the incoming pulses to tube 71, in which case a wave train produced by any incoming pulse formation would be damped out after the appropriate delay by the same incoming pulse formation acting on tube 71. The invention is applicable to other types of time-modulated-pulse systems, e.g: (1) where one of the pulses of each pair is fixed in time and the other one is varied in accordance with the modulation, Fig. 5 (J, K) (not shown). In this case, the " fixed " pulse may be suppressed at the transmitter and then reproduced locally at the receiver. (2) Where the duration of single pulses is varied to indicate the time modulation, Fig. 5 (L, M) (not shown). The incoming pulses need not be re-shaped before application to the tuned circuit 12, Fig. 5 (N, o) (not shown). The interval P may be arranged, alternatively, to be either a maximum or minimum in the absence of modulation. In the former case, a local demodulating carrier wave will be necessary in the amplitude detector 13. The pulse recurrence frequency R may be continuously varied at the transmitter for secrecy purposes without necessitating any modification in the receiver.