Patent GB-627462-A (GB)
Title: Improvements in or relating to electric impulse communication systems
IPC: H03M3/02
Filed: 1947-08-08
Abstract: 627,462. Secret transmission. STANDARD TELEPHONES & CABLES, Ltd. Aug. 8, 1947, Nos. 21925 and 21926. Convention dates, Aug. 10, 1946. [Class 40 (iv)] [Also in Group XL (c)] The amplitude of a varying signal voltage is communicated by a series of recurrent pulses which are of one sign when the amplitude is rising and of opposite sign when the amplitude is falling. In one method, the time derivative of the signal voltage is used to send positive pulses to line when the derivative is positive and above a predetermined value; and negative pulses when the derivative is negative and above that value. At the receiver, the incoming pulses are integrated and passed to the telephone or other signal-receiving instrument. In another method, the sign of each pulse is determined by the sign of the difference between the instantaneous signal amplitude and the integrated amplitudes of all the preceding pulses. When radio transmission is used, the positive and negative pulses may be sent on different carrier frequencies. The pulses are all of the same amplitude, and their sign indicates whether the signal voltage is rising or falling. If the rise or fall is of small amount, no pulse is sent. By using pairs of pulses in combination, four different conditions can be signalled, indicating both the sense of the voltage change and whether it is slow or rapid. In the transmitter shown in Fig. 3, the signal wave P is applied through a differentiating network C7, R25 to the grid of a valve V4, the output of which is connected in push-pull to two tilting-relays V2, V3. These relays comprise reciprocally-coupled triode pairs of which the left-hand valve is normally conducting and the right-hand valve blocked. If sufficient negative bias is applied to the left-hand valve, the condition is momentarily reversed, and restores itself after a short interval. The grids are supplied with negative pulses from a multivibrator V1 which are insufficient by themselves to tilt the relay. If, however, one of the grids is receiving from the valve V4 negative voltage above a certain limit, that relay is tilted and a pulse is sent to one of the grids of a double-amplifier V5 and thence to the line Q. As the right-hand anode of valve V3 and the left-hand anode of valve V2 are connected respectively to the differently-biased grids of valve V5, the pulses sent to line Q will be positive or negative according to the sign of the voltage applied to the grid of the valve V4, that is according to whether the original signal voltage P is falling or rising. If the rise or fall is below a certain value, no pulse is produced. At the receiver, Fig. 4, the incoming pulses P1 are applied in parallel to the differently-biassed inner grids of two pentode valves V6. Negative pulses do not affect the blocked right-hand valve, but block the left-hand valve and cause its anode to send positive pulses to the integrating filter F1. Positive pulses do not affect the left-hand valve, but open the right-hand valve to deliver negative pulses from its anode to the filter Fl. The filter output is amplified in a valve V7 and delivered to the receiving instrument at Q<SP>1</SP>. Noise is reduced by gating the valves V6 by pulses applied at A, B to their suppressor grids and derived from a pulse generator synchronized with the multi-vibrator V1 of Fig. 3. The transmitter shown in Fig. 5 is similar to that of Fig. 3, except that the signal is applied directly to the valve V4 instead of through a differentiating circuit, and the valve V4 is also fed from the output pulse at Q through an integrating circuit H similar to the receiver circuit of Fig. 4. The latter feed is of the nature of a negative feed-back and the resultant voltage on the grid of valve V4 is the difference between the signal and the integrated resultant of all the preceding pulses. The effect is shown in Fig. 8, in which the curve P represents the signal, and the curve a the integrated pulses, the sign of the pulses Q being reversed whenever the curves cross. Fig. 6 shows a further transmitter operating in the same general manner as Fig. 5. The initial regular pulses are applied at G to the grids of two triode valves V10, V11. The signal P is applied through valve V8 to the grid of V11 and the cathode of V10. The integrated resultant of outgoing pulses is applied through a valve V9 to the grid of V10 and the cathode of V11. According to which valve V8 or V9 has the predominating effect, one or other of the valves V10, V11. is unblocked and passes a pulse from the input G either directly to line Q if valve V11 is operative or through an inverting valve V12 if valve V10 is operative. The pulses are integrated by applying them to the grids of a valve V13, the anode and cathode of which feed the input grids of two valves V14, V15 connected in series across a H.T. supply. The midpoint of a high resistance Z2 also connected across the H.T. supply is connected to the grid of the valve V9, to one side of a condenser C, and to the anode-cathode junction of the valves V14, V15. According to the sign of the pulse at Q, one or other of these valves is unblocked and a unit of charge passes into or out of the condenser. At the receiving station the pulses Q are fed to an integrating circuit similar to that enclosed at I, and the condenser voltage is applied to the receiving instrument through an amplifying valve and a low-pass filter (Fig. 7, not shown). Specification 535,860 is referred to.