Patent GB-659020-A (GB)
Title: Improvements relating to electrical systems of transmission by code pulses
IPC: H03M1/00
Filed: 1949-07-25
Abstract: 659,020. Pulse code modulation circuits. LIBOIS, L. J., and GLOESS, P. F. M. July 25, 1949, No. 19559. Convention date, Aug. 5, 1948. [Class 40 (v)] Duration modulated pulses are translated into groups of binary code pulses by a succession of circuits in each of which the input pulse duration is compared with a reference time interval, a code pulse being generated if the duration of the input pulse exceeds the reference interval and a pulse of the excess duration being passed to the next circuit, while if the reference interval exceeds the input pulse duration no code pulse is generated and the input pulse is passed unchanged to the next circuit. The reference intervals in the successive circuits are each one half of that in the preceding circuit. In the embodiment described, the negative input pulse 23, Fig. 2, having a fixed leading edge and a modulated trailing edge, is inverted in valve 24, to produce pulse 28 which is fed to valve 34. It is also displaced by the reference time interval, T/2 in this case, where T is the maximum duration of the input pulses, this being effected by a delay line 25. The pulse is applied to the input 27, which is correctly terminated and is reflected from the unterminated end 26 to reappear at the input end where it is added to the pulse 28 to produce the composite pulse 32 at the grid of valve 34. As shown, the input pulse duration t is greater than T/2 so that the pulse 32 has a part 33 of greater amplitude than pulse 28 and of duration t-T/2. Valve 34 acts as a clipper passing pulse 33 to produce pulse 38 which is fed to the next coding stage input valve 70. In addition, the composite pulse 32 is fed to the control grid of pentode 39 which is cut off for the duration of the input pulse 23 by applying this pulse to its suppressor grid. The output from valve 39 is thus a pulse 41 of duration T/2 delayed by t with respect to pulse 23. The transformer 44 in the anode circuit of valve 34 inverts and differentiates the pulse 38 to produce the pair of pulses 46, 47, that corresponding to the leading edge of 38 being used to trigger a flip-flop circuit 48. The latter produces a negative pulse 50 of duration T which with the positive pulse 41, which occurs within it, is applied to the grid of valve 42. The positive pulse 41 is thus unable to unblock valve 42 to pass the input pulse 23 to the next stage, which would have occurred if it had been of less duration than T/2. The code pulse is generated by differentiating the pulse 57 on the right-hand anode of valve 48 and using the resultant leading edge pulse 61 which is fed to the mixer 64, the negative trailing edge pulse being clipped by rectifier 65. The remainder of the circuit shows a second coding stage for deriving the next digit, and pulses are shown which occur when the input pulse duration is less than T/4, the reference time interval for this stage. In this case, the composite pulse 80, corresponding to pulse 32, is unable to unblock valve 76, so that no pulse therefrom reaches the input valve 90 of the next stage. Also, the flip-flop valve 84 remains untriggered so that the pulse 86, which is a delayed version of the input pulse 38 derived from the second element of the composite pulse 80 in valve 78, is able to unbreak valve 87, to transmit pulse 89 to the next input valve 90. No code pulse is fed to the mixer 64 by the flip-flop valve 84 in the case considered. The final and nth coding stage comprises the input valve 101, Fig. 5, and a delay line 104 by which the composite pulse 107 is produced as in the other stages. As shown, the input pulse duration exceeds the reference time interval T/2n so that pulse 107 has a portion able to unblock valve 108 and produce a pulse 111 which after differentiation provides a code pulse 113 to be fed to the mixer 64. The mixer 64 consists of a delay line, to tappings on which the code pulse outputs are fed, and which are selected to time and arrange the code pulses to form a group 69 having the desired spacing and order of the pulses, which are also shaped in circuit 67.