Patent GB-657251-A (GB)
Title: Improvements in or relating to electric pulse code group decoding systems
IPC: H03M1/00
Filed: 1949-01-14
Abstract: 657,251. Pulse code modulation circuits. WESTERN ELECTRIC CO., Inc. Jan. 14, 1949, No. 1086. Convention date, Jan. 17, 1948. [Class 40 (v)] For decoding pulse groups based on numbers having a radix r a network includes a feed-back arrangement having a feed-back factor rand a delay equal to the pulse intervals, means being provided to sample the output of the network at the end of the receipt of a pulse group. In one embodiment, the network comprises an amplifier 40, Fig. 3, with a positive feed-back factor or Á# characteristic of two, the delay round the feed-back loop being equal to the pulse intervals of the group to be decoded. This network is used, for example, with a four-unit code of "on-off " pulses the four "on" pulses representing, successively, amplitudes of eight, four, two and one respectively. By the time the last pulse is received the first has been increased in amplitude by repeated passage round the feed-back loop by a factor of eight, the second by a factor of four and the third by a factor of two so that, at that time, the combined amplitude of the output pulse from the amplifier is equal to the amplitude which the code group represents. When the code element pulses are transmitted in ascending denominational order the amplifier 40 is arranged to employ negative feedback instead of positive. In order that the amplifier may be restored to quiescence after the output has been sampled at the end of a code group, two amplifiers 40, 42 are provided and brought into use alternately. The input code groups from terminals 44 are distributed alternately to the two amplifiers 40, 42 by valves 46, 48 alternately rendered conductive by a wave A, Fig. 4, applied to their screen grids in push-pull from transformer 64. At the same time, the output of the amplifier which is out of use is earthed through one of the diodes 66, 68 which are rendered conductive alternately by the same wave A fed through transformer 74. The amplifier outputs are sampled alternately at the end of the pulse code period by valves 84, 86, the screen grids of which are fed in push-pull from transformer 90 with a pulse wave-form B, Fig. 4. The anodes of valves 84, 86 are connected in parallel to the output transformer 92 so that the decoded signal amplitudes are re-assembled in the correct order. These switching operations may be performed mechanically, Fig. 1 (not shown). In a second embodiment, the decoder employs passive elements only, the feedback network consisting of a delay line 100, 102, Fig. 5, of electrical length such that it introduces a delay equal to half the pulse interval. The pulses, after regeneration in circuit 104, control a constant current generator 106 to produce voltage pulses across the series resistance 108 which are applied through resistance 112 to the input of the delay line. The pulses of the code group are in ascending denominational order and the far end of the line is open-circuited to reflect them without change of amplitude. Resistances 108, 112 are so proportioned that one half of the pulse returning to the input is absorbed and the other half reflected back into the line. The summation pulse at the completion of the code group is applied through condenser 114 to a cathode follower stage 116 and output circuit 118.