Patent GB-713614-A (GB)
Title: Electric pulse communication system
IPC: H03M1/00
Filed: 1952-10-17
Abstract: 713,614. Pulse code modulation systems. STANDARD TELEPHONES & CABLES, Ltd. Oct. 17, 1952 [Oct. 26, 1951], No. 26102/52. Class 40 (5). [Also in Group XXXVIII] In a pulse code communication system, sample amplitudes of a signal wave are represented by groups of pulses using a " constant duty ratio " code, in which each code signal has the same number of pulses which may occur at any of a greater, but fixed, number of instants. The coder described comprises a source of a number of standard waves of decreasing voltage with which the signal sample is compared in succession and each comparison is repeated until the sample voltage exceeds the standard wave voltage, whereupon a pulse is produced, the sample is diminished by the standard wave voltage and the comparison is switched to the next standard wave. The decoder is somewhat similar in operation. As an example a seven element code, in which three pulses are transmitted for each value, has 35 levels. Thus zero may be 0000111, one is 0001011, two is 0001101 and so on, to 34 which is 1110000. This code is used in the system described. For each coding operation, seven comparisons are made, the forms of the three standard wave voltages during this period being shown as V7, V8, V9 respectively, Fig. 3. These waves are produced by the generators 7, 8, 9, Fig. 2. A synchronizing pulse source triggers the sawtooth generator 43 which, after its output has been delayed by one comparison interval in circuit 44, produces V9. The generator 8 comprises an integrator 45, to which the undelayed output of generator 43 is applied, and its output is delayed by half a comparison interval in circuit 46 and a suitable bias added in circuit 47 to produce wave V8. The generator 7 comprises an integrator 48, to which the output of integrator 45 is applied, and produces wave V7. These wave generators 7, 8, 9 are shown in Fig. 1, being controlled by synchronizing pulse source 10. The latter also controls a timing pulse generator 11, a pulse from which opens the sampling gate 13 to store a signal sample from source 4 on condenser 14 which is discharged at the end of a coding operation by circuit 23, energized by the same timing pulse after delay in the network 16. The comparison operations are controlled by pulses arriving in succession at tapping points on the network 16 and fed to the comparison circuit 6, in which the voltage on condenser 14 and the standard wave voltages are compared. At the commencement of the coding wave generator 7 is connected to circuit 6 through the open gate circuit 15. When, after a comparison, the sample exceeds the standard wave voltage, a code pulse is produced and fed to the line 2. It also is fed to a subtraction circuit 17, to effect a reduction in the voltage on condenser 14 by the instant voltage of the wave source, and to a ring counter circuit 18 which closes gate 15 and opens gate 19 to connect the second wave generator 8 to the comparison circuit 6. This process is repeated until all seven comparisons have been made and three code pulses have been derived by using all three wave generators 7, 8, 9 brought into use successively by the control of the gate circuits 15, 19, 20 by the counter 18. At the receiver the three code pulses in the line 2 are applied to a ring counter circuit 31 which distributes them to the gate circuits 28, 29, 30, respectively. The three wave generators 24, 25, 26 produce standard waves similar to V7, V8, V9 at the coder and are connected respectively to the gate circuits which when opened for short periods under control of the incoming pulses allow the appropriate samples of the standard wave voltages to pass to the addition circuit 32. The latter comprises three H series-connected storage condensers 33, 34, 35 fed respectively with said samples through isolating rectifiers 36, 37, 38 and connected, at the end of the decoding operation and in additive combination through gate circuit 39, to the signal reproducing circuit 42. The wave generators 24, 25, 26 are controlled by a synchronizing source 27 timed in known manner from the input signal, which also controls the gate circuit 39 through a pulse generator 41. Specification 630,292 is referred to.