Patent GB-754758-A (GB)
Title: Improvements in or relating to electric pulse code modulation systems
IPC: H03M1/00
Filed: 1954-04-28
Abstract: 754,758. Pulse code modulation systems; pulse delaying circuits. SOC. FRANCAISE RADIO-ELECTRIQUE. April 28, 1954 [April 28, 1953], No. 12386/54. Classes 40 (5) and 40 (6). Relates to circuits for deriving pulse code groups in the cyclic progression binary code, described in Specification 649,376, from position-modulated pulses, and for converting them to groups in the conventional simple addition code. Each position-modulated pulse is paired with an unmodulated pulse and in n successive coding stages the first oncoming pulse of a pair is delayed by a time T/2<SP>m</SP>, where m is the number of the stage. In the first stage a code pulse is emitted if the delayed pulse precedes the undelayed pulse and conversely in the subsequent stages. A coding stage 4, Fig. 2, comprises a bi-stable multivibrator 20, 21 to the cathodes of which are applied the incident pulse pair from lead 8. The two outputs therefrom are applied respectively to the grid of triode 30 through a differentiating circuit 23, 24 and a diode 27 and to the grid of triode 31, through circuit 25, 26, diode 28 and the appropriate delay network 29. The output pair of pulses for the next coding stage are taken from the common anode circuit of triodes 30, 31 over the lead 9. The cathodes of valves 30, 31 are connected respectively to the grids of a bi-stable multivibrator 32, 33 which is set in one position or the other depending on whether the delayed or the undelayed pulse precedes the other. In the stages other than the first, the code pulses are taken from the anode of valve 33 over lead 11, but in the first stage they are taken from the anode of valve 32 over lead 11<SP>1</SP>. The code output of each stage is fed to the grid 35b of a tetrode 35, all these valves being gated open simultaneously by a pulse from the master generator 3, to deliver code pulses to the equally-spaced tappings of a delay line 6, of which one section 36 is shown, and which delivers at its output the required group of code pulses. Thereafter, a pulse from generator 3, suitably delayed in circuit 37, resets the multivibrator 20, 21 through diode 22 ready for the next coding operation. In an embodiment in which the cyclic progression code is converted into the conventional code, the circuit 2, Fig. 6, produces the pulse pairs, one pulse of which comes from the master generator 3, the other being position-modulated by the signal source 1. The coding stages 41, 4<SP>2</SP>, 4<SP>3</SP>, &c. have an additional diode 34, Fig. 2, for resetting multivibrators 32, 33 after each coding and feed their outputs to the inputs 65<SP>1</SP>, 65<SP>2</SP>, 65<SP>3</SP>, &c. of a circuit 50 which has outputs 66<SP>1</SP>, 66<SP>2</SP>, 66<SP>3</SP>, &c. The differentiating networks 51, 52 and the diodes 53, 55 in circuit 50 are so connected that an input applied at 65q causes pulses to appear at the outputs 66q, 66q<SP>+1</SP> ... 66n, no pulse being delivered at outputs 66<SP>1</SP>, 66<SP>2</SP> ... 66q<SP>-1</SP>. The outputs are connected to bi-stable multivibrators 561, 562, 56<SP>3</SP>, &c. forming a counter, the state of which at the end of the coding operation represents in conventional code the number previously coded in cyclic progression code. When all the multivibrators are reset by a pulse from the source 3 through diode 59, those which are in the " 1 " position produce a code pulse at the anode of valve 58 which is fed through cathode follower 62 to the distributer delay line 63, the pulse group being taken from the output 64. In another embodiment a single coding stage is used to provide all the code pulses, its output being continually returned to the input after an appropriate delay, Fig. 9 (not shown). This necessitates variation of the internal delay for each passage of the pulse pair through the coding stage. A circuit for this purpose comprises a multivibrator 95, Fig. 11, with one stable state, triggered to execute one cycle by a pulse on the input 79, a delayed output pulse being derived from the differentiating circuit 90, 91 and diode 92. At the beginning of a coding cycle, a condenser 86, the voltage of which determines the grid bias of the valve 88 of the multivibrator through cathode follower 85, is charged by applying a pulse to the diode gate 81, 82 to connect the.voltage source Vo to the condenser. Between each operation of the delay circuit a pulse is applied to the diode gate 83, 84, to allow the condenser to lose half its charge through the resistance 87 so that the grid voltage of valve 88 is reduced in steps producing a proportional reduction in the multivibrator period.