Patent GB-688938-A (GB)
Title: Improvements in or relating to electric pulse modulation signalling systems
Inventor: LEVY MAURICE MOISE
IPC: H03M3/02
Filed: 1950-09-28
Abstract: 688,938. Pulse code modulation systems; pulse circuits. GENERAL ELECTRIC CO., Ltd. Sept. 6, 1951 [Sept. 28, 1950], No. 23810/50. Classes 40 (v) and 40 (vi). A pulse modulation signal is generated by comparing the amplitude of the modulating signal with a reference level, which may vary in a known manner with respect to time, selecting for transmission one of a series of regularly repeated pulses when the difference between the two levels exceeds a predetermined value, and reducing the difference to a further predetermined value which may be zero, when a pulse is selected. In one embodiment, the modulating signal is fed to the grid 2 of valve 3, Fig. 1, the anode 4 of which is connected through condenser 6 to the parallel oppositely-directed diodes 8, 9, which are normally cut off. A reference potential is set up by rendering conducting by a pulse the diode clamp circuit 7 which earths the junction between condenser 6 and the diodes 8, 9. Thereafter when the potential at this junction varies by more than a predetermined amount, due to a change in signal amplitude, one of the diodes 8, 9, conducts to trigger one of the flip-flop multivibrators 15, 34. The output pulse is applied to circuit 17, which thereupon generates a voltage to operate the clamp 7, and is stored for a sufficient time to allow the next pulse from generator 36 or 38, as the case may be, to pass to the output through gating circuit 16 or 35 respectively. The storage element is the condenser 22, Fig. 3, in the anode circuit of valve 24, to which the multivibrator pulse is applied. The stored voltage on the condenser is applied to grid 27 of gate valve 28 and the pulse generator 36 is connected to another grid 29. The next pulse from the generator is therefore passed to the output conductor 30 and after a slight delay in circuit 37 renders valve 31 conducting to discharge condenser 22 and restore the circuit to its quiescent state. In another embodiment, for multichannel working, each channel has a transformer 40, Fig. 4, to which the modulating signal is applied. The transformer secondary 43 is connected to the grid 44 of valve 45, through condenser 41, across which a rising reference voltage is produced by a charging current through resistance 46. Valve 45 is normally cut off by the anode current of valve 50 in the common cathode resistance 53 and its anode is connected through condenser 47 and diode 48 to the multivibrator 49. These three elements are common to all the channel circuits. Valve 50 is periodically cut off by pulses from a generator 39 to render valve 45 conducting and the multi-vibrator 49 is triggered if at that time the grid 44 has a potential greater than a predetermined value. When an output signal pulse is produced in conductor 55 by the multivibrator 49 it is also passed through the common amplifier 56 to the clamp circuit 57 to discharge the condenser 41. The circuit 57 comprises a valve 58 normally held non-conducting by the anode current of valve 59 in the common cathode resistance 60. Valve 59 is periodically cut off by pulses from generator 39 delayed by network 65, so that valve 58 can then be rendered conducting to discharge condenser 41 by the pulse from amplifier 56. The other channel circuits produce signal pulses interlaced with each other, the pulse generators being suitably phased, or a common generator may supply the individual channel circuits from the tappings of a delay line distributer. In a modification of the system of Fig. 4, more than one incremental change can be signalled by output pulses of different amplitude, a plurality of multivibrators and differently-biased diodes being connected to the valve 45, Fig, 6 (not shown). A receiver for the multi-channel system of Fig. 4 comprises a pulse generator 174, Fig. 5, feeding a tapped delay network 173 serving as a channel distributer. The channels are gated on in turn by rendering the shunt diodes 165 non- conducting so that a positive channel pulse can pass over diode 166 to the integrating condenser 170, to raise the potential of the demodulatedsignal output conductor 172. This potential falls between pulses, the condenser 170 being connected to a negative potential source through resistance 171.