Patent GB-715809-A (GB)
Title: Improvements in or relating to pulse modulation communication systems
Inventor: FEDIDA SAMUEL
IPC: H03M1/00
Filed: 1950-07-06
Abstract: 715,809. Pulse code modulation system; gating circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. April 26, 1951 [July 6, 1950], No. 16950/50. Classes 40 (5) and 40 (6) In a pu'se code modulator system the speech signal to be transmitted is applied to a sampler A, Fig. 1, to produce pulses (a) at a predetermined frequency, which correspond to the instantaneous amplitude of the signal at the time the pulse is produced. The sampler is controlled by a master multivibrator B which provides pulses (b) at the sampling frequency. The sample pulses are fed to one valve of a unit C, which consists of two valves with a common anode load, and the output from C, consisting of pulses (c), is fed via an amplifier D to a delay line E having a central tap which feeds an amplifier K which contro's a multivibrator L. The output from the delay line, consisting of pulses (e), is fed via an amplitude doubler F and cathode follower stage G into a subtracting gate H. When the input to the multivibrator L exceeds a predetermined voltage a pulse (l) is produced which is passed through a shaper M' to produce a code output pulse and also produces a reference voltage pulse (m, l) at the gate H. The gate H has two conditions of operation, in one it passes any signal fed thereto, and in the other it subtracts from the signal fed thereto the reference voltage provided by the pulses (m, l) when the signal exceeds the reference voltage, the output from H being fed back to the other valve in the unit C so that voltage doubling is obtained round the feedback loop. The master multivibrator B also feeds pulses (bb) through a shaper M to produce pu'ses (m) which short out any residue at the gate H and make the feedback inoperative when the predetermined number of pulses in a train have been produced. At the decoder, Fig. 7, the received signal is amplified at D4 and passed to a delay line DDL having a number of spaced taps, the time delay between taps being equal to the time interval between the pulses of a train of pulses. Each tap feeds a gate DG1 to DG5 which is normally closed, all gates being opened simultaneously by a pulse generator operating at the repetition frequency of the pulse trains, to which synchronizing pulses are fed at SYNC. Each gate produces an attenuation appropriate to the time position of the pulse to which it corresponds, and the outputs of the gates are fed in parallel to a unit DMN which produces an output corresponding to the original sample pulse. The sample A and subtracting gate H of Fig. 1 may be variations of the basic circuit shown in Fig. 2. The crystal diode DD is initially conducting, due to the potential +V applied to the resistor R1. When a negative going pulse v is applied through the resistor R no output is produced until the current through R equals the current through R1, when the crystal is cut off. The circuit acts as a shorting switch until the crystal is cut off, and above this value acts as a subtracting gate. In a sampler unit, Fig. 3, the valve BV represents one valve of the master multivibrator B, Fig. 1, and when it is cut off the diode AD and crystal diode ADD are conducting and the signal input applied to the resistor A4 produces no output at C. Each time the valve BV conducts, the two diodes are cut off and an output pulse is produced which is proportional to the input voltage at that instant. The diode AD isolates the anode circuit of the valve BV from the crystal diode. In a modification, Fig. 4 (not shown) the resistor A2 is replaced by a capacitor, and the anode of the diode AD is connected through a resistor to HT+. In a subtracting gate, Fig. 6, a negative-going control pulse is applied to the grid of the valve MV, and in its absence the valve conducts and an input pulse applied to the resistor H4 is passed on, the crystal diode H.D. being cut off. When the control pulse occurs the valve MV is cut off and the junction of resistors H2 and H3 is clamped at a potential of 2HT+ due to the action of the diode HDD3 and the circuit is then in a subtracting state. A muting control valve MMV is normally conducting and the diode 2HD cut off, but when a negative-going control pulse is applied to the grid of the valve MMV, the diode 2HD conducts, causing the crystal diode HD to conduct and short out the input voltages.