Patent GB-852848-A (GB)
Title: Improvements in or relating to an encoding apparatus
IPC: H03K7/08
Filed: 1958-03-04
Abstract: 852,848. Analogue-to-digital converters. DAYSTROM Inc. March 4, 1958 [March 12, 1957], No. 6962/58. Class 40 (1). An analogue voltage or current is converted to a pulse-time code by comparison with a standard reference voltage or current by time switching in repeated cycles, the reference being switched on for such a time in each cycle as to balance the input analogue as an average over the cycle. The time of switching off the reference in each cycle represents the input analogue and when used to generate a pulse for transmission is referred to as a pulse-time coding. The circuit of Fig. 1 converts a current analogue to a pulse-time code; the value of the input current is to be represented by the time at which a pulse occurs in a regularly repeating cycle. The input current I is applied on leads 11 and 12 to a D.C. amplifier 13, having two feed-back circuits, one of which is an A.C. circuit consisting of a capacitor 14 and the other is a negative D.C. feed-back loop. This loop is controlled by a relay 19 and consists of relay contacts 22, 23, primary transformer winding 24, and a standard reference current source 30 comprising a variable resistor 27 and a source 28. When relay 19 closes, the standard current Io flows to the input of the amplifier in the opposite sense to the input current. The transformer 26, condenser 33 and resistance 34 comprise a pulse generating network, the output of which is connected to a standard telephone line for transmission of the pulses. In operation the current input I applied to point j is balanced by the current Ic passing through the capacitor 14 and periodically when relay 19 is energized by current in the negative feed-back loop. When the current I is first applied the only balancing current is through capacitor 14 as the result of a rising potential e<SP>1</SP> at the amplifier output. This potential rises until relay 19 operates and the reference current Io is switched to point j. The potential e<SP>1</SP> therefore oscillates as shown in Fig. 3. The repetition period To (sum of the times during which the relay is open and closed, T 1 , T 2 ) depends on the capacitor 14 and the value of e<SP>1</SP> required to operate the relay. The input current I is given by Io is four to ten times the input current I. A modified circuit is described (Fig. 2, not shown) to translate a voltage in the same way, the circuit being similar except that a resistance is included in the standard current feed-back line to produce a voltage opposing the input voltage. As before, the amplifier output is as shown in Fig. 3 and the feed-back current Io has a squarewave whose magnitude is set by resistor 27. The pulse generating network 31 which consists of a critically damped inductance capacitance circuit produces pulses from this square wave, as shown in Fig. 3, marking the beginning and end of each current flow. These appear across the resistor and are applied to the telephone line. The differential timing of the negative pulse within a cycle (defined by two positive pulses) represents the input analogue current. In a modification the use of moving parts is avoided by applying the standard current through an electronic gate which is enabled by a signal from a bi-stable trigger. The trigger is set and reset by signals from two peaking circuits, one of which operates when the saw-tooth amplifier output begins to fall and the other operates when it begins to rise again. Specification 852,849 is referred to.