Patent GB-678726-A (GB)
Title: Electric pulse code circuits
Inventor: OXFORD ALAN JOHN HENRY
IPC: H03M1/00
Filed: 1949-12-06
Abstract: 678,726. Electric selective signalling. MINISTER OF SUPPLY. Jan. 22, 1951. [Dec. 6, 1949] No. 31339/49. Class 40 (i). [Also in Group XL (c)] Relates to coding or decoding circuits using a signal having two or more digit pulses in the scale of notation N to represent the magnitude of a given electrical potential. The signal may be used to communicate speech &c. or for application to a digital computor. The arrangements operate in a succession of stages wherein the potential existing prior to the stage and stored on a condenser is changed by a potential selected from N potentials and the resulting potential increases in a ratio of N in the interval between each code digit, due to the connection of a negative resistance across the condenser. The N potentials progressively differ from one another by a constant potential when N is greater than two. Coders operating on the binary and ternary scales and a decoder operating on the binary scale are described in detail. The binary coder, Fig. 4, has a varying signal potential applied to terminals 1, 3, which is periodically sampled by applying the pulses of graph (a), Fig. 2, to the clamp circuits 6, at the primary of transformer 10. The bridge-connected diodes are thereby rendered conducting to charge condenser Cl and in the intervals between pulses the diodes are biased in the non-conducting direction by a potential developed across condenser 13 shunted by the leak resistance 14. The coder produces three-digit code groups and in graph (c), the line A ... J represents the signal potential to be coded, A, B, C &c. being the eight possible sample potentials which can be transmitted. Graph (d) shows the corresponding output code pulses from each sample. Condenser C1 is connected to the input and output circuits of the linear amplifier comprising valves 16, 17 so that its potential rises exponentially, doubling itself in the period between two code pulses. A slicer circuit S, fed from the anode of valve 16, charges a condenser C2 to a fixed positive or a fixed negative potential depending upon whether the potential of condenser C1 is below or above a threshold value XX, graph (c). In each recurrence period of the sampling pulses, graph (a), three pulses from the train of graph (b) are applied to the clamp circuit 7, to share the charges of condensers C1, C2. The slicer circuit comprises the amplifier stages 23, 24, with positive non-critical feedback adjustable by resistance 31, and a trigger pair 25, 26 with its feedback resistance 32 adjusted beyond the point of instability. Thus in coding the sample A, graph (c), the initial potential of the sample is above the threshold XX and the first code pulse at the anode 8 of valve 26 is negative. The potential of condenser C1 doubles to point a1 when clamp 7 operates to reduce it to point a2. It is still above the threshold value so the second pulse at 8 is negative. It now doubles to point a3, is reduced to point a4 by operation of clamp 7, and the third code pulse is negative, as it is still above the threshold. Subsequent values of the potential of condenser C1, during the coding of the samples B ... J, are as shown in graph (c), and the resulting code pulses in graph (d). In the corresponding binary pulse decoder, Fig. 3, the incoming code pulses are applied through resistance R2 to condenser C2, the charge on which is shared with condenser C1 when clamp 2 is closed once per input code pulse recurrence period. The potential of condenser C1 increases exponentially to double its value in the ensuing intervals due to the feedback amplifier A to which it is connected. When. a pulse group has terminated, clamps 1 and 3 are closed in quick succession to produce an output decoded signal sample pulse, at the resultant potential of condenser Cl, which is fed through a low pass filter to the signal reproducer and then to discharge condenser C1 in preparation for the next decoding cycle. The circuit for coding on a ternary scale pro duces positive, zero or negative output pulses which are developed across the condenser C2, Fig. 7, from the signal samples produced on condenser C1 by clamp 1. The potential on condenser C1 rises exponentially between code pulses due to the negative resistance -R thereacross, and is fed to two slicers S1, S2, in parallel. The threshold levels of these slicers is +V and -V respectively and the centre point P of the resistance R3 connecting their outputs may thus assume one of three fixed values, which are positive, zero and negative respectively. Once per code pulse period clamp 4 is closed to charge condenser C2 to the potential of point P through a cathode follower stage CF, and to provide an output code pulse. Subsequently clamp 5 closes to share the charge of condenser C2 with condenser C1 A coding diagram similar to Fig. 2 is described, Fig. 8 (not shown).