Patent GB-362472-A (GB)
Title: Improvements in electrical transmission devices
IPC: H04B3/40
Filed: 1930-07-30
Abstract: 362,472. Artificial lines. BLUMLEIN. A. D., 57, Earl's Court Square, London, July 30, 1930, No. 23045. [Class 40 (iii).] In an artificial line which can be tapped at various points along its length and is so designed that the impedances facing the input and output terminals are invariable, while the attenuation varies in a regular and definite manner with the position of the tapping points, variable tappings are provided at both ends of the line and II-sections are employed. Fig. 1 shows a pair of resistance lines in series. The first line comprises sections A, B, and has a high attenuation-constant for coarse adjustment, while the second line comprises sections C, D, and has a low attenuation-constant for fine adjustment. The characteristic impedance is 2Zo throughout, and the lines are terminated by resistances R1, R2 having this value. If, then, the resistance of either the source of power connected at 1, 2 or that of the load connected at 3, 4, has a very high value, or if both have a fairly high value, the resistance of the line facing these terminals is Zo whether the tappings 5, 6 be taken at 7, 8 or 9 and 10, 11 or 12 respectively. The sections may be asymmetrically dimensioned. Instead of a doubly tapped line, a a line connected at one end to a load or source, the impedance of which is matched, may be employed. The parallel pairs of resistances a, a, c, c, may in practice be replaced by single resistances, Fig. 2 (not shown), and the sections may be balanced, with resistances distributed in all four arms ; in the latter case a central earthed conductor may connect together the middle points of all the shunt resistances, Fig. 3 (not shown). The switches may be rotary, sliding or plug switches. Reactance networks. In a corrector for varying the frequency-characteristic of a circuit, Fig. 7, the impedance arms are reactive. The switch S may be mechanically coupled to switches controlling pure-resistance attenuators in series with the corrector shown. Lines for producing variable phase-changes and delay may be similarly designed. Several lines may be connected in cascade, each terminating upon the tapping contacts of another line having twice its characteristic impedance. Telephone transmission systems. Two microphones 17, 18, Fig. 5, feed jointly into two gramophone recorders (e.g. a wax and an optical recorder) 19, 20', through four artificial lines shown diagrammatically at 20 .. 23. The lines are so designed that their attenuation constants can be varied by alteration of the tapping-points, while the impedances which they offer at the connections 17, 18, 19, 201 are independent of the tapping- points. In a modification, Fig. 6 (not shown), four microphones feed into four such lines and tappings from these lines form the arms of a Wheatstone bridge, the diagonals of which are formed by tappings from two lines feeding into two amplifiers. If the impedance of each microphone and the characteristic impedance of its line is twice the characteristic impedance of each of the amplifier lines, each line can be adjusted independently without disturbance of the impedance balance. Alternatively one arm of the balanced bridge may be connected to a monitoring receiver as well as to a microphone, so that a microphone in one of the diagonals will influence the monitoring receiver but will not affect the amplifier in the conjugate diagonal.