Patent GB-396046-A (GB)
Title: Improvements in or relating to electrical signal transmission systems
IPC: H04B1/64
Filed: 1932-01-25
Abstract: 396,046. Transmission systems. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.- (Assignees of Norman, N. C. ; 1217, Nelson Avenue, New York, U.S.A.) Jan. 25, 1932, No. 2254. Convention date, Jan. 27, 1931. [Class 40 (iv).] A volume compressor comprises one or more thermionic valves, connected in shunt of a transmission line, the impedance of which is adapted to be varied inversely as a root of the applied signal current by a control device whose output is proportional to a root of the input, and a corresponding expander for restoring the volume range, comprises one or more valves in series with the line and the impedance of which is controlled by a device having a linear input -output characteristic. The characteristics of the compressor and expander are such that there is in each a linear relationship between the input and output levels expressed in logarithmic units. The Specification contains a mathematical investigation of the problem. The drawings show a transmission line for speech and music embodying a compressor 16 at the sending end TC and an expander 33 at the receiving end RC. The compressor may be used alone on telephone lines, in wireless circuits or in recording on films or discs to improve the signal to noise ratio, and to enable the sensitivity of echo-suppressors to be improved without danger of being operated by noise. The expander may be used alone on long telephone lines which act as partial compressors by introducing relatively higher loss at high speech levels, the effect varying slightly with frequency. Several stages of compression and expansion may be used. In the arrangement shown in Fig. 1, the control circuit VC1 tapped off.input 2 feeds an amplifier 7 through potentiometer 6. The output from 7 is taken to a rectifier 9, the output of which is proportional to the square root of the input voltage, and is amplified at 12 and put into a linear rectifier 14 from which the output is filtered at 15 to remove frequencies exceeding say 40 cycles and applied to the grids of valves 17, 18 arranged in push-pull, their anode circuits being coupled in shunt of the line by impedances 19, 20. The line circuit associated with the compressor has an impedance high relatively to the shunt impedance of the compressor. The rectifier 9 comprises a pair of opposed triodes 10, 11 operating as diodes, the constants of the circuit being arranged so that the desired output is obtained. The compressed range is amplified at 1 and sent out over line ML to the receiving circuit RC which includes expander 33 in series with the line. The control circuit VC2 leads to an amplifier and linear rectifier from which the output is filtered at 32 and applied to the grids of the expander valves 34, 35. The circuit RC is of low impedance relatively to the expander. In a modified form, Fig. 2, of the rectifier 9, the ungrounded filament battery of the rectifier 9, Fig. 1, is avoided by the use of two separate secondary windings for the transformer. Specification 255,221, [Class 40 (iv), Telephones &c.], is referred to.