GB452084A - Improvements in or relating to systems for recording, reproducing or transmitting signals by means of electric currents - Google Patents

Abstract

452,084. Automatic volume control. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A. Jan. 10, 1936, No. 948. Convention date, Jan. 15, 1935. [Class 40 (iv)] [See also Group XL] In signal transmission, recording or reproduction using compression and/or expansion of the range of volume variations, the compressor (and/or expander) is designed to be ineffective at input volumes above and/or below a definite level or levels. As shown in Figs. 4 and 5 for example, below and above given limits, variations in input volume produce equal variations in output volume. Between these limits however the range of variations is compressed or expanded. Different limits, indicated by the numerals 1 ... 5, are given by a preliminary adjustment of the apparatus. An application of the invention for improving the signal/noise ratio, Fig. 3 (not shown), consists in arranging for expansion only when the received level is below the lowest necessary signal level. Figs. 6 and 7 show circuit arrangements of a compressor and expander respectively. Signals received at 18, 19 if of sufficiently low level suffer a fixed amount of attenuation in the apparatus 23 ... 28. At a given level, determined by the setting of loss networks 32 .. 36 and 42 .. 46, the voltage drop produced in resistance 49 by rectifying signals tapped from the transmission path overcomes the battery 51 and the valves 27, 28 begin to cause the attenuation of the network 23 ... 28 to vary with the signal level. A condenser 50, connected to resistance 49 by a variable tapping, controls the time taken for a given variation to take effect. At a second (higher) volume level, the peak voltages from the transformer 37 overcome the paralyzing effect of batteries 54, 55 on valves 52, 53 which hitherto have been without effect, signals in the control path having been simply amplified by the pair 38, 39. The pair 52, 53 opposes the pair 38, 39, and consequently, variations in input level when a definite level has been passed do not alter the attenuation of the network of 23 ... 28 and are reproduced in the output circuit by variations of the same extent. The expander shown in Fig. 7 operates in a similar way except that the transmission equivalent of the signal path is adjusted by varying the gain of a repeater instead of the loss of an attenuator and that amplifiers 57, 58 are added to the control path. Distortion due to the fact that the impedance of the variable-gain repeater varies whereas that of the output transformer does not, is minimized by the resistances 70 ... 73 and condenser 74.