GB330642A - Improvements in or relating to electrical signalling systems - Google Patents

Abstract

330,642. Western Electric Co., Ltd., (Bell Telephone Laboratories, Inc.). March 16,1929. Amplifying systems; simultaneous telegraphy and telephony.-In a signalling system, particularly for telephony over a submarine cable, the signals are pre-distorted in such a manner that the amplitudes of the several components are rendered more nearly equal, at a point in the transmission channel e.g. at the receiving end, and at least equal to the amplitude of corresponding interfering or noise frequencies, the original relationship between the amplitudes of the signal components being restored at the receiving end. Fig. 1 shows a two-way terminal arrangement associating a land line 11 u'ith a submarine cable 18, and Figs. 2 and 3- show amplitude-frequency curves for various stages at the transmitting and receiving ends, respectively. Curve a represents the amplitude distribution in the air before exciting microphone 9 and curve b the distribution after passing through the line 11. A filter 13 suppresses all frequencies below 400 and above 2500 (curve b<1>), thus permitting more energy to be put into the remaining frequencies. Two attenuation equalizing amplifiers 14, Hi give respectively the curves c, c', the latter introducing the required distortion, and a uniform amplifier 16 then gives the curve d which represents the currents actually put into the cable. The unequal attenuation produced by the cable gives a reception curve e, Fig. 3, which owing to the pre-distortion is more nearly uniform and is at all points above the noise-distribution curve f by a minimum found necessary for intelligible reception. A uniform amplifier 19 and a distorting amplifier 21 produce too curves g, h, and i, k, the latter representing the currents actually put into the receiving telephone which gives acoustic waves represented by m, n. A filter 20 suppresses frequencies below 400 and <Lbove 2500 in the noise currents. The frequencyband below 400 may be used for simultaneous telegraphy. Specifications 151,140 and 202,284 are referred to.