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Improving Long-Distance Telephone Transmission

Nature volume 135, page 301 (23 February 1935) | Download Citation



THE rapid improvement of the technique of radio communication during the last ten years is now having a beneficial influence in the development of long-distance telephone transmission. In particular, the improvements made in vacuum valves due to the demands made by broadcasting engineers have led directly to great improvements in the design of the repeater valves used in long-distance telephony. It is well known that during conversation over long lines by means of carrier frequency equipment the -sounds heard sometimes vary greatly in loudness. This is attributed to the fact that the attenuation of the line, especially when overhead wires are used, changes with climatic conditions, temperature, etc. With cable circuits, the loudness remains much more constant. The phenomenon is analogous to the well-known phenomenon of ‘fading’ in radio transmission. Successful attempts have recently been made to mitigate this trouble. A paper by H. Sterky and R. Stalemark which appears in Ericsson Technics, No. 3, 1934, describes an automatic method of compensating for these variations which has been used in practice for the last two years with good results. The development of the method is due to the telephone firm of Ericsson, Stockholm. It depends on the well-known mathematical theorem that a carrier wave modulated by a wave of voice frequency is equivalent to three separate simultaneous oscillations. One of these has the frequency of the carrier wave; the others, called side-band waves, are of higher and lower frequency respectively. In the Ericsson device, during conversation, the carrier and one side-band wave are transmitted. Signalling is done by modulating the carrying wave with the ‘ringing’ current. It is stated that the volume control of the sound obtained in this way is very good.

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