Recent developments in long-distance telephony

The paper deals with some of the research and development work carried out by the Post Office during the last few years, in connection with the transmission of speech over long lines. With the rapid growth of international telephony since the advent of the thermionic valve, many new problems have arisen, requiring to be solved in accordance with geographical and other conditions peculiar to this country. A brief mention is made of aerial lines, but the greater part of the paper is devoted to transmission over long cables provided with repeaters. The principal subjects dealt with are:¿ Two-wire and four-wire repeaters. The operation of two-wire circuits at an overall equivalent of zero. Echo-suppressors and voice-operated stabilizers. Systems of carrier-current telephony developed by the Post Office for use on aerial lines, land cables, and sea cables. Recent investigations regarding such general problems of long-distance telephony as cross-talk and transient effects are described.