Theory of E-type repeaters

IN THE exchange telephone plant, speech is transmitted largely at voice frequencies over a single pair of wires which carries both directions of conversation. Until recently, these circuits were mostly unrepeatered, adequate transmission being assured through a suitable choice of coil loading and conductor size. The need for low-cost repeaters for the exchange plant had long been recognized, but it had not been economical to use widely the conventional hybrid type of repeater with separate amplifiers for the two directions of conversation. The E1 repeater introduced in 1949 was the first device designed specifically to meet the needs of the exchange plant. It is a bilateral device, which provides gain in both directions of conversation without the use of hybrid coils or line filters and with the use of but a single transmission network.1 This simplicity, together with the fact that it imposes little impairment on the signaling of the exchange plant, makes it well suited to introducing amplification in 2-wire circuits.