Abstract :
Successful telephone conversation depends upon the persons involved, their ambient conditions, the transmission characteristics of the subscribers´ sets and the local lines to the terminal exchanges, the main lines between exchanges, and the exchanges themselves. Against the extreme variability of the telephone habits and abilities of the first, random variations in line and exchange speech transmission are of little account. Mean loss is important. Electronic exchanges may permit substantial reductions in mean loss throughout a network by providing the means, not present in existing exchanges, of correct and invariable impedance termination of lines and of accurate line balancing at 2-wire/4-wire terminations, by eliminating the impedance mismatches and reflections at switching points and by providing amplifiers in all lines. These same conditions may also be applied to improving the economy and possibly the transmission of local lines. Electronic exchanges using analogue speech transmission are known and exchanges using p.c.m. are being examined. There is little to choose between the two types of transmission so far as network planning is concerned. The invariant-loss feature of p.c.m. gives it some advantage in main-line transmission, but this may be a handicap in local-line provision, where some means of varying the gain according to the connection made can lead to economy and transmission improvement.
