tailieunhanh - Thông tin thiết kế mạch P9

SIGNAL PROCESSING IN THE TELEPHONE SYSTEM Until the introduction of the digital telephone, there was virtually no signal processing on the subscriber loop. Indeed, there was no need for it. The majority of subscriber loops were able to transmit voice signals with no particular difficulty and in cases where the lines were longer than usual, line ‘‘loading’’ was used with success. Signal processing has two major aims: (1) To improve the quality of signal transmission over the telephone communication channels | Telecommunication Circuit Design Second Edition. Patrick D. van der Puije Copyright 2002 John Wiley Sons Inc. ISBNs 0-471-41542-1 Hardback 0-471-22153-8 Electronic 9 SIGNAL PROCESSING IN THE TELEPHONE SYSTEM INTRODUCTION Until the introduction of the digital telephone there was virtually no signal processing on the subscriber loop. Indeed there was no need for it. The majority of subscriber loops were able to transmit voice signals with no particular difficulty and in cases where the lines were longer than usual line loading was used with success. Signal processing has two major aims 1 To improve the quality of signal transmission over the telephone communication channels. 2 To lower the cost of communication by improving the efficiency of channel use. In general the quality of a communication channel tends to deteriorate with distance. In addition long distance channels are expensive to establish and maintain. It follows that the more messages that can be transmitted in a given time the lower the cost per message. It is therefore on the long-distance channels trunks or tolls that signal processing techniques have proven to be most successful. In this chapter the common signal processing techniques used in the telephone system and some of the circuits employed will be examined. FREQUENCY DIVISION MULTIPLEX FDM Frequency division multiplex FDM is a technique in which a number of signals can be transmitted over the same channel by using them to modulate carrier signals with 267 268 SIGNAL PROCESSING IN THE TELEPHONE SYSTEM different and appropriate frequency so that they do not interfere with each other. The assignment of specific carrier frequencies to radio stations for broadcasting and other purposes is in fact FDM. It can be used with amplitude modulation as well as other forms of modulation. In the context of the telephone FDM is used in conjunction with amplitude modulation. In normal amplitude modulation the carrier upper and lower sidebands are .

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