Thông tin thiết kế mạch P9

SIGNAL PROCESSING IN THE TELEPHONE SYSTEMUntil 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....
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Thông tin thiết kế mạch P9 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 SYSTEM9.1 INTRODUCTIONUntil the introduction of the digital telephone, there was virtually no signalprocessing on the subscriber loop. Indeed, there was no need for it. The majorityof subscriber loops were able to transmit voice signals with no particular difficultyand in cases where the lines were longer than usual, line ‘‘loading’’ was used withsuccess. Signal processing has two major aims: (1) To improve the quality of signal transmission over the telephone commu- nication 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. Itfollows that the more messages that can be transmitted in a given time, the lower thecost per message. It is therefore on the long-distance channels (trunks or tolls) thatsignal processing techniques have proven to be most successful. In this chapter thecommon signal processing techniques used in the telephone system and some of thecircuits employed will be examined.9.2 FREQUENCY DIVISION MULTIPLEX (FDM)Frequency division multiplex (FDM) is a technique in which a number of signals canbe transmitted over the same channel by using them to modulate carrier signals with 267268 SIGNAL PROCESSING IN THE TELEPHONE SYSTEMdifferent and appropriate frequency so that they do not interfere with each other. Theassignment of specific carrier frequencies to radio stations for broadcasting and otherpurposes is, in fact, FDM. It can be used with amplitude modulation as well as otherforms of modulation. In the context of the telephone, FDM is used in conjunctionwith amplitude modulation. In normal amplitude modulation, the carrier, upper and lower sidebands aretransmitted. When the depth of modulation is 100%, the amplitude of the carriervoltage is twice that of the sidebands. The power in the carrier is therefore 2 of the 3total. Unfortunately, the carrier has no information content. Each of the sidebandscontains 1 of the total power. In radio, 100% modulation is almost never used so the 6power content of the sidebands is much less than described. It is noted that theinformation content is duplicated in the two sidebands. It is clear that one way tobeat the corrupting influence of noise on the information content of the transmissionis to put as much as possible, if not all, of the available power into one of thesidebands. An added advantage to this scheme is that the required bandwidth isreduced to one-half of its original value. Clearly, this would allow twice as manymessages to be sent on the same channel as before. The transmission of only onesideband in an AM scheme is called single-sideband (SSB) modulation. The price tobe paid for this advantage is that to demodulate an SSB signal, it is necessary toreinstate the carrier at the receiver. The reinstated carrier has to be in synchronismwith the original carrier, otherwise demodulation yields an intolerably distortedsignal. Providing a synchronized local oscillator requires complex equipment at thetransmitter as well as at the receiver. In SSB radio, an attenuated form of the carrieris transmitted with the signal. This is used to synchronize a local oscillator in thereceiver. In the telephone system, a centrally generated pilot signal is distributed toall offices for demodulation purposes. In some cases, a local oscillator withoutsynchronization is used. If the frequency error is small (approximately Æ5 Hz),successful demodulation can be achieved [7].9.2.1 Generation of Single-Sideband SignalsA block diagram of the SSB generator is shown in Figure 9.1. Figure 9.1. The block diagram for SSB modulation. 9.2 FREQUENCY DIVISION MULTIPLEX (FDM) 269 The signal and the carrier are essentially multiplied by the balanced modulator togive a DSB-SC output. The bandpass filter removes either the lower or uppersideband. f ðtÞ ¼ A cos os t cos oc t ð9:2:1Þ A f ðtÞ ¼ ½cosðoc þ os Þt þ cosðoc À os ÞtŠ: ...