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First, in the case of digital signalling, the detected data should be used. Forexample, if the modem performance is good, an error probability of 10 2 stillsuggests that the desired output produces bits 99% every time. Algorithms thatuse the detected data as the desired output are called decision-directed.The answer to (2) is to use an adaptive equalization method: a procedurethat allows the initial guess for the coefficients to be corrected according to arecursive relationshiphðn þ 1Þ ¼hðnÞ k½z c ðnÞ z d ðnÞŠZ c ðnÞ ð3:44aÞwherek ¼ Filter weight or adjustment parameter.23z c ðnDÞz c ½ðn 1ÞDŠ½Z c ðnÞŠ ¼ 6 . 74 . 5z c ð2NDÞð3:44bÞThe expressions in (3.44) suggest that a training sequence can be periodicallysent through the channel, or the equalizer can make use of the incoming datato carry out minimization adjustments.In summary, the resulting weights for either the zero forcing techniqueor the minimum mean-square error technique can be precalculated and preset,or adaptive circuitry can be used to automatically adjust the weights.3.3.2 CodecA codec is an essential part of a communications network. The function of acodec is the reverse of a modem; that is, a codec converts analog input intodigital format (A=D)—the coder and digital input analog format (D=A)—thedecoder. Basically, a codec consists of an ADC (analog-to-digital converter), aDAC (digital-to-analog converter), an internal clock, encoders and decoders,and filtering stages. The basic building block of a codec is shown in Fig. 3.15.The coder portion of Fig. 3.15 digitizes the transmitted voice signal, andthe decoder reconstructs a received voice signal. In the encoder=decoder units,the digitized signal is decorrelated to reduce redundancies in the signalFIGURE 3.15General structure of a codec.Copyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.