Signal Processing and Linear Systems-B.P.Lathi copy

I t is digitized by rounding off its value t o one of

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Unformatted text preview: d signal. A/2 ( a) -A/2 f\ D t- ~ 7V ~C I \. f'v:-\ A :7\J0~'0 t- ~qJ I R= ( b) (c) ( d) F ig. 5.12 Digital signal: (a) transmitted (b) received distorted signal (without noise) (c) received distorted signal (with noise) (d) regenerated signal a t the receiver. 2. T he g reatest a dvantage of digital signal transmission over analog transmission, however, i s t he viability o f regenerative repeaters in t he former. In a n analog transmission system, a message signal, as it travels along t he c hannel (transmission p ath), grows progressively weaker, whereas t he channel noise a nd t he signal distortion, being cumulative, become progressively stronger. Ultimately t he signal, overwhelmed by noise a nd d istortion, is m utilated. Amplification is of little help because it enhances t he signal a nd t he noise in t he s ame proportion. Consequently, the distance over which a n analog message can be t ransmitted is limited b y t he t ransmitted power. I f a transmission p ath is long enough, the channel distortion a nd noise will accumulate sufficiently to overwhelm even a digital signal. T he trick is t o s et up repeater stations along t he transmission p ath a t d istances short enough t o be able t o d etect signal pulses before t he noise a nd d istortion have a chance t o a ccumulate sufficiently. At each repeater station t he p ulses are detected, a nd new, clean pulses are t ransmitted t o t he next 5.1 T he Sampling Theorem 335 repeater station, which, in turn, duplicates t he same process. I f t he noise a nd d istortion remain within limits (which is possible because of t he closely spaced repeaters), pulses can be detected correctly.t This way t he digital messages can be t ransmitted over longer distances w ith g reater reliability. In contrast, analog messages cannot be cleaned up periodically, a nd t he t ransmission is therefore less reliable. T he m ost significant error in P CM comes from quantizing. This error can be reduced as much as desired by increasing t he n umber of quantizing levels, t he price of which is p aid in a n increased bandwidth of t he t ransmission medium (channel). 3 . Digital hardware implementation is flexible a nd p ermits t he use of microprocessors, miniprocessors, digital switching, a nd large-scale integrated circuits. 4 . D igital signals can b e coded t o yield extremely low error rates a nd high fidelity, as well as privacy. Also, more sophisticated algorithms can be used to process digital signals. 5 . I t is easier a nd more efficient t o multiplex several digital signals. 6 . D igital signal storage is relatively easy a nd inexpensive. I t also has t he ability t o s earch a nd select information from d istant electronic storehouses. 7 . R eproduction with digital messages is e xtremely reliable without deterioration. Analog messages such as photocopies a nd films, for example, lose quality a t each successive stage o f r eproduction, a nd have to be t ransported physically from one distant place t o a nother, often a t relatively high cost. S....
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This note was uploaded on 04/14/2013 for the course ENG 350 taught by Professor Bayliss during the Spring '13 term at Northwestern.

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