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Unformatted text preview: d signal.
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: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
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.
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