In section 3.2, the filtering effects of a
channel are analysed for their impact on streams of data pulses bits and symbols. What
at first sight appears to be a 'show stopper' for data transmission the intersymbol
interference caused by channel filtering is shown to be unfounded if steps are taken to
achieve a Nyquist filter response for the whole system. Section 3.3
introduces the eye diagram as a powerful visual tool for observing and diagnosing problems
within the modem portion of a digital communications link. This is followed in Section 3.4 by a look at a very popular type of channel filter, the
raised cosine filter, which is found in almost all modern modem implementations. Section 3.5 discusses the concept of a matched filter this in
fact describes an overall channel filter response that should result in the optimum
performance of the modem in the presence of noise. Finally, Section
3.6 goes against the stance taken in Section 3.3 by advocating the introduction of a
controlled amount of intersymbol interference into the system by careful choice of filter
(termed partial response signalling). Surprisingly, perhaps, this can achieve improved
performance under certain conditions.
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