In some digital communications applications, digital radio for
example, the transmission channel itself (the ether) may not impose any significant
filtering effect across the modulation bandwidth, and the main filtering is performed by
transmitter and receiver circuitry.
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This transmitter filtering is employed largely to constrain the
modulation to the regulated bandwidth, while in the receiver, the filtering is necessary
to remove a multitude of other signals entering the receiver, and to minimize the noise
entering the demodulator. Often the Nyquist filtering response needed for zero ISI is
split equally between the TX and RX systems using a root raised
cosine filter pair.
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