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For simplicity, if we make the pulse shaping filter an ideal
brick-wall filter with cut-off at 0.5 x 1/Tb, then the impulse response and
frequency response of the combined duo-binary encoder and pulse-shaping filter are as
shown (see in-depth). As expected, energy from one input bit is seen to be spread over two
bit periods in the transmitted signal. The composite frequency response, however, is the
key to the success of duo-binary signalling. A filter with this response is relatively
easy to synthesize and can be used to replace the duo-binary
encoder and unrealizable brick-wall filter and realistically
achieve a data throughput of 2 bits/second/Hz.
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