A useful extension of the duo-binary signalling method is modified duo-binary which has an overall frequency response which
contains a spectral null at dc. This approach is thus well suited to channels which have
poor low-frequency response (see Section 4.4), where an
alternative, Manchester coding, is discussed.
The encoder involves a two-bit delay, causing the intersymbol interference to spread over
two symbols. As for the basic duo-binary method, error propagation necessitates the use of
a pre-coder which can again be implemented using an exclusive-or gate.
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