TABLE OF CONTENTS The correlation span for binary signals can be extended to more than three digits, resulting in waveforms that have most of their energy at low frequencies. For duobinary and modified duobinary systems, the correlation span extends over two and three bits, respectively. Longer memories are possible; such systems have been termed polybinary [7.2].
Suppose a binary message with two signaling levels is transformed into a signal with b signaling levels, numbered consecutively from zero to ( b ?1), starting at the bottom. All even-numbered levels are identified as binary 0 and all oddnumbered ones as binary 1 (of course, this labeling can be reversed easily). Both the original message and the polybinary signal have an identical symbol duration of T seconds. There are no restrictions on the number of b levels.
A binary message is transformed into a polybinary signal in two steps. In the first step (encoding), the original sequence a n , consisting of binary 1 s and 0 s, is converted into another binary sequence d n in such a manner that the present binary digit of sequence d n represents the modulo-2 sum of the ( b ?2) immediately preceding digits of sequence d n and the present digit a n . For example, let
Here, a n represents the input binary digits and d n the encoded bits. Since the encoding expression involves 3 bits preceding d n , then...
