undant signal set with three possible amplitude levels as shown in Figure 28.11(c). The zero d.c. property is achieved by sending marks as a positive or negative symbol 28.3.3.1 Manchester code alternately. An estimate of the link error performance is obtained by counting violations of this alternating mark inversion rule. We note This is also called diphase or WAL1 code and is a simple example that AMI has a potential weakness; if many successive zero-level of a bit-by-bit code. Each information bit is coded into a two-bit symbols are transmitted, then the clock will fail. Thus, either the number of successive zeros has to be restricted, which is undesirable Binary Example (Alternating) (Alternating) Figure 28.11 Some bit-by-bit line codes: (a) Manchester code; (b) code mark inversion (CMI); (c) alternate mark inversion (AMI) Line codes 28/11 for a transparent data link, or a code substitution has to be made to needed, this redundancy provides the necessary design flexibility. prevent this potential hazard. Such AMI based substitution codes The selection of words and their mappings from input-to-output is are used in 24 and 30 channel PCM telephone systems, where they made on the basis of ensuring: good timing content, error monitor- are known as B6ZS and HDB3 respectively (Waters, 1983). ing, word alignment and of minimising the opportunity for trans- mission error multiplication in the decoding process. A computer search may be used to optimise this mapping. 28.3.3.4 Many such binary-to-binary balanced codes have been designed. A similar design philosophy can also be used for bandwidth efficient This (Yoshikai et al., 1984) is an example of a block code of the bit multi-level codes, of which 4B3T (4 binary bits converted to 3 insertion type. A block of m information bits has added to it a single ternary symbols) (Catchpole, 1975) and subsequent variations on it extra bit which is selected in an attempt to provide a long term are perhaps the best known. polarity balance and SO no d.c. component. For this to wor