k reliably, the source data may need to be scrambled first to ensure that it is adequately randomised. The added bit also ensures a minimum 28.3.3.6 Partial response clock content and some redundancy to permit transmission error monitoring. Binary codes of this type feature high efficiency (m has ranged in practice from 7 to at least 23) and simple hardware, and These codes (Lender, 1981), known also as correlative or multiple they are thus attractive for high speed applications such as in optical response codes, deliberately introduce a controlled amount of inter- fibre systems. Codes such as mB1P (Dawson and Kitchen, 1986) symbol interference into the received signal. This known amount of and DmB1M (Kawanishi et. al., 1988) are similar in concept but correlated interference produces a multi-level received signal offer slightly different features. which, with suitable pre-transmit coding, can be designed to provide the requisite line code features. A simple example of the class is duobinary (Lender, 1966) in 28.3.3.5 which a binary signal is transmitted at twice the rate required to give zero intersymbol interference. With suitable coding and equalisa- These (Brooks and Jessop, 1983) are block codes where m binary tion the resulting intersymbol interference can be arranged to give source bits are mapped into n binary bits for transmission. Redun- a 3-level received signal. Figure 28.12 gives waveforms for this dancy is built into the code to provide the desired transmission code. Figure 28.12(a) shows an input step applied to a transmission features by making n > m. Several such codes have been proposed channel which, after appropriate equalisation, produces the requisi- (and used), in particular where n = m + 1. They differ from the te output at the receiver after time T (ignoring channel delay) Figure previous category in that the m-bit transmitted block may bear little 28.12(b). Now, if signals are launched into the channel with a time similarity to its input source block. This gives greater flexibility for s