pacing of less than T, the output will have time to only partially providing the desired line code features for a given level code respond to the input stimuli. Figure 28.12(c) shows such an input efficiency, but is achieved at the expense of increased encoder and where the symbol period is only T/2, that is, signalling is at twice decoder circuit complexity. the conventional rate. With duobinary, some pre-transmit coding is By way of example, part of the translation table for the balanced used as detailed in Figure 28.12(d), the waveform being shown in polarity 7B8B code (Sharland and Stevenson, 1983) is illustrated in Figure 28.12(e). This signal is launched into the channel and its Table 28.3. Each 7-bit input source word is mapped into one or two associated equaliser, the result is shown in Figure 28.12(f). This possible 8-bit output words depending on the polarity balance, or received signal can then be applied to a 3 level digital decision circuit and decoded in accordance with the rules given in Figure Table Part of the translation table for 7B8B 28.12(g) to yield an output which is the same as the original binary input. Thus, a binary data signal has been transmitted at twice the conventional rate, the price paid being a 3-level signal at the re- Input (7 Output as transmitted (8 bits) ceiver. The code features adequate transitions for clock extraction bits) and also some redundancy suitable for transmission error monitor- Word disparity ing, by noting that the output cannot change between the two outer Negative Positive levels in adjacent decision times. 1111111 For well behaved channels (where intersymbol interference can be accurately controlled), partial response codes can provide a 1010001 bandwidth efficient transmission technique. They have been used in 0101001 radio systems and also to convert existing 24-channel PCM systems into 48-channel systems with a minimum of equipment change. 0000000 28.3.3.7 Other line codes disparity, of the transmitted words. Output words which are bal- anced in themsel