Digital communication includes any system which can deliver data over distance. Figure 9.1 shows some of the ways in which the subject can be classified. The simplest is a unidirectional point-to-point signal path shown at (a). This is common in digital production equipment and includes the AES/EBU digital audio interface and the serial digital interface (SDI) for digital video. Bidirectional point-to-point signals include the RS-232 and RS-422 duplex systems. Bidirectional signal paths may be symmetrical, i.e. have the same capacity in both directions (b), or asymmetrical, having more capacity in one direction than the other (c). In this case the low-capacity direction may be known as a back channel ...