ibre. 57.22. 2. The video performance requirement, which will obviously in- A number of devices can be used as the crosspoints. Relays make fluence the quality and cost of a link's components, but may a direct wire connection, and coaxial versions are capable of very also limit the channel capacity and the type of modulation high performance and bandwidths; even miniature versions are method that can be used. relatively large and mainly suited to head end or studio uses. 3. The input and output interface requirements. For example, are Semiconductors, such as the CMOS and DMOS FET devices, can the signals baseband or already modulated in a form suitable for provide analogue switches with a good degree of integration on a optical transmission? Also how is TV sound to be carried? chip. Although 'on' resistances are at least a few tens of ohms and The length of link, and hence optical loss to be incurred. capacitive signal leakage occurs in the 'off' state, good quality 5. Optical power output, and whether there are any safety impli- compact matrices for cable TV use can be constructed able to switch cations. baseband video signals. Higher frequency switching is quite possible, but crosstalk problems will imply very careful board The designer may not need to know optical technicalities in any design and probably lower integration. detail, but some aspects worth being aware of are: Digital crosspoints are relatively straight forward; a simple AND gate performs this function with one input as signal and one as 1. Fibre type, only single mode fibre will be relevant nowadays control. Both ECL and CMOS technologies are used, the latter in for video links. Multi mode is appropriate for shorter, lower particular being capable of high integration and high speeds (16 capacity links. Single mode (core diameter < 10u) is a little in/16 out chips working at 200Mbit/s have been produced). Their more difficult to joint and connectorise than multi mode (core use for cable TV has been limited because nearly all transmission diameter