 > 50u). has been analogue. 2. Wavelength 1300nm (1.3u) or 1550nm optical devices could be used with single mode fibre. 1550nm offers slightly lower loss, 57.3.5.2 Frequency agile switching but possible bandwidth limitation (caused by 'dispersion') which increases with transmission distance. The second approach used has been frequency agile switching 3. Source output level, measured in milliwatts, or fractions (Figure 57.23), utilising the tuner function described above. This is thereof, for lasers, or microwatts for lower power LEDs. Alter- popular where the incoming feed is a frequency division multiplex 57/22 Video transmission Buffers Input Control lines Switch control Switch element Output lines Figure 57.22 Crosspoint matrix with buffered inputs Compared to audio, there is for the uninitiated a bewildering array Switch control Agile of video performance parameters, and it is a major topic in its own oscillator right. There are in fact two ways that a measured impairment may be quoted, in objective or subjective terms. The objective version is Shifted spectrum the physical measurement taken (e.g. in dBs, % etc.); sometimes it is converted into its subjective equivalent (using standard formulae) Wanted Filter channel measured in 'imps'. This latter was derived originally from real Incoming viewing assessments made of different levels of each parameter for multiplex Mixer their annoyance value, and allows one to compare the impairment from different parameters in the same terms. To other switch elements It is not sensible here to explain each type of parameter, but Table 57.7 does classify them into types and lists the objective value Figure 57.23 Frequency agile switch equivalent 1/8th imp. In this table a weighting filter is used to Table 57.7 Video performance parameters and their of many channels, as on tree and branch systems. A frequency agile equivalent to an impairment of one-eight imp, measured at oscillator is mixed with the multiplex, such that the desired channel baseband for PAL-I signals is trans