rs (Slater, 1988), push-pull, internal from intermodulation). Unitl recently it was not con- feed forward, and power doubling are three main types, each of sidered appropriate for fibre transmission, because of the intrin- which has different features in terms of noise, distortion, and of Cable distribution systems 57/21 course cost. They are designed to match different parts of the natively dBm is used, which is the power relative to 1mW; thus network, there being trunk, distribution and line extender versions for example - 3dBm is 0.5mW, and +6dBm is 4mW. as the network progresses outwards. Some are also combinations of Receiver sensitivity, the minimum optical power that can be amplifier(s) and splitter(s), e.g. a mainline bridger combination received whilst maintaining the required performance. giving amplification in both the trunk and branch line. Relative intensity noise (RIN), the noise within the optical In modern systems impairment has been reduced by having for source itself, which could degrade the signal prior to launching each channel incrementally related carriers (IRC, equal spacing onto the fibre. It is measured as dB/Hz; better than - 120dB/Hz between each channel) or harmonically related carriers (HRC, each might be all right for a digital or FM link, but for AM better than carrier an exact multiple of the spacing). This ensures that carrier 150dB/Hz is required. intermodulation products fall exactly on other carrier frequencies Optical budget, the difference in dBs between source output (2nd and 3rd order for HRC, just 3rd order for IRC) rather than level and receiver sensitivity. It shows what is available for within the actual video signal, where they would give subjectively fibre loss, splice and connector loss, splitter (if used) loss, and annoying interference. a margin for temperature, ageing etc. Another important feature to bear in mind is signal level. Ampli- fier inputs and outputs are designed for specific levels (though with 57.3.5 Switching some tolerance); differences may affect 