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- The Difference between AM and FM and Digital Video Transmission
Multiplexer Knowledge
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- How to Install a Fiber Optic Modem?
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Multiplexers can be used to connect PBX, Hot line and other devices of network from central site to user site through fiber optical cable.
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The Difference between AM and FM and Digital Video Transmission
There is some confusion in the marketplace as which technology is most appropriate for different applications. While there is a lot of marketing hype about digital technology the fact remains that for many applications AM offers very adequate performance at extremely competitive pricing.
FM has many key advantages. The following attempts to give a flavor of the tradeoffs between the main technologies available today.
AM is very simple technology which can provide extremely good performance for many situations. It is easy to get wide bandwidths without excessive circuit complexity. Also, over low optical loss budgets typical of most security type CCTV which only occasionally operates over more than 2 to 3km, the signal-to-noise ratio attainable with AM can be very good. This does degrade from some maximum value with increasing fiber length, so some care is required to ensure that your optical loss budget is under control.
However, linearity as measured by parameters such as differential gain and phase, while perfectly adequate for single hop CCTV links is not usually good enough for systems where 3 or 4 links may have to be put in for the series. It can be difficult to achieve the linearity obtained with multimode AM systems on single mode AM links because the optical devices required for the latter are usually not as linear as those for multimode.
FM has two key advantages over AM:
1. Performance is not so strongly dependent on optical loss: the signal to noise ratio will degrade slowly with increasing loss to a threshold point after which it degrades very quickly.
2. Performance is not dependent on the linearity of the optical components since it is almost completely dominated by the FM modulator and demodulator.
Digital system video performance is dominated by the analog to digital conversion at the transmit end and the digital to analog conversion at the receive end. The actual transmission path will have virtually no visual effect on the picture if the Bit Error Rate is better than 1 error in 1 billion bits. In fact, BERs down to as low as 1 X 10-4 still allow quite useable signals to be received and monitored on screens.
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