P1f(ii) Data Transmission You will learn about: How Optical Fibres are used How interference is reduced

Optical Fibres RECALL: Infrared waves or laser beams are able to be transmitted through an optical fibre. Each time the wave hits the boundary it is reflected back because the angle of incidence is greater than the critical angle. Optical fibres can transmit lots of information at speeds up to 200,000 km/s which is the speed of light in glass. Imagine talking to someone on the other side of Earth using a fibre optic connected telephone wire. It would not take long to receive the signal and very little transmitted energy would be lost. Fibre optic technology allows more information to be communicated at much higher speeds than ever before. Pulses of light are transmitted to a receiver which decodes the signal. This is an example of a digital signal.

Digital Switchover Broadcasters decided to switch from analogue to digital transmissions because: Signal quality would be vastly improved (sound as well as image) Consumers would have a greater choice of programmes Consumers could interact with the programme Information services would be improved such as television guides and subtitles. A huge campaign in 2009 reached across the UK readying us for the digital switchover. Since those efforts in 2009 practically the whole of the UK Is now digital ready. The Government pledged that the whole of the UK would be 100% ready for digital broadcasts by 2015 … this includes the Scottish isles.

Analogue Wave Noise Normal human speech frequency is around 200 Hz. But to transmit our sounds more efficiently we need more energy. Our 200 Hz analogue sound wave is carried by a higher frequency 200,000 Hz carrier wave. This provides the energy needed to transmit the signal. Our sound wave and the carrier wave are transmitted together. At the receiver, the receiver subtracts the carrier wave and the original sound wave would then be reproduced. However, any interference, or noise, would be transmitted also. Simply amplifying the wave would amplify the noise. Look at the analogue wave. You can see that by increasing the amplitude of the wave any interference is simply increased too. This causes interference when the receiver decodes the signal. REMEMBER: If you increase the amplitude of a noisy analogue wave you increase the noise too.

Multiplexing REMEMBER: Digital transmission has lots of benefits over analogue. This is because multiplexers allow lots of information to be transmitted at once very, very quickly. Signal A Signal B Signal C Signal B Signal A Signal C Transmitted as Multiplexer: Allows lots of digital signals to be sent simultaneously. Each section is divided into individual sections each of a short pulse De-multiplexer: Separates the transmitted code as separate signals A digital signal can only have two values – on and off. If a digital signal is interfered the decoder in the receiver corrects the signal as either on and off or as a 1 or a 0. When the signal is corrected it is called regenerated. It is very easy for the decoder to rid of any noise. You can see in the image how a regenerated signal has its noise subtracted. An analogue wave does not.

Questions 1.Why are telecommunication companies changing their traditional copper wires for glass fibre optics? 2.What happens to the noise on an analogue signal if the signal is amplified? 3.What are three advantages of the Digital Switchover? 4.How can multiplexing enable so much information to be transmitted at once?

Questions 1.Why are telecommunication companies changing their traditional copper wires for glass fibre optics? Fibre Optics enable the signal to be transmitted at very high speeds – 200,000 km/s with very little energy loss at very large distances. 2.What happens to the noise on an analogue signal if the signal is amplified? The noise simply gets amplified also. 3.What are three advantages of the Digital Switchover? Improved signal quality. Better choice of programmes – television as well as radio. Can interact with the programme. Services such as guides and subtitles are improved. 4.How can multiplexing enable so much information to be transmitted at once? Multiple signals are broken down in small fast pulses. These are all transmitted in a sequence. The de-multiplexer unscrambles the signal and separates them into their original signals.