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Loss and noise

One of the facts of life is that nothing's perfect. There is no such thing as a perfect cable, where everything you put in comes out at the other end. In real life, a cable has resistance and there are stray electrical effects on the line. This means that a signal won't be either as powerful or as clear as it started.

It may not even be powerful enough to make a device work at all, or it may be too distorted to read. These are the problems of 'loss' and 'noise'.

Loading pot

Loading pot (1900s) : this load makes things lighter

By the early 1900s it seemed as if the telephone companies had reached the best performance that they could hope to achieve from the telephone wires. Thicker wires, with lower resistance, helped the signals travel further but the communication still faded over distance.There was also a limit to how thick and heavy they could make the wires.

Happily the discovery of 'loading' meant they could overcome this problem. By inserting small coils of wire every so often along a long-distance telephone line, to balance out the capacitance that weakened the strength of the signals, the signals could be sent sucessfully over far greater distances.

This loading pot is essentially a densely packed container, full of 'loading coils' of wire.

Submersible telephone repeater - Connected Earth artefact, now at the Science Museum, London

Submersible repeater (circa 1943) : shooting through the water

Most telephone cables laid under the sea have to carry signals over long distances. The further a signal travels the weaker it is likely to become, so amplifiers, known as repeaters, are added to the cable at strategic points to boost each conversation to ensure it arrives loud and clear at the far end.

This submersible repeater was the first experimental model used to assess the technology. It was inserted into a large loop of cable that spiralled out on the bed of the Irish Sea, from Anglesey and back again. It was used to see whether the repeater worked underwater in live test conditions. It proved successful and the results were integrated into the new generation telephone submarine cables.

The equipment wasn't built to withstand the high water pressure experienced at the bottom of an ocean and its use would have been restricted to the connections on the Continental Shelf.

Amplification and loss

Amplification and loss : from a whimper to a bang

An amplifier makes a small signal larger and more powerful.

An electrical signal sent through a cable or over the airwaves gets smaller as the distances get longer. Before the signal becomes too weak it has to pass through an amplifier to make it usable.

Why not just start with a larger signal so as to have more at the receiving end? The problem is that you just lose even more power along the way -assuming that more power is practical anyway. In most cases it's simply better - also safer and cheaper - to accept a small signal and amplify it where needed.

Sometimes, however, repeater amplifiers are used at relay stations along the transmission path to boost the signal back up to full strength. This intermediate amplifier picks up a very weak signal and passes on a very strong one.


Noise : creating a disturbance

All communication channels suffer to some degree from noise or interference - try talking when someone's drilling the road next to you.

Noise makes it difficult to understand the actual information. In any electrical system, 'noise' is used to describe any electrical interference that degrades the original signal - the stuff you want.

Noise can come from an outside source - as when a nearby electric motor interferes with your radio station with a buzzing noise - or from the inside - for example the hiss of an audio amplifier that gets louder as you turn up the volume.

Modern communication systems use a wide range of techniques - 'noise reduction systems' - to try to eliminate noise, and the widespread move to digital systems is because the effect of noise is eliminated automatically.

fun and games

Can you beat our games? Explode equipment to see what's inside, hear the changing sounds of telecommunications, see how telecommunications designs have changed over time or send an e-postacard.

what's on

The UK's first permanent gallery dedicated to the history of information and communication technologies opens in the new Information Age gallery at London’s Science Museum.

audio history

Take a trip down memory lane with extracts of the interviews which have been recorded as part of the Connected Earth oral history programme.

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