A pure wave of energy carries no information but modulators can be used to impose another wave (the input signal) which changes the shape of the carrier wave in order to encode data or information to be transmitted. This process keeps communications cheap, convenient and fast and requires less power to carry as much information as possible. The word 'modem' in fact means modulator and demodulator together so this is their most common use. The modem allows computers to communicate with each other over telephone lines. There are other types of modulators but they can be categorised as amplitude (AM), frequency (FM), phase or polarisation. The phase modulator can be further divided into analogue, which varies continuously like a sine wave or digital, when the sound is sampled, compressed and turned into a stream of 1/0. Polarisation is carried over free space or optical fibres. Amplifier modulators are actuators which hold the intensity of a laser beam. Electromagnetic signalling modulators are mainly used to transmit radio and television stations. The digital version allows multiple signals to be sent simultaneously by assigning each channel its own frequency. Power line networking can be applied to AC or DC current by turning it on and off.

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