Background: Radio

Radio waves are just like light, except that their frequency is much lower - so low that they are completely invisible to our eyes. A radio transmitter is like a lighthouse, shining in invisible radio "light" over great distances.

If a metal rod of the appropriate length - an aerial - is placed in line-of-sight of the transmitter, the energy in the passing radio waves will set up a tiny oscillating electric current it. A suitably sensitive electronic circuit can then amplify this signal until it is powerful enough to drive a loudspeaker, producing the sound or music that we actually hear.

The radio wave is not the sound itself, of course. The signal representing the sound is carried by the wave, hitching along for the ride so to speak. Two main ways of impressing the signal onto the carrier wave are used, AM and FM.

AM, or amplitude modulation, means that the amplitude - or strength - of the carrier is varied in time with the signal to be transmitted. The carrier has a fixed frequency. AM: amplitude plotted against time

The second method is to keep the amplitude constant while varying the frequency of the carrier slightly, again in time with the signal. This is Frequency Modulation (FM):

The same signal Frequency Modulated. The amplitude is constant, while the frequency is varied around the average. The variation in frequency has been greatly exagerated; it is usually a tiny fraction of the central carrier frequency.

Both amplitude and frequency modulations can be separated from the carrier using a relatively simple electronic circuit called a demodulator. For both systems, the carrier frequency has to be higher than the maximum frequency in the original signal, and generally speaking, a higher carrier frequency means a higher quality signal.

Tuning

The transmitter at a radio station doesn't just broadcast one signal, but many. Each signal is at a slightly different carrier frequency, so that a receiver can select one station from amongst them, simply by ignoring everything other than the required frequency. This is what is happening as you move the tuning dial on your radio: you are adjusting the tuner to select a higher or lower carrier frequency.

Frequencies are measured in Hertz, where one Hertz (written 1Hz) means one cycle per second. The stations on your radio dial are marked in KHz or MHz, meaning kilohertz (thousands of cycles per second) or megahertz (millions), but TV signals are much higher, around one Gigahertz (thousands of millions). This is because a picture contains far more information than a simple sound wave, and a TV signal has to transmit scores of pictures every second.

A typical signal uses more than one frequency. For example, an FM signal deviates from the carrier frequency by a certain amount, so the signal as a whole contains frequencies both below and above the carrier frequency. The spread of frequencies used by a signal is known as the bandwidth, and in general the higher the bandwidth the more information can be transmitted.

No tuner can completely isolate one frequency. This limits how "near" to each other different radio stations can be: the closer the stations' frequencies, the harder it is to ignore those each side of the one you want. An AM signal uses a single frequency so has the smallest possible bandwidth, which explains why there are hundreds of stations on an AM dial, but only a few dozen on an FM dial. For TV, which uses a mixture of FM and AM, and has a very large bandwidth in order to transmit all the information required, only a few stations can be fitted into the waveband allocated to them.

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