How Do...

[ck1]

...radio receivers convert the electromagnetic waves back into the original sound / picture that was transmitted?

[heathfield]

A full explanation would take up a lot of space. Try Google for 'How AM radio works', and 'How FM radio works' to get links that describe it in more detail. But have a look here...

http://www.nrao.edu/index.php/learn/radioastronomy/radiocommunication

[bert_h]

It's called 'demodulation'. A steady electromagnetic wave carries no information at all. The sound or picture is applied to modulate it, and when the receiver separates the modulation from the carrier wave, it gets the picture or sound. The details of all this are a miracle of technical complexity, of course.

[ck1]

Thanks both, I think I get the concept now, I guess the 'miracle of technical complexity' is what I would like to understand but am thinking that is going to be way beyond me!!!

[Morris8]

Sound, such as speech or music, is carried through air as waves, or changes in pressure, at relatively low frequencies detectable by the human ear, typically up to about 12,000 Hertz (cycles per second) for an adult. A microphone converts such sound waves into electrical waves, or changes in voltage at the same frequencies. If fed directly to an aerial (antenna) such low-frequency electrical waves would not travel far. They are therefore superimposed on a high frequency “carrier” signal, say 150 to 1,600 KHz (thousands of cycles per second) which can travel long distances. In AM (amplitude modulation) the “height” (amplitude) of the carrier wave is changed (modulated) in accordance with the low frequency waves, obtained from the original sound, which are superimposed on them. A receiver detects these electrical changes in amplitude (or voltage), discards the carrier wave and feeds the changing voltages into a loudspeaker which changes them back into sound by causing a cone to vibrate. In FM (frequency modulation) instead of changing the size (amplitude) of the carrier wave, the superimposed low frequency signals slightly vary the frequency of the carrier wave but the principal remains the same. In digital broadcasting, the receiver effectively “counts” the number of electrical pulses (generated by a microphone from the original sound) transmitted per second and feeds the result into the loudspeaker. The same principles (of modifying a carrier signal) apply to TV with the original picture being scanned point by point and rebuilt point by point at the receiver but done so quickly that, with persistence of vision, the eye appears to see all the points simultaneously.