Digital radio signals maintain their quality better than analogue signals. They are less prone to interference. All signals become weaker as they travel long distances, and they may also pick up random extra signals. This is called noise, and it is heard as crackles and hiss on radio programmes.
Noise adds extra random information to analogue signals. Each time the signal is amplified, the noise is also amplified. Gradually, the signal becomes less and less like the original signal. Eventually, it may be impossible to make out the music in a radio broadcast against the background noise, for example.
Noise also adds extra random information to digital signals. However, this noise is usually lower in amplitude than the amplitude of the ON states. As a result, the electronics in the amplifiers can ignore the noise, and it does not get passed along.
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A radio receiver does not need to be directly in view of the transmitter to receive programme signals. The lowest frequency radio waves are reflected from an electrically charged layer of the upper atmosphere, called the ionosphere. This means they can reach receivers that are not in the line of sight, because of the curvature of the Earth’s surface.
Signals are diffracted as they leave a transmitter dish. This causes them to spread out. Diffraction can be reduced by using larger dishes.
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