Amplitude modulation is a method of embedding information in radio waves by changing the amplitude of a carrier signal with an audio signal. It is used in two-way radios and radio broadcasting, but has been replaced by more efficient methods like frequency modulation. Amplitude modulation is inefficient and subject to background noise. Demodulation is necessary to remove the carrier signal and hear the audio signal.
In order for information to be transmitted over radio waves, the information must somehow be embedded in the radio waves. The first method developed for incorporating information is known as amplitude modulation. Amplitude modulation is a way of changing a carrier signal with an audio signal so that the audio signal can be transmitted to a receiver. It is used today in two-way radios and radio broadcasting. AM radio in AM stands for amplitude modulation.
Amplitude is the amount of energy in a sound wave. Loud sounds are produced by high amplitude waves while quieter sounds are produced by lower amplitude waves. Frequency is the number of waves that pass through a fixed point during a given period of time. During amplitude modulation the frequency of the carrier signal remains constant but its amplitude changes to match the amplitude of the audio signal. To avoid distortion, the amplitude of the carrier should be greater than that of the audio signal.
The device used to produce the amplitude modulation is a non-linear device known as a modulator. It is called a non-liner device because it has two inputs, the carrier and the audio signal, but only one output, the modulated signal. This output is transmitted to a receiver, where it then has to be broken down again into its component parts in a process known as demodulation.
Demodulation removes the audio signal from the carrier signal. Without this step it would be impossible to hear the audio signal through the interference created by the carrier signal. There are several types of demodulators, but all of them are for stripping the carrier signal so that the audio signal can be heard.
The main advantage of amplitude modulation is that it is a cheap way to transmit radio signals because it requires only a few easily acquired parts. Yet, in many cases, it has been replaced with frequency modulation (FM) and other forms of broadcasting. The reason is twofold: it is inefficient and is subject to high levels of background noise.
Amplitude modulation is inefficient in using both power and bandwidth. A large percentage of the required power is consumed by the carrier signal, which is then removed. It also uses nearly twice the bandwidth of other transmission methods. A practical consequence of this is that fewer radio stations are able to operate in a given area. Amplitude modulation is also sensitive to background noise which can result in poor audio quality.
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