Difference Between Amplitude Modulation and Frequency Modulation

Main Difference – Amplitude Modulation vs. Frequency Modulation

Amplitude modulation and frequency modulation are both methods of modifying a carrier signal in order to transmit data. The main difference between amplitude modulation and frequency modulation is that, in amplitude modulation, the amplitude of the carrier wave is modified according to the data whereas, in frequency modulation, the frequency of the carrier wave is modified according to the data.

What is Modulation

When several different sets of data need to be transferred using the same medium, each set of data can be sent along using waves of different frequencies. This is how radio broadcasts are transmitted. So, the “frequency” of a radio station refers to the frequency chosen by the radio station to broadcast its music. The chosen frequency is referred to as the carrier frequency. If a simple wave of the carrier frequency is sent, it would be just a monotonous beep! In order to transmit some data, a sine wave with the carrier frequency needs to be modified in some way. Modulation refers to this process of modification.

What is Amplitude modulation

In amplitude modulation, the amplitude of the carrier signal is modified in order to send the data. This is most easily understood through a diagram:

Amplitude modulation

Whenever the amplitude of the information signal increases, the amplitude of the carrier signal is increased in proportion. The receiver of the signal should be connected to a demodulator, which would then monitor the changes in amplitude and recreate the information signal.

AM radio is transmitted using amplitude modulation. AM radio stations use lower carrier frequencies compared to those used by FM radio stations. As a result, they are able to travel farther than FM signals. AM signals also have the ability to bounce off the ionosphere and travel over much larger distances. This is how shortwave radio stations can be heard from thousands of kilometres away (Even though in radios, shortwave signals (SW) are separated from AM signals, they both use amplitude modulation)!

What is Frequency Modulation

In frequency modulation, the frequency of the carrier wave is modified according to the information signal. Here, the instantaneous frequency of the carrier wave is shifted slightly, depending on the amplitude of the data signal. FM radio signals have a larger bandwidth compared to AM radio signals and therefore, the sound quality offered by FM is much better. In addition, the larger bandwidth in FM radios also allows for the transmission of “stereo” signals. This is something that AM signals cannot do. Since the amplitude of FM signals does not change, it is also very easy to pick out any noise that get added to the signal along the way (any noise would change the amplitude of the signal).

The diagram below depicts frequency modulation:

Frequency modulation

Difference Between Amplitude Modulation and Frequency Modulation

Type of Modulation

In amplitude modulation, the amplitude of the carrier wave is modified in order to transmit information.

In frequency modulation, the instantaneous frequency of the carrier wave is modified in order to transmit information.

Sound Quality

The sound quality offered by AM radio stations is poorer, having low fidelity and easily being affected by noise.

The sound quality of FM radio stations is better, and stereo signals can be transmitted using FM.

Range

AM radio stations have a larger range.

FM radio stations have a smaller range.

Image Courtesy

“The illustration of amplitude modulation (AM) which depicts comparison between information signal, carrier signal, and AM signal.” by Ivan Akira (Own work) [CC BY-SA 3.0], via Wikimedia Commons

“The illustration of frequency modulation (FM) which depicts comparison between information signal, carrier signal, and FM signal.” by Ivan Akira (Own work) [CC BY-SA 3.0], via Wikimedia Commons

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