Advantages of FM over AM

Here is a list of advantages and disadvantages of FM and AM radio systems. These systems rely on a carrier wave, which is an electromagnetic wave that travels through the air. Hence, these systems were termed wireless. These waves are high in frequency and contain energy enabling them to travel instantaneously at the speed of light. By encoding them, using either the FM, or AM encoding methods, they are able to carry useful information. These encoding systems, known as modulation, are not perfect and both have advantages and disadvantages.

Advantages of FM

  • FM radio systems employ frequency modulation of the carrier wave to encode information. Since the information is stored in the frequency variation, it is immune to any corruption in amplitude.
  • The quality of an FM signal remains high even when the transmitter power is low.
  • The FM band is used for the purposes of entertainment broadcasting in the 88 MHz to 108 MHz VHF band.
  • The FM Bandwidth is approximately 200 kHz, which is much wider than that of AM, allowing it to carry more information.
  • FM signals usually have low amplitude and therefore the carrier does not require much power, and almost all the power goes into the sideband where the information is. This makes it a more power efficient system over AM.

Disadvantages of FM

  • Frequency modulation is more complex and requires two additional radio stages. A limiter stage corrects any corruption in amplitude. The discriminator converts variations in frequency into variations in amplitude thereby recovering the original audio signal.
  • Mountains and large buildings easily block FM signals.
  • An FM signal occupies large bandwidth, which places a serious limit on the number of broadcast stations on the FM band. FM transmissions occupy a wide bandwidth because the information is stored by varying the frequency, which inherently results in the production of more sidebands.
  • The FM hiss usually increases when the signal is weak.
  • FM signals have a small range and tend to be local stations.

Advantages of AM

  • AM radio systems employ amplitude modulation of the carrier wave to encode information. This is the simplest method to encode and decode radio signals and achievable with small scale electronic circuits.
  • AM signals in the LW band have a very long range and can travel far to other countries.
  • AM transmission in the range 540 kHz to 1600 kHz occupies a very narrow bandwidth, which allows many broadcast frequencies to operate close to each other.

Disadvantages of AM

  • Since useful information is contained within the amplitude of the signal, it is very easy for it to corrupt because the amplitude of an electromagnetic wave can change easily through interaction with other electromagnetic noise.
  • To keep the signal quality high, the transmitter has to be high power.
  • The AM bandwidth is very narrow at approximately 10 kHz.
  • The carrier signal takes a huge amount of power during transmission, which in the end the receiver discards. Approximately ¾ of the power is within the carrier, whilst only a ¼ in the sideband where the data is.

Further Reading

The modern day DAB transmissions are also in the VHF band. They occupy the range 217.5 MHz to 230 MHz. However, these transmissions do not carry raw audio as in FM. Instead, they carry compressed data that is very similar to an MP3 data file. In DAB broadcasting, they group the channels in multiplexes. These transmissions are not immune to electromagnetic noise either, however since the information sent is binary data, the receivers use error correcting algorithms to remove corruption in the data. This enables the signal to remain in near perfect condition.