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Amplitude Modulation (AM)

Amplitude Modulation (AM) is a modulation technique used in radio transmission where the amplitude of the carrier wave is varied in accordance with the amplitude of the modulating signal (audio signal). This technique allows information (such as voice or music) to be transmitted over a radio carrier wave.

Key Concepts

Modulation Process

  • Carrier Wave: A high-frequency sine wave generated by the transmitter.
  • Modulating Signal: The audio signal containing the information to be transmitted. For example, the voice from a microphone or music from an audio source.
  • Amplitude Variation: The amplitude of the carrier wave is varied or modulated based on the instantaneous amplitude of the modulating signal.

Amplitude Modulation Equation

The amplitude-modulated signal can be represented mathematically as:
\[ A(t) = A_c [1 + m \cdot \sin(2\pi f_m t)] \cdot \cos(2\pi f_c t) \]

  • \( A(t) \) is the amplitude-modulated signal.
  • \( A_c \) is the amplitude of the carrier wave.
  • \( m \) is the modulation index, representing the depth of modulation.
  • \( f_m \) is the frequency of the modulating signal.
  • \( f_c \) is the frequency of the carrier wave.

Signal Transmission

  • Broadcasting: AM signals are used for broadcasting radio signals, especially for long-distance transmission due to their ability to travel farther compared to higher-frequency signals like FM.
  • Demodulation: At the receiver, the AM signal is demodulated to extract the original modulating signal (audio) by recovering the amplitude variations imposed on the carrier wave.


Radio Broadcasting

  • AM Radio: Historically, AM radio has been widely used for broadcasting news, music, and other content over long distances.
  • Shortwave Radio: AM is also used in shortwave broadcasting, where signals can propagate over large distances by bouncing off the ionosphere.

Communication Systems

  • Telecommunication: AM is used in various communication systems where simplicity and long-distance propagation are critical factors.

Advantages and Disadvantages


  • Coverage: AM signals can cover long distances and penetrate obstacles better than higher-frequency signals.
  • Cost-effective: AM transmitters and receivers are simpler and less expensive compared to FM counterparts.


  • Signal Quality: Susceptible to noise and interference, which can degrade the quality of the transmitted audio.
  • Bandwidth Efficiency: Less efficient use of bandwidth compared to FM, limiting the number of channels available in a given frequency range.

Amplitude Modulation (AM) is a fundamental modulation technique used in radio transmission to encode information on a carrier wave by varying its amplitude. Despite its limitations in signal quality and bandwidth efficiency, AM remains crucial in broadcasting and communication systems, offering reliable long-distance transmission capabilities for audio content and other data.

See Related Terms:

  • Quadrature Amplitude Modulation
  • Frequency modulation
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