Amplitude modulation

Amplitude modulation (AM) is a fundamental signal‐modulation technique used in radio communication to encode information by varying the instantaneous amplitude of a carrier wave. It has played a central role in the development of broadcasting and telecommunication systems and remains in use across a wide range of analogue and digital applications. AM represents one of the earliest forms of radio modulation, with its origins in experimental radiotelephony at the start of the twentieth century. Although many modern communication systems employ more efficient or more noise-resistant modulation methods, amplitude modulation continues to be important in broadcasting, aviation communication, amateur radio, and digital modem technologies.

Principles and General Description

In electronic communication, modulation involves varying a property of a high-frequency carrier wave—usually a sine wave—according to a lower-frequency information signal. For amplitude modulation, the carrier’s amplitude is altered in direct proportion to the instantaneous value of the message signal, which may be analogue (such as speech) or digital (such as binary data). The carrier itself is of much higher frequency than the information signal and serves as the transport medium for the encoded information.
A generic mathematical expression for a modulated sine wave highlights the role of each component: the time-varying amplitude term represents the modulating influence, while the angular frequency and phase describe the carrier. When amplitude modulation is used, the amplitude term varies with the input message whereas the frequency and phase remain constant; conversely, in angle modulation techniques such as frequency or phase modulation, the amplitude remains fixed while the message alters the frequency or phase.
Amplitude-modulated radio signals consist of a strong carrier component accompanied by two sidebands equal in bandwidth and symmetrical around the carrier frequency. These sidebands contain the encoded information. The presence of both upper and lower sidebands defines standard double-sideband amplitude modulation (DSB-AM), the form most familiar from broadcast AM radio.

Types and Variants of Amplitude Modulation

Standard AM has several important variants developed to improve efficiency and reduce bandwidth:

• Double-sideband amplitude modulation (DSB-AM): The original and most widespread form, in which both sidebands and the full carrier are transmitted.
• Single-sideband modulation (SSB): One sideband is removed, and optionally the carrier is suppressed. This halves the required bandwidth and greatly improves power efficiency.
• Double-sideband suppressed-carrier (DSB-SC): Both sidebands are retained but the carrier is removed, increasing efficiency but requiring more complex receivers.
• Reduced-carrier transmissions (DSB-RC): A pilot carrier is transmitted at reduced power for demodulation purposes.
• Quadrature amplitude modulation (QAM): A form of two-dimensional amplitude modulation widely used in digital modems, combining amplitude and phase variations to encode multiple bits per symbol.

In the digital domain, amplitude-shift keying (ASK) maps binary states to the presence or absence of a carrier, or to discrete amplitude levels. A simple variant, on–off keying, forms the basis of Morse code transmission in radio systems.

Frequency-Domain Characteristics and Performance

In the frequency domain, amplitude modulation produces a spectrum with three main components: the carrier at the original frequency and two sidebands offset by the modulating frequency. The combined bandwidth equals twice that of the message signal. Because the carrier contains no information, traditional AM is power-inefficient, with the majority of transmitted energy residing in the carrier. Nevertheless, the carrier’s presence allows straightforward envelope detection, which is a significant advantage for inexpensive and simple receivers.
A principal drawback of amplitude-modulated systems is their sensitivity to noise. Any unwanted variation in amplitude caused by atmospheric interference or electrical noise is indistinguishable from intentional modulation, leading to reduced audio quality. As a result, AM is better suited to speech-based communication than to high-fidelity audio broadcasting. Frequency and phase modulation systems, by contrast, are less susceptible to amplitude-related noise when the received signal is sufficiently strong.

AM in Analogue and Digital Telephony

A basic form of amplitude modulation appears in the analogue telephone system. The direct current supplied through the local loop from the central office acts as a zero-frequency carrier. Speech signals from the telephone’s microphone modulate this current, producing an amplitude-varying signal that is separated into its alternating component for onward transmission. Although primitive in concept, this mechanism demonstrates the universality of AM as a method of encoding information.

Demodulation and Signal Recovery

At the receiving end, demodulation is used to extract the original message from the modulated carrier. For standard AM with an intact carrier, envelope detectors provide a simple means of recovering the signal by tracking the outline of the modulated waveform. More sophisticated demodulators are needed for suppressed-carrier systems, which require precise carrier regeneration. Techniques such as the Costas loop generate the necessary reference signal in DSB-SC systems, though this approach is ineffective for single-sideband suppressed-carrier signals, which require highly accurate tuning to avoid distorted audio.
The carrier in traditional AM also provides an amplitude reference for automatic gain control (AGC) in receivers. AGC maintains a consistent output volume despite variations in signal strength. Suppressed-carrier systems lose this reference, so AGC must operate from peaks in the modulation, which is adequate for speech but unsuitable for faithful music reproduction.

Applications and Broadcasting Uses

AM broadcasting has served as a cornerstone of radio communication since the early twentieth century. Medium-wave and shortwave AM stations remain widespread due to their long-distance propagation characteristics and compatibility with inexpensive receivers. Beyond broadcasting, AM is integral to aviation communication in the VHF airband, citizens’ band radio, maritime communication, and various two-way radio services. In digital communication, QAM remains a dominant modulation method for cable modems, Wi-Fi, and other broadband systems.

Historical Development

Experiments with amplitude-modulated signals date to the late nineteenth century, particularly in telegraphy and early telephony. The first recognised AM transmission of intelligible speech occurred on 23 December 1900, when Reginald Fessenden transmitted a short audio message across a short distance using a spark-gap transmitter and specially designed induction coil. Though the spark-generated carrier produced substantial noise, the demonstration marked a milestone in wireless communication.
Early AM transmitters used high-power arc devices, such as those developed by Telefunken in the early 1900s, before the invention of thermionic valves made continuous-wave transmission and amplification more feasible. The introduction of triode valves enabled smaller, more efficient AM transmitters, such as those built by Meissner in 1913, paving the way for commercial broadcasting. By the early twentieth century, AM radio had become the dominant radio format worldwide, establishing the foundations for modern wireless communication.