What Is Amplitude Modulation?

Amplitude modulation (AM) is a method of encoding information onto a carrier signal by varying the carrier's amplitude in proportion to the instantaneous value of a message signal. A sinusoidal carrier at frequency fc is multiplied by a function that includes the message signal, producing a waveform whose envelope follows the message waveform. The resulting spectrum contains the original carrier frequency and two symmetric sidebands, each displaced from the carrier by the baseband frequencies of the message. Amplitude modulation was the first widely deployed analog radio transmission technique and remains the basis of AM broadcast radio, short-wave communications, and several broadcast television vestigial sideband standards.

The theoretical framework draws on Fourier analysis and linear systems theory developed in the early twentieth century. In contrast to angle modulation methods, AM changes only the envelope of the carrier, leaving the instantaneous frequency constant, which simplifies certain demodulation schemes but makes the signal more susceptible to amplitude noise from atmospheric and man-made interference.

Modulation Variants

Conventional double-sideband full-carrier AM (DSB-FC) transmits both sidebands along with the full carrier, which allows simple envelope detection at the receiver but wastes power because the carrier itself carries no information. Double-sideband suppressed-carrier (DSB-SC) transmission removes the carrier, improving power efficiency to 100 percent of transmitted signal power going into the sidebands, but requires a coherent carrier reference at the receiver for demodulation. Single-sideband (SSB) modulation, a further refinement, eliminates one sideband entirely because each sideband already contains all of the message information; SSB occupies half the bandwidth of DSB-SC and is the standard for high-frequency (HF) voice communication and amateur radio, as described in the Stanford EE179 course materials on modulators and AM variants. Vestigial sideband (VSB) modulation suppresses most of one sideband while retaining a residual "vestige," offering a bandwidth compromise that proved practical for analog television broadcast, where the steep filter required for SSB would have distorted the sharp edges of video signals.

Demodulation

Recovering the message from an amplitude-modulated carrier requires separating the envelope from the carrier frequency. For conventional AM, envelope detection uses a diode and a low-pass filter to track the positive peak of the modulated waveform, forming a rectified output that approximates the message. This approach is incoherent: no carrier phase reference is needed. For DSB-SC and SSB, product detection (synchronous or coherent detection) multiplies the received signal by a locally generated carrier at the same frequency and phase, then low-pass filters the result. Phase errors in the local carrier introduce distortion, and phase-locked loops are commonly used to recover the carrier from pilot tones or from the signal itself. In intensity modulation of optical carriers, used widely in fiber-optic systems, the power of a laser is varied by the message signal, and direct detection at the receiver uses a photodiode as an envelope detector.

Applications

Amplitude modulation has applications in a wide range of fields, including:

• Medium-wave and short-wave AM broadcast radio
• Single-sideband HF communication links for aviation and maritime services
• Vestigial sideband modulation in analog television standards (NTSC, PAL, SECAM)
• Intensity modulation with direct detection in optical fiber communication systems
• Quadrature amplitude modulation (QAM) as the basis for digital cable, DSL, and wireless standards
• Amplitude shift keying in low-power wireless and RFID data links