Übermodulation

Übermodulation, or overmodulation, refers to a condition in amplitude modulation where the modulating signal causes the instantaneous amplitude of the carrier to be modulated beyond the carrier amplitude itself. This leads to envelope distortion and, in many cases, distortion in the recovered signal at the receiver. The phenomenon occurs when the modulation index exceeds unity (|m| > 1).

In the standard AM model, the transmitted signal can be written as s(t) = Ac [1 + m cos(ωm t)] cos(ωc t), where Ac is the carrier amplitude and m is the modulation index defined as the peak amplitude of the modulating signal relative to the carrier (m = Vm/Ac). When m ≤ 1, the envelope never reverses sign; when m > 1, the envelope crosses zero and can become negative, causing distortion in envelope detectors and changing the sideband content.

Consequences of übermodulation include distorted audio on the received channel, cliping effects in the envelope detector, and inadvertent spectral components that may extend beyond the intended sidebands, potentially causing interference with adjacent channels. Detectability and intelligibility degrade, particularly in conventional AM receivers that rely on proper envelope recovery.

Prevention involves keeping the modulation index at or below unity, using limiting or pre-distortion, careful design of the modulator chain, and monitoring with suitable test equipment to ensure the envelope remains within bounds. In broadcasting practice, moderate headroom is often provided to avoid übermodulation and preserve signal quality.