Modulation is the process of encoding information (in aviation, primarily voice) onto a radio wave carrier signal. The carrier signal is a pure sine wave at the assigned frequency (e.g., 118.100 MHz). Without modulation, the carrier alone carries no information—it is just a pure tone.

In aviation radiotelephony, the standard modulation technique is AM (Amplitude Modulation). In AM, the amplitude (strength) of the carrier wave is varied in synchrony with the audio signal (your voice). As you speak, the microphone converts sound waves into electrical signals, and these signals vary the amplitude of the carrier. The receiver detects these amplitude variations and converts them back into audio for the listener.

The relationship between the audio signal and the carrier is quantified by the modulation index. The modulation index represents how much of the carrier's power is used to encode the audio information. A modulation index of 100% is ideal—it means the modulation is optimized to use the carrier power efficiently. If the modulation index is too low (under-modulation), the receiver hears a weak voice signal. If it is too high (over-modulation), the transmitted signal splitters into adjacent channels, causing interference.

Your aircraft's microphone is equipped with a push-to-talk switch that activates the transmitter only when you are speaking. When you release the switch, transmission stops. This half-duplex operation ensures that you do not accidentally interfere with others while listening. It also conserves power and prevents continuous transmission of noise.

The modulation quality is affected by: