A “carrier” refers to a primary signal that is being modulated or altered by another signal, typically called the “modulator.” The term “carrier” is most commonly used in the context of frequency modulation (FM) and amplitude modulation (AM) synthesis techniques.
In frequency modulation synthesis, the carrier is the primary oscillator whose frequency is being modulated by the output of another oscillator, the modulator. The modulator signal’s frequency and amplitude determine the amount of frequency deviation and the complexity of the resulting sound.
In amplitude modulation synthesis, the carrier is the primary audio signal whose amplitude or volume level is being modulated by another signal, usually an oscillator or an envelope. The modulator signal’s amplitude and frequency shape the resulting amplitude variations of the carrier signal, creating tremolo-like effects or more complex modulation patterns.
In both cases, the carrier signal is the base sound or waveform that is being transformed by the modulator, resulting in new and complex timbres.