Modulation is a process whereby an information signal modifies a second signal called the carrier. In demodulation, the information signal is recovered from the modulated carrier.
Since most carriers are sinusoids, we can write the carrier as
Various modulation schemes have been designed to vary A (amplitude modulation), ωc (frequency modulation), or ø (phase modulation). Varying ωc or φ is sometimes called angle modulation. Note that with angle modulation, the amplitude of the modulated carrier does not vary. This leads to the language that an angle-modulated carrier has constant envelope.
In analog ...