Carrier waves are waveforms that have been modified to transmit information, though an unmodified wave can also be called a carrier. Modification can occur in a number of different ways, such as the amplitude modulation (AM) and frequency modulation (FM) methods used for radio transmissions. Carrier waves are typically sine waves, but non-sinusoidal square waves can also be used in some cases. It is also possible for carriers to propagate as electromagnetic waves, or to be transmitted through physical mediums. Radio and over the air (OTA) television broadcasts are examples of technologies that rely on electromagnetically propagated carrier waves, while cable television uses a carrier wave that travels through a physical data infrastructure.
The most common type of carrier wave is a simple sine wave, which has a frequency that can be described in hertz (Hz), or oscillation per second. If it is not modified, this sine wave will simply repeat itself and never change. In radio and television applications, the amount of times a carrier wave oscillates each second is what determines the frequency of a station or channel. The FM band in many parts of the world consists of carrier waves that oscillate between about 88 million and 108 million times each second. This means that a radio station that broadcasts a frequency of 100mHz is sending out a carrier wave that oscillates 100 million times per second.
In order to convey useful information on a carrier wave, it must be modified in some way. This modification process is typically referred to as modulation, and it can be applied to either the frequency or amplitude of a carrier wave. In the case of frequency modulation, a data signal is used to modify the frequency of the carrier. When this occurs, the frequency of the carrier will increase as the amplitude of the data signal increases. Amplitude modulation works in a similar way, though the carrier amplitude is modified to conform to the amplitude of the data signal.
Carrier waves can be used in a number of different applications. In the case of radio and OTA television broadcasts, the carrier propagates in the form of an electromagnetic wave. When such a wave contacts the antenna of a receiver device, it can be demodulated to reconstruct the original data signal. It is also possible for a carrier wave to be sent through a physical medium, such as a cable television network. In this case, a number of carrier waves with different frequencies can share the same physical cable, which is known as frequency division multiplexing.