subcarrier

Subcarrier refers to a secondary carrier frequency used to convey information in a communication system, derived from a primary carrier or generated alongside it. A subcarrier enables multiplexing of multiple signals, allowing a channel to carry additional data, control signals, or stereo information without requiring a separate spectrum. Subcarriers are modulated with their own baseband signals, and their spectra are offset from the main carrier's frequency.

In analog broadcast, subcarriers are used to carry auxiliary channels. A well-known example is FM stereo, where the left and right audio difference signal is transmitted on a 38 kHz subcarrier that is double-sideband suppressed-carrier, with a 19 kHz pilot tone used to aid demodulation and synchronization. In television, subcarriers carry audio and other data signals alongside the main video signal; different standards allocate subcarriers at specific frequencies within the allocated channel.

In digital communication, the term subcarrier is prominent in two contexts. In subcarrier multiplexing (SCM), multiple signals modulate separate subcarriers that share a higher-frequency carrier to increase throughput. In orthogonal frequency-division multiplexing (OFDM), a large number of closely spaced orthogonal subcarriers carry parallel data streams, enabling efficient use of bandwidth and robustness to multipath. Subcarriers may also be used in powerline, radio, or optical communications for multiplexing.