subcarrier
A separate signal carried on a main radio transmission, often carrying additional information such as voice or data.
subcarrier: a signal riding inside another signal
A subcarrier is a secondary radio frequency (RF) signal that modulates a primary carrier wave. In broadcast and communication systems, it allows transmission of multiple information streams on a single main frequency without interference. The subcarrier sits at a lower frequency than the main carrier but occupies bandwidth within the main transmission's spectral envelope.
FM stereo radio is the most familiar example. The main carrier at 88 to 108 MHz carries the left plus right (L+R) audio as baseband mono. A 38 kHz subcarrier then carries the left minus right (L-R) stereo difference signal. A receiver detects both, recovers them separately, and reconstructs stereo sound. The 19 kHz pilot tone helps the receiver lock to the subcarrier frequency and decode it correctly.
Television systems use subcarriers extensively. Analogue TV broadcasts carry video on the main carrier and audio on a separate sound subcarrier offset by a fixed amount, typically 4.5 MHz for NTSC. Chroma (color) information travels as another subcarrier above the luminance signal. Digital video broadcasting systems use multiple orthogonal frequency-division multiplexing (OFDM) subcarriers to transmit data in parallel across a channel, improving spectral efficiency.
Subcarrier variants and applications
In satellite and deep-space communications, subcarriers modulate the main RF signal carrying telemetry, command data, or ranging tones. The subcarrier itself may be modulated with phase-shift keying (PSK) or frequency-shift keying (FSK) to encode the actual information. This two-stage modulation isolates the modulating signal from the raw RF domain and improves demodulation and signal recovery at the receiver.
Frequency stability and separation matter critically. Subcarrier frequencies must be chosen to avoid cross-talk; they are often harmonically related to a reference frequency or spaced according to filter bandwidth limits. In systems with multiple subcarriers, phase coherence between them must be maintained to prevent bit errors or crosstalk. Equipment failure usually shows as loss of subcarrier lock, audible noise in audio channels, or missing color information in video, symptoms that help technicians isolate whether the problem lies in subcarrier generation, transmission, or detection.