Reduced-carrier transmission

Lore & Background

Reduced-carrier transmission is a form of amplitude modulation where the carrier signal level is intentionally lowered to conserve electrical power. In the extreme case of suppressed-carrier transmission, the carrier is reduced below the level needed for demodulation by a normal receiver. This reduction allows more power to be allocated to the sidebands, improving efficiency compared to conventional AM. To recover the original signal, the receiving station must restore the carrier power, typically using a beat frequency oscillator (BFO). If the BFO does not precisely match the original carrier frequency, a heterodyne results. Suppressed carriers are commonly employed in single sideband (SSB) transmissions, notably in amateur radio on shortwave, where the system is referred to as SSB suppressed carrier (SSBSC) or SSB-SC. International broadcasters agreed in 1985 to adopt SSBSC entirely by 2015, though the anticipated widespread adoption of IBOC and IBAC digital radio (specifically Digital Radio Mondiale) was expected to make this plan irrelevant. In FM stereo transmissions, a double-sideband suppressed carrier (DSBSC) signal is generated from a stereo generator, accompanied by a pilot tone at exactly half the original carrier frequency, enabling reconstitution of the stereo carrier and signal.

Reader's Guide

The significance of reduced-carrier transmission lies in its efficient use of transmitter power, as reducing or suppressing the carrier allows more energy to be directed into the sidebands that carry the information. This is particularly valuable in applications like amateur radio on shortwave, where power conservation and signal clarity are critical. International broadcasters agreed in 1985 to transition to SSB suppressed carrier (SSBSC) by 2015, but this plan was potentially rendered moot by the anticipated rise of digital radio standards such as IBOC, IBAC, and specifically Digital Radio Mondiale. The legacy of reduced-carrier transmission is also evident in FM stereo broadcasting, which uses a double-sideband suppressed carrier (DSBSC) signal with a pilot tone to reconstruct the stereo carrier. The technique requires careful carrier recovery at the receiver, often via a beat frequency oscillator, and any frequency mismatch introduces a heterodyne. Overall, reduced-carrier transmission represents a practical compromise between power efficiency and compatibility, with its most notable implementations in SSB and FM stereo systems.

Did You Know?

• Suppressed-carrier transmission is a special case of reduced-carrier transmission where the carrier level is below that required for normal receiver demodulation.
• Failure of the beat frequency oscillator to match the original carrier frequency when receiving a suppressed-carrier signal causes a heterodyne.
• International broadcasters agreed in 1985 to adopt SSB suppressed carrier (SSBSC) entirely by 2015.
• FM stereo transmissions use a double-sideband suppressed carrier (DSBSC) signal with a pilot tone of exactly half the original carrier frequency.