Reduced-carrier transmission
AM transmission reducing carrier level to save power and boost sidebands.
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Reduced-carrier transmission is a form of amplitude modulation where the carrier signal's strength is lowered to conserve electrical power. When the carrier is reduced so much that a standard receiver can no longer demodulate it, this is known as suppressed-carrier transmission. Lowering the carrier level allows more power to be directed into the sidebands compared to conventional AM. To recover the original signal, the receiving station must restore the carrier, typically using a beat frequency oscillator.
If this oscillator does not precisely match the original carrier frequency, a heterodyne will result. Suppressed carriers are commonly used in single sideband transmissions, such as amateur radio on shortwave, a system called SSB suppressed carrier (SSBSC or SSB-SC). In 1985, international broadcasters agreed to adopt SSBSC entirely by 2015, though the expected rise of IBOC and IBAC digital radio—specifically Digital Radio Mondiale—likely makes this plan irrelevant.
FM stereo transmissions employ a double-sideband suppressed carrier signal from a stereo generator, along with a pilot tone at exactly half the original carrier frequency, which enables the original stereo carrier and thus the stereo signal to be reconstructed.
Quick Facts
- Type
- amplitude modulation
- Special case
- suppressed-carrier transmission
- Demodulation requirement
- carrier power must be restored by receiving station, usually via beat frequency oscillator (BFO)
- Application
- single sideband (SSB) transmissions, such as for amateur radio on shortwave
- Full name ssb
- SSB suppressed carrier (SSBSC) or SSB-SC
- International broadcaster agreement year
- 1985
Facts from the source article.
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.
More in Radio Modulation Modes
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Reduced-carrier transmission (CC BY-SA 4.0).
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