Types of Modulation
Further information: ModulationSeveral different types of modulation are used to impress information on a short-wave transmission.
Amplitude modulation is the simplest type and the most commonly used for shortwave broadcasting. The instantaneous amplitude of the carrier is controlled by the amplitude of the signal (speech, or music, for example). At the receiver, a simple detector recovers the desired modulation signal from the carrier.
Single sideband transmission is a form of amplitude modulation but in effect filters the result of modulation. An amplitude-modulated signal has frequency components both above and below the carrier frequency. If one set of these components is eliminated as well as the residual carrier, only the remaining set is transmitted. This saves power in the transmission, as roughly 2/3 of the energy sent by an AM signal is unnecessary to recover the information contained on it. It also saves "bandwidth", allowing about one-half the carrier frequency spacing to be used. The drawback is that the receiver is more complicated, since it must re-recreate the carrier to recover the signal. Small errors in the detector process can greatly affect the pitch of the received signal, so single side band is not usual for music or general broadcast. Single side band is used for long-range voice communications by ships and aircraft, Citizen's Band, and amateur radio operators. LSB (lower sideband) is generally used below 9 MHz and USB (upper sideband) above 9 MHz.
Vestigal sideband transmits the carrier and one complete side-band, but filters out the redundant side-band. It is a compromise between AM and SSB, allowing simple receivers to be used but requiring almost as much transmitter power as AM. One advantage is that only half the bandwidth of an AM signal is used. It can be heard in the transmission of certain radio time signal stations.
Continuous wave (CW) is on-and-off keying of a carrier, used only for Morse code communications.
Narrow-band frequency modulation (NBFM) is mainly used in the higher HF frequencies (typically above 20 MHz). Because of the larger bandwidth required, NBFM is much more commonly used for VHF communication. Regulations limit the bandwidth of a signal transmitted in the HF bands, and the advantages of frequency modulation are greatest if the FM signal is allowed to have a wider bandwidth. NBFM is limited to short-range SW transmissions due to the multiphasic distortions created by the ionosphere.
Digital Radio Mondiale (DRM) is a digital modulation for use on bands below 30 MHz.
Radioteletype, fax, digital, slow-scan television and other systems use forms of frequency-shift keying or audio subcarriers on a shortwave carrier. These generally require special equipment to decode, such as software on a computer equipped with a sound card.
Read more about this topic: Shortwave Radio
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