Modulation

Modulation is a technique for impressing information (voice, music, picture, or data) on a radio-frequency carrier wave by varying one or more characteristics of the wave in accordance with the signal. There are various forms of modulation, each designed to alter a particular characteristic of the carrier wave. The most commonly altered characteristics include amplitude ( AM), frequency (FM) and phase (PM).

Amplitude Modulation

Amplitude modulation (AM) is the modulation method used in the AM radio broadcast band. In this system the intensity, or amplitude, of the carrier wave varies in accordance with the modulating signal. When the carrier is thus modulated, a fraction of the power is converted to sidebands extending above and below the carrier frequency by an amount equal to the highest modulating frequency. This form of modulation is not a very efficient way to send information; the power required is relatively large because the carrier, which contains no information, is sent along with the information.
In a variant of amplitude modulation, called single sideband modulation (SSB), the modulated signal contains only one sideband and no carrier. The information can be demodulated only if the carrier is used as a reference. This is normally accomplished by generating a wave in the receiver at the carrier frequency.
The upper and lower sidebands contain the same information. By eliminating the duplicated information in the lower sideband and the carrier, the transmitter efficiency is greatly increased. In effect, the space taken within the frequency band is reduced and so more stations are able to transmit. A narrower bandwidth for the transmitted signal means that less noise and interference is apparent at the receiver, resulting in a relatively smaller masking effect on the wanted transmission. Also, for the same transmitter power the effective range of the transmission will be greatly extended by using SSB modulation.

Frequency and Phase Modulation

In frequency modulation (FM) and phase modulation (PM), the frequency of the carrier wave is varied in such a way that the change in frequency is proportional to another signal that varies with time. Its principal application is also in radio, where it offers increased noise immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth.

The International Telecommunication Union uses an internationally agreed system for classifying radio frequency signals. Each type of radio emission is classified according to its bandwidth, method of modulation, nature of the modulating signal, and type of information transmitted on the carrier signal. It is based on characteristics of the signal, not on the transmitter used. Emissions shall be classified and symbolized according to their basic characteristics and any optional additional characteristics.

The basic characteristics are:

1) first symbol - type of modulation of the main carrier;

N - Unmodulated carrier
A - Double-sideband amplitude modulation
H - Single-sideband with full carrier
R - Single-sideband with reduced or variable carrier
J - Single-sideband with suppressed carrier
F - Frequency modulation
G - Phase modulation

2) second symbol - nature of signal(s) modulating the main carrier;

0 - No modulating signal
1 - One channel containing digital information, with no subcarrier
2 - One channel containing digital information, using a subcarrier
3 - One channel containing analogue information

3) third symbol - type of information to be transmitted.

N - No transmitted information
A - Aural telegraphy, intended to be decoded by ear, such as Morse code
B - Electronic telegraphy, intended to be decoded by machine
C - Facsimile
D - Telemetry or Telecommand
E - Telephony (voice or music intended to be listened to by a human)
F - Video (television signals)

Some of MF/HF maritime radios give the operator the possibility to choose emission. In GMDSS for RT on MF and HF band, emission "J3E" is used. DSC (Digtal Selective Calling) system utilizes "F1B/J2B" emissions on MF/HF bands.