Modulation is the process in which the information to be conveyed by radio is added to a radio 'frequency carrier'. The information is the message that you want to send, either voice or data. The radio 'frequency carrier' is the frequency to which you tune your receiver and transmitter. This modulation process can be carried out in several different ways.
Frequency and Modulation, FM
VHF transmissions use Frequency Modulated signals which vary the frequency of the radio signal to convey their information. This technique allows a great deal of information to be transferred which, together with the 'capture effect' associated with VHF, makes it ideal for the transmission of high quality audio, such as music. The disadvantage is the large bandwidth required and the limited range.
Amplitude Modulation, (AM)
As the name suggests, Amplitude Modulated, (or AM) radio signals are transmissions where the frequency of the signal remains the same but the amplitude of the radio wave has been changed, (or 'modulated' ), It is these changes in amplitude that are used to convey information. A.M.radio signals have two sub-divisions; 'Double Side-Band' (DSB) and 'Single Side Band' (SSB) transmissions.
Double Side Band, (DSB)
The above illustration shows a 'Double Side-Band Amplitude Modulated' signal which was the easiest and simplest form of AM radio. The disadvantage of using a DSB transmission is that it takes up a comparatively large bandwidth which in turn means it requires a great deal of power to transmit the signal and is prone to interference from other AM stations. DSB is no longer used in Maritime communications.
Single Side Band, (SSB)
Looking again at the Double Side-Band illustration it can be seen that the signal is symmetrical, one half of the signal exactly mirrors the other half. It follows that only one half, (or band), of this signal is actually required to convey information. A Single Side-Band transmission is a signal where one half of the radio signal has been electronically filtered out. This halves the bandwidth required and reduces the power needed by the transmitter with no real loss of signal information. The disadvantages are that it requires more sophisticated equipment than is necessary with Double Side-Band. The full name for this type of transmission is an Amplitude Modulated, Single Side-Band, Full Carrier signal. It is more simply referred to as H3E mode and is now no longer used.
An even greater saving in bandwidth can be made by using even more sophisticated equipment and reducing the carrier wave to nothing and only transmitting the changes in amplitude. This type of AM signal is referred to as a Single Side-Band, Suppressed Carrier signal, or J3E mode, and is used in all Lifeboat MF/HF transmissions. Telex transmissions, (such as NAVTEX), are made using F1B mode.