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Exploring communications technology
Exploring communications technology

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1.3 Digital signals and modulation

Radio waves are naturally sinusoidal, with frequencies covering a wide range. They are capable of travelling through space, and are widely used for communication. This is a brief explanation of how they are able to carry information. Many of the same principles apply to other communication media, such as optical signals and electric currents.

Activity 1.3 Exploratory

Radio waves cover a wide range of frequencies, some of which are more suitable than others for a particular service. You can explore some uses of radio with this interactive chart.

Click on the image of the electromagnetic spectrum below to learn more about the highlighted part of the spectrum (radio and microwave frequencies). You will see that this part of the spectrum is conventionally divided into bands, each covering a decade in frequency (or wavelength). Make a note of the frequencies and wavelengths and the typical uses of each band.

[The radio and microwave frequencies interactive will open in a new window. After you have viewed the interactive, click on the link 1.3 Digital signals and modulation, to return to this page.]

Interactive feature not available in single page view (see it in standard view).

Generally a medium used for communication (such as radio waves) needs to be processed in some way to carry information. The process is called modulation. Two signals are combined in modulation:

  • The message signal, called the modulating signal. (Often this is non-periodic.)
  • A signal of the right frequency for transmission, called the carrier signal.

When they are combined, the modulating signal changes the carrier signal in some way, such as by changing its amplitude or frequency. This creates a new signal that contains the message information and is also at the correct transmission frequency. Note that although modulation of some kind is essential for wireless transmission, it is also used in much wired transmission, for example broadband and optical fibre.

In the next section, assume that the message to be sent is in the form of a digital signal (that is, a signal that is interpreted as a sequence of discrete values). In fact, most communications fall into this category; computer networks and almost all telephony, as well as digital TV and radio. Analogue signals such as speech are converted to digital form at one end of a communications link and back to analogue at the other. When the message signal is digital, modulation produces distinct states of the carrier wave that can be distinguished by the receiver and can be used to represent ones and zeros, or groups of ones and zeros. Next you will see some basic digital modulation schemes.