Radio Modulation

Audio-frequency waves must be combined with carrier waves in order to be transmitted over the radio. Either the frequency (rate of oscillation) or the amplitude (height) of the waves may be modified in a process called modulation. This accounts for the option on the radio dial for AM or FM stations; the signals are very different, so both kinds may not be received simultaneously.

The shaping of a signal to convey information is known as modulation. Modulation can be used to represent a digital message as an analogue waveform. This is known as keying and several keying techniques exist (these include phase-shift keying, frequency-shift keying and amplitude-shift keying). Bluetooth, for example, uses phase-shift keying to exchange information between devices. The simplest form of modulation is keying, interrupting the carrier wave at intervals with a key or switch used to form the dots and dashes in continuous-wave radiotelegraphy.

Modulation can also be used to transmit the information of analogue signals at higher frequencies. This is helpful because low-frequency analogue signals cannot be effectively transmitted over free space. Hence the information from a low-frequency analogue signal must be superimposed on a higher-frequency signal (known as the carrier wave) before transmission. There are several different modulation schemes available to achieve this (two of the most basic being amplitude modulation and frequency modulation). An example of this process is a DJ's voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel "96 FM").