Modulation is the process of encoding information from a message source in a way that is suitable for transmission. This is achieved by altering the characteristics of a wave. By superimposing a message on to a high frequency signal known as a carrier wave (or sinusoidal signal), video, voice and other data can be transmitted.
In the modulation process, a parameter of the carrier wave (such as amplitude, frequency or phase) is varied in accordance with the modulating signal. This variation acts as a code for data transmission.
This modulated signal is then transmitted by the transmitter.
The receiver demodulates the received modulated signal and gets the original information signal back.
There are two main types of modulation that are used in telecommunications; analog and digital.
In analog modulation, a continuously varying sine wave is used as a carrier wave that modulates the message signal or data signal.
How analog modulation works
A carrier wave has three defining properties, which are amplitude, frequency and phase. These three defining properties are used to create three types of modulation:
A high energy wave is characterized by a high amplitude; a low energy wave is characterized by a low amplitude.
Amplitude modulation or AM is the method of varying the instantaneous amplitude of carrier signal accordingly with instantaneous amplitude of message signal.
The frequency of a wave is the number of waves passing a point in a certain time.
FM or Frequency modulation is the process of varying the in instantaneous frequency of Carrier signal accordingly with instantaneous amplitude of message signal.
Phase is the position of a point in time (an instant) on a waveform cycle.
PM or Phase modulation is the process of varying the instantaneous phase of Carrier signal accordingly with instantaneous amplitude of message signal.
Digital modulation is similar analog except base band signal is of discrete amplitude level. For binary signal it has only two levels, either high or logic 1 or low or logic 0.
The modulation scheme can be broken down in to three main types.
ASK or Amplitude Shift Key
This form of modulation is when the carrier amplitude is varied in proportion to message signal.
FSK or Frequency Shift Key
This type of modulation transmits data by varying the frequency of the carrier. In this modulation, the carrier has two predefined frequencies.
PSK or Phase Shift Key
With PSK, the phase of the carrier is shifted to transmit data.
Why is modulation used in telecommunication?
There are many key problems in telecommunications that modulation helps to solve, including:
Problem: Certain types of data such as voice, video, and bit streams from computers have lower frequency bands and therefore they cannot travel very far wirelessly because they cannot propagate through space.
Solution: When using modulation, the carrier wave (which has a much higher frequency) is used to carry the signal, meaning it can travel further and more quickly through space.
Problem: Antenna length would be impractical for voice, video and bit streams without modulation.
Solution: By using the high frequency carrier waves, the necessary antenna size is greatly reduced.
Problem: Without modulation, transmission of video, voice or data bit streams being sent on the same frequency range would cause overlapping of channels (resulting in interference).
Solution: Carrier radio waves have a wide range of frequencies which allow you to select individual non-overlapping channels to carry the information.