Normal communication signals loose strength as they travel to the large distances. Hence, we often transmit the signals through electromagnetic waves and we use antennas to recover them at a remote point. To send transmitting message signals effectively for long distances, we use Modulation. At the receiver end, after receiving the signal, we need to “move” them back to the original frequency band (baseband) through demodulation. Therefore, we can see the modulation task as “giving wings” to the information message. However, the original information is retrieved at the receiver end.
Often, the message being communicated is itself a signal, e.g., an audio signal, and to produce a signal that is suitable for transmission through the channel, we effect some transformation on the message signal. Modulation is the Process by which a property or a parameter of a signal is varied in proportion to another signal. The original signal is normally referred as the modulating signal and the high frequency signal, whose properties are changed, is referred as the carrier signal. The resulting signal is finally referred as the modulated signal.
For example in case of the amplitude modulation, the amplitude of the carrier wave is varied in accordance with the amplitude of the message signal, whereas in the angle modulation, phase angle of the carrier is varied with respect to the message signal.
- Modulation can shift the frequency spectrum of a message signal into a band which is better suited to the channel. Antennas only efficiently radiate and admit signals, whose wavelength is similar to their physical aperture. Hence, to transmit and receive, say, voice, by radio we need to shift the voice signal to a much higher frequency band.
- Modulation permits the use of multiplexing. Multiplexing means allowing simultaneous communication by multiple users on the same channel. For instance, the radio frequency spectrum must be shared and modulation allows users to separate themselves into bands.
- Modulation can provide some control over noise and interference. For example the effect of noise can be controlled to a large extent by frequency modulation.
Modulation can be classified into two categories Analog Modulation and Digital Modulation.;