The process of processing a signal to make it suitable for transmission is called modulation. Most of the information bearing signals in day-to-day communication are audio signals of frequency less than 20 kHz. For small distances, you can form direct link. But it is not practical to transmit such signals to long distances.
This is because of the following reasons:
The signal should have an antenna or aerial of size comparable to the wavelength of the signal so that the time variation of the signal is properly sensed by the antenna. It means that for low-frequency or long-wavelength signals, the antenna size has to be very large.
The power carried by low frequency signals is small and can not go far. It is because of continuous decline due to absorption or radiation loss. It means that for long distance transmission high frequencies should be used. But these can not carry useful information.
For signal transmission, audio signal acts as message and high (radio) frequency acts as the carrier. By super imposing a low frequency signal on a high frequency carrier wave, signal is made suitable for transmission.
The original signal is converted into an electrical signal, called the base band signal using a signal generator. Next, super impose the base band signal over carrier waves in the modulator. The change produced in the carrier wave is known as modulation of the carrier wave and the message signal used for modulation is known as modulating signal.
The carrier wave can be continuous or pulsed. Since a sinusoidal wave is characterized by amplitude, frequency and phase, it is possible to modulate (or modify) either of these physical parameter. This is known as analog modulation. There are different types of analog modulation:
For pulsed carrier waves, Pulse Code Modulation (PCM) is the preferred scheme.