CBSE NOTES CLASS 12 PHYSICS
CHAPTER 15 COMMUNICATION SYSTEMS
The original low frequency message/information signal cannot be transmitted to long distances because of various reasons.
Therefore, at the transmitter, information contained in the low frequency message signal is superimposed on a high frequency wave, which acts as a carrier of the information. This process is known as modulation. There are several types of modulation, e.g. AM, FM and PM.
The process of retrieval of information from the carrier wave at the receiver is called demodulation. This is the reverse process of modulation.
Signal representing the band of frequencies of the original signal, as delivered by the source of information, is called baseband signal. The range of frequencies is called the signal bandwidth.