. M ODULATION AND ITS N ECESSITY As already mentioned, the purpose of a communication system is to transmit information or message signals. Message signals are also called baseband signals , which essentially designate the band of frequencies representing the original signal, as delivered by the source of information. No signal, in general, is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth .
Suppose we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than kHz) over a long distance directly. Let us find what factors prevent us from doing so and how we overcome these factors,