Digital and analog signals to be transmitted are usually of low frequency and hence cannot be transmitted as such. These signals require some carrier to be transported. These carriers are known as carrier waves or high-frequency signals. The process of placement of a low-frequency (LF) signal over the high-frequency (HF) signal is known as modulation various types of modulations are shown in Fig. 8.
Need for modulation
A sound wave (20 Hz to 20 KHz) cannot be transmitted directly from one place to another for the following reasons:
Height of antenna For efficient radiation and reception , the height of transmitting and receiving antennas should be comparable to a quarter of wavelength of the frequency used. For 15 kHz it is 5000 m (too large) and for 1 MHz it is 75 m. The energy radiated from an antenna is practically zero, when the frequency of the signal to be transmitted is below 15 Hz.
Detecting signals All audible signals are in the range of 20 Hz to 20 kHz. So the signals from all sources remain heavily mixed up in air. It will be very difficult to differentiate or detect the broadcast signal at the receiving station. Thus, modulation is necessary for a low-frequency signal especially when it is to be sent to a distant place, so that the information may not die out in the way itself (Fig. 9).