( i ) Energy carried by low frequency audio waves ( 20 Hz to 20000 Hz ) is very small .
( ii ) For efficient radiation and reception of signal . the transmitting and receiving antennas should be very high approximately 5000 m .
( iii ) The frequency range of audio signal is so small that overlapping of signals create a confusion .
Types of Modulation
( i ) Amplitude Modulation In this type of modulation , the amplitude of high frequency carrier wave is varied in accordance to instantaneous amplitude of modulating signal .
Band width required for amplitude modulation = twice the frequency of the modulating signal .
( ii ) Frequency Modulation In this type of modulation , the frequency of high frequency carrier wave is varied in accordance to instantaneous frequency of modulating signal .
( iii ) Pulse Modulation In this type of modulation , the continuous waveforms are sampled at regular intervals . Information is transmitted only at the sampling times .
Demodulation
The process of separating of audio signal from modulated signal is called demodulation .
Antenna
An antenna converts electrical energy into electromagnetic waves at transmitting end and pick up transmitted signal at receiving end and converts electromagnetic waves into electrical signal .
Modem
The term modem is contraction of the term modulator and demodulator . Modem is a device which can modulate as well as