. Explain the Need for Modulation
Need of Modulation:
Frequency of signal
The audio frequency signals (20 Hz to 20 kHz) cannot be transmitted without distortion over long distance due to less energy carried by low frequency audio waves. The energy of a wave is directly proportional to square of its frequency. Even if the audio signal is converted into electrical signal, the later cannot be sent very far without employing large amount of power.
Height of antenna
For efficient radiation and reception, the height of transmitting and receiving antennas should be comparable to a quarter wavelength of the frequency used.
For 1 MHz it is 75 metre; for 15 kHz frequency, the height of antennas has to be about 5000 metre, which size is unthinkable.
c. Number of channels
Audio frequencies are concentrated in the range 20 Hz to 20 kHz. This range is so narrow that there will be overlapping of signals. In order to separate the various signals it is necessary to convert all of them to different portions of the electromagnetic spectrum.
The radiation of electromagnetic energy is practicable only at high frequencies, e.g., above 20 kHz. The high frequency signals can be sent over large distances even with comparatively small power. This is achieved by superimposing electrical audio signal on high frequency carrier. The resultant waves are known as modulated waves or radio waves and the process is called modulation.