Ans-:
Need for modulation:
The baseband signals are incompatible for direct transmission. When the signal is transmitted without modulation they cannot travel longer distances as low frequency signal get it attenuates, so its strength has to be increased by modulating with a high frequency carrier wave, which doesn’t affect the parameters of the modulating signal.
Modulation is needed to achieve the following basic needs:
1. Practicability of antennas: For the transmission of radio signals, the antenna height must be multiple of λ/4 ,where λ is the wavelength.
λ = c /f
Where c: is the velocity of light
f: is the frequency of the signal to be transmitted
The minimum antenna height required to transmit a baseband signal of f = 10 kHz is 7.5 Km.
The antenna of this height is practically impossible to install.
Now, let us consider a modulated signal at f = 1 MHz The minimum antenna height is 75 meters. This antenna can be easily installed practically. Thus, modulation reduces the height of the antenna.
2. Avoids mixing of signals
If the baseband sound signals are transmitted without using the modulation by more than one transmitter, then all the signals will be in the same frequency range i.e. 0 to 20 kHz. Therefore, all the signals get mixed together and a receiver cannot separate them from each other. If each baseband sound signal is used to modulate a different carrier then they will occupy different slots in the frequency domain i.e. through different channels. Thus, modulation avoids mixing of signals.
3. Multiplexing is possible:
Multiplexing is a process in which two or more signals can be transmitted over the same communication channel simultaneously. If transmitted without modulation, the different message signals over a single channel will interfere with each other. So multiplexing helps in transmitting a number of messages simultaneously over a single channel which reduces cost of installation and maintenance of more channels.
4. Narrow banding: The frequency translation through modulation converts a wideband signal to a narrowband, which is termed as narrow banding.
Let us assume a system is radiating directly with the frequency range from 50 Hz to 10 kHz, the ratio of highest to lowest wavelength is 200. If antenna is designed for 50 Hz, it will be too long for 10 kHz and vice versa. But if signal is translated to higher frequency of 1 MHz range using modulation, then the ratio of lowest to highest frequency will be 106+ 50/106+ 104 ≈ 1 and the same antenna will be suitable for the entire band.
5. Improves Quality of Reception
6. Increase the Range of Communication