Need Of Modulation In Communication Systems - Practice Questions & MCQ

Modulation and its Necessity

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.

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 a high frequency (HF) signal is known as modulation.

A transmitter is a device that converts electrical signals into electromagnetic waves for communication over distances. It modulates the signal onto a carrier wave, amplifies it, and transmits it via an antenna. Transmitters are used in radio, television, telecommunications, and wireless data transmission systems to send information to a receiver.

Need for modulation: The sound wave (20 Hz to 20 kHz) cannot be transmitted directly from one place to another for the following reasons :

1. Size of the antenna or aerial: For efficient radiation and reception, the height of transmitting and receiving antennas should be comparable to a quarter of the wavelength of the frequency used. For 15 kHz it is 5000 m (too large) and for 1 MHz it is 75 m. 
2. Effective power radiated by an antenna: The power radiated is proportional to $\left(\frac{l}{\lambda}\right)^2$. It shows that for the
   same antenna length, the power radiated increases with decreasing  i.e., increasing frequency. Hence, the effective power radiated by a long-wavelength baseband signal would be small. For a good transmission, we need high powers and hence this also points out the need of using the high-frequency transmission.
3. 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 the 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.í