Modulation – Definition, Types and Need for Modulation:
Any wave has three significant characteristics viz. amplitude, frequency and phase, and modulation is a process of impressing information to be transmitted on a high frequency wave, called the carrier wave, by changing its one of the characteristics (amplitude, frequency or phase angle).
Modulation may also be defined as the process of altering some characteristic (amplitude, frequency or phase angle) of the carrier wave in accordance with the instantaneous value of some other wave called the modulating wave.
Carrier Wave:
Carrier wave is a high frequency, constant amplitude, constant frequency and non-interrupted wave generated by radio-frequency oscillators. These waves are inaudible i.e., by themselves they are not able to produce any sound in the loudspeaker. As their name indicates, their function is to carry the audio or video signal from transmitting station to the receiving station. The resulting wave (audio signal superimposed on the carrier wave) is called the modulated wave.
Need For Modulation:
Low-frequency signals cannot be transmitted over long distances if radiated directly into the space. This is because of the following hurdles:
1. Short Operating Range: The energy of any wave depends on its frequency—the larger the frequency of the wave, the greater the energy associated with it. Obviously the audio signal having small frequency and consequently small power cannot be transmitted over long distance when radiated directly into the space. However, modulated wave (audio signal with high frequency) can be transmitted over long distances.
2. Poor Radiation Efficiency: At audio frequencies, radiation is not practicable as efficiency of radiation is poor. However, electrical energy can be radiated efficiently at high frequencies (above 20 kHz).
3. Mutual Interference: If low frequency signals are transmitted directly from different sources, all of them would be mixed up and completely blanket the air. However, by modulation different messages of different frequency levels can be transmitted simultaneously without any interference.
4. Huge Antenna Requirement: For efficient radiation of a signal, the length of transmitting and receiving antenna should be at least one quarter wavelength i.e.,
Thus, for transmitting a signal of frequency 2 kHz, an antenna of length 37.5 km will be required, practically impossible. On the other hand, for transmitting a signal of frequency 2 MHz, an antenna of about 37.5 metres would be required which can be easily constructed.
The carrier wave alone also cannot be employed for transmission of message, though it has high frequency and enormous energy. The reason is that an unmodulated carrier wave has all of its parameters constant (constant amplitude, constant frequency and constant phase relationship with respect to some reference) while any message has changing quantities.
Hence, the solution lies in modulation that enables a low frequency signal transmission over long distances through space with the help of a high frequency carrier wave. These carrier waves need reasonably sized antenna and produce no interference with other transmitters operating in the same area.
Types of Modulation: Classification of Modulation
The sinusoidal carrier wave may be represented as
where Vc is the maximum value, fc is the frequency and θ is the phase relation w.r.t. some reference of the carrier wave. As obvious from above equation, the sinusoidal carrier wave has basically three characteristics, viz. amplitude, frequency and phase. In amplitude modulation, the amplitude of the carrier wave is varied in accordance with the modulating signal, keeping the frequency and phase of the carrier wave unchanged.
Instead of amplitude, either frequency or phase of the sinusoidal carrier can be changed according to the message, keeping the amplitude constant. This is another method of modulation, called the “angle modulation“. The angle modulation is further subdivided into frequency modulation (FM) and phase modulation (PM).
In frequency modulation, the frequency of the carrier wave is varied in accordance with the modulating signal, keeping the amplitude and phase of the carrier wave unchanged.
In phase modulation, the phase of the carrier wave is varied in accordance with the modulating signal, keeping the amplitude and frequency of the carrier wave unchanged.
However, the modulation may also be classified, according to the nature of carrier wave, into continuous wave modulation and pulse modulation.
1. Continuous Wave Modulation: When the carrier wave is continuous in nature, the modulation process is called the continuous wave modulation or analog modulation. Amplitude modulation and angle modulation (frequency modulation and phase modulation) fall under this category.
2. Pulse Modulation: When the carrier wave is a pulse type waveform, the modulation process is referred to as pulse modulation. In pulse modulation, the carrier wave consists of a periodic sequence of rectangular pulses. Pulse modulation may be of an analog or digital type.
In India, amplitude modulation and frequency modulation are employed in radio broadcasting while in television transmission frequency modulation is employed for audio signals and amplitude modulation for video signals.