The picture by TV, sound, and speech are transmitted over hundreds of kilometers by radio transmitters. The message signal due to its low signal strength cannot travel long distances plus there are other factors such as external noise, friction, long-distance which also reduce the signal strength.
In these cases, the carrier of speech and pictures are high-frequency radio waves. The information is superposed on the radio wave. It carries along with it to its destination.
The process of combining low-frequency signals with high-frequency radio waves is called modulation. The waveform as a result is called a modulated carrier wave.
Carrier Wave: High-frequency radio wave with high energy in modulation.
Modulation Signal: the low-frequency signal in modulation.
Types of Signals in Modulation
There are three types of signals in modulation used to send messages from source to destination.
The signal having a message to be transmitted from source to destination is a message signal. It is also called modulating or baseband signal.
It is the high frequency and high energy signal which carries no information. It has characteristics like amplification, frequency to carry modulating signal from transmitter to receiver.
When the low signal and high-frequency signal combined, the resultant signal formed is called modulated signal.
Types of Modulation
Amplitude Modulation (A.M)
In this process of modulation amplitude of carrier wave changes according to the signal that is the amplitude of carrier waves changes in amplitude modulation. When there is no signal, the amplitude of the carrier wave is equal to the modulated amplitude but when the signal is present the amplitude of the carrier modifies according to the inconstant value of the signal.
During the positive half cycle of the signal amplitude of the carrier increase to the sum of the amplitude of carrier and signal. During the negative half cycle of the signal amplitude of the carrier increases and becomes equal to the amplitude carrier and the signal.
The transmission frequency ranges from 540 kHz to 1600 kHz.
- They are better to send the signal for large ranges.
- These have poor quality transmission of sound.
- Broadcast transmissions: AM is still extensively used for transmitting on the long, medium, and short-wave bands.
- Airband radio: VHF transmissions for numerous airborne applications still use AM.
Frequency Modulation (F.M)
Frequency modulation is a type of modulation where the information (message signal) is transmitted over a carrier wave by varying its frequency according to the amplitude of the message signal.
In frequency modulation, the amount of change in frequency of the carrier signal is determined by the amplitude of the message signal. The frequency of the modulated wave is high when the message signal reaches its maximum amplitude.
The carrier wave does not carry any information so even if we alter the frequency of the carrier wave, there will be no information loss. Nevertheless, if we change the frequency of the modulating signal, some amount of information loss will happen due to the fact that the modulating signal contains the information. So, the frequency of the modulating signal should not be changed.
The transmission frequency ranges from 88MHz to 108MHz.
- These are less impacted by electrical interference than AM waves.
- Offers higher quality transmission of sound.
- They are less able to travel around obstacles such as high structures, buildings and hills.
- It needs broader bandwidth than amplitude modulation.
- FM broadcasting
- Magnetic tape-recording systems
Phase modulation (PM), in which the phase of the carrier wave is varied to reflect changes in the frequency of the information. In PM, the frequency is unchanged while the phase is altered relative to the base carrier frequency. It is similar to FM.
Polarization modulation, in which the angle of rotation of an optical carrier signal is varied to reflect transmitted data.
Pulse-code modulation, in which an analog signal is sampled to obtain a data stream that is used to modulate a digital carrier signal.
Quadrature amplitude modulation
Quadrature amplitude modulation (QAM), which uses two AM providers to encode 2 or more bits in a single transmission.
Importance of Modulation
The message signal is low strength signal so cannot travel long distance. The carrier wave cannot carry information. In order to send data in form of audio, pictures, or other forms, the message signal is superimposed on a high-frequency carrier wave and this process is modulation. By modulation, data can easily be transmitted over long distances without loss.