# Modulation and Demodulation

Modulation is performed at the broadcast station to relay audio signals to the receiver over a greater distance. Since the modulated wave is picked up by the radio's receiver, the audio signal needs to be retrieved from it. In a radio receiver, this phase is received and is called demodulation.

Modulation:

• Modulation is the process by which we change some characteristics carried signal (high frequency signal) according to the instantaneous value of the message (information signal base band signal).

• Need of modulation:- 1) Practicality of antenna height – To send or receive a signal the dimension of (height of) antenna should be λ/2.
Height = λ/2 (wave length of antenna)

Typical audio signal is having 3.4Khz frequency. So antenna height to send this signal.

H= λ/2

λ = C/F Where, C is the speed of light and F is the frequency of signal
f=3.4Khz
f=3.4*103 hz
h = 3*108/2*3.4*103
= 44.1*103

= 44km (It is not practical.)​

If we take carrier signal of 100Mhz and superimpose are message signal then antenna a height.
h = 3*108/2*100*106

h = 115m (That is practical.)

2) Due to modulation we can multiplex the multiple signal and can send by same channel. Example F.M

In this we modulate different audio signal at different carrier frequency and send by the same channel at receiver we demodulate and get the original signal.

3) It is less costly and more reliable.

4) At high frequency less are less are compare to low frequency. So it saved a power also.

Communication:- Communication is the process to sender or receiver information from transmitted and receive and vice verse and sender to the receiver or one entity to other entity.

As shown in above figure there is a information source it may be generating, audio, video, text, picture, which is converting into electrical signal with the help of input transducer.

Now electrical signal is process by transmitter (modulation DTC) and transmitted into the channel. This electrical reached of the receiver and receiver process converted into original with the help of output transducer.

Noise a attack mostly at channel.

Noise:- is a unwanted signal which distort information signal.

Type of noise:

1. External noise:- it occur due to external disturbances or surrounding like lightning, thunderstorm, solar noise, motor, arcs in industries.
2. Internal noise:- it is again divide first Johnson and white noise it occur due to thermal vibration . among electron, atom, molecule at receiver register.
3. Shot noise:- it is due to random movement of charge carrier electron and holes.
4. Transit time noise:- It occur during transit ‘on’ and ‘off’ of a device.

Signal to noise ratio:- SNR =) Signal power/ noise power

Spectrum of  a signal:-

• A signal to represented a time domain as well as frequency domain.
• Graphical representation  of frequency component is called spectrum.
• Spectrum of above signal gt will be fourier transformer V(f).

• Note:- bandwidth is define as range of frequency over which a signal s communicated.

Types of Modulation:

There are two types of modulation :-

1. Continuous wave modulation
2. Pulse modulation
• Continuous wave modulation have two types:-
1. Amplitude Modulation
2. Angle modulation
• Angle modulation have two types:-
1. Frequency Modulation
2. Phase modulation.
• Pulse modulation have six types:-
1. PAM
2. PWM
3. PPM
4. Delta modulation
5. Differential modulation
6. PCM

Analog modulation:- Analog modulation refers to the process of transferring an analog baseband (low frequency) signal, such as an audio or TV signal over a higher frequency signal, such as a radio frequency band. Several amplitude modulation variants are used in practice. Now( Amplitude modulation, Phase modulation, PAM,PWM, and  PPM) they all are Analog.

Digital Modulation:- Digital modulation is the process of encoding a digital information signal in the amplitude, phase or frequency of the transmission signal. In general, a modulation technique encodes several bits in a symbol, and the transmission speed of symbols determines the bandwidth of the transmission signal. Now (Angle modulation, frequency modulation, Delta modulation, Differential modulation, and PCM ) they all are Digital.

• PAM is stand for (Pulse amplitude Modulation)
• PWM is stand for (Pulse width Modulation)
• PAM is stand for (Pulse Position Modulation)
• PCM is stand for (Pulse Code Modulation)

Amplitude modulation:- It is the process in which carrier signal amplitude is very according to the instantaneous value of the message signal.

Pulse modulation:- In pulse modulation we use pulse train signal as a carrier.

Phase modulation:- It is the process in which carrier signal phase is vary according to instantaneous value of message signal.

Frequency modulation:- It is the process in which carrier signal frequency is vary according to the instantaneous value of the message signal.

Demodulation:

Demodulation is the reverse process of modulation. The process of recovering the original message signal from the modulated wave is called Demodulation. Hence, Demodulation is the process of converting modulated carrier into a current( or voltage) that is equivalent to the original signal.

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