A message signal cannot travel a long distance because of its low signal strength. In addition to this, physical surroundings, the addition of external noise and travel distance will further reduce the signal strength of a message signal. So in order to send the message signal to a long distance, we need to increase the signal strength of a message signal. This can be achieved by using a high frequency or high energy signal called carrier signal. A high energy signal can travel to a larger distance without getting affected by external disturbances. We take the help of such high energy signal to transmit the message signal. This high energy or high frequency signal is known as carrier signal.

HDMI Encoder Modulator,16in1 Digital Headend, HD RF Modulator at Soukacatv.com

The low energy message signal is mixed with the high energy or high frequency carrier signal to produce a new high energy signal which carries information to a larger distance.

The question arises how the message signal should be added to the carrier signal. The solution lies in changing some characteristics (amplitude, frequency or phase) of a carrier signal in accordance with the amplitude of the message signal. This process is called modulation. Modulation means to “change”.

The Message signal contains information whereas the carrier signal contains no information. Carrier signal is used just to transmit the information to a long distance. At the destination, the message signal is consumed whereas the carrier signal is wasted.

In modulation process, the characteristics of the carrier signal is changed but the message signal characteristics will not be changed. The carrier signal does not contain any information so even if we change the characteristics of the carrier signal, the information contained in it will not be changed. However, the message signal contains information so if we change the characteristics of the message signal, the information contained in it will also changes. Therefore, we always changes the characteristics of the carrier signal but not the message signal.

Modulation allows the transmission to occur at high frequency while it simultaneously allows the carrying of the message signal.

Modulation Definition

Modulation is the process of mixing a low energy message signal with the high energy carrier signal to produce a new high energy signal which carries information to a long distance.

Or

Modulation is the process of changing the characteristics (amplitude, frequency or phase) of the carrier signal, in accordance with the amplitude of the message signal.

A device that performs modulation is called modulator.

Example:

The modulation process can be understood with a simple example. The below figure shows the amplitude modulation.

The first figure shows the modulating signal or message signal which contains information, the second figure shows the high frequency carrier signal which contains no information and the last figure shows the resultant amplitude modulated signal.

From the above three figures, it can be observed that the amplitude of the carrier signal is varied in accordance with the instant amplitude of the message signal.

Types of Signals in Modulation

In modulation process, three types of signals are used to transmit information from source to destination. They are:

1) Message signal

2) Carrier signal

3) Modulated signal

1) Message signal

The signal which contains a message to be transmitted to the destination is called a message signal. The message signal is also known as a modulating signal or baseband signal.

The original frequency range of a transmission signal is called baseband signal. The message signal or baseband signal undergoes a process called modulation before it gets transmitted over the communication channel. Hence, the message signal is also known as the modulating signal.

2) Carrier signal

The high energy or high frequency signal which has characteristics like amplitude, frequency, and phase but contains no information is called a carrier signal. It is also simply referred to as a carrier. Carrier signal is used to carry the message signal from transmitter to receiver. The carrier signal is also sometimes referred to as an empty signal.

3) Modulated signal

When the message signal is mixed with the carrier signal, a new signal is produced. This new signal is known as a modulated signal. The modulated signal is the combination of the carrier signal and modulating signal.

Need for Modulation

Modulation is extremely necessary in communication system because of the following reasons:

1) Avoids mixing of signals

2) Increase the range of communication

3) Wireless communication

4) Reduces the effect of noise

5) Reduces height of antenna

1) Avoids mixing of signals

One of the basic challenges facing by the communication engineering is transmitting individual messages simultaneously over a single communication channel. A method by which many signals or multiple signals can be combined into one signal and transmitted over a single communication channel is called multiplexing.

We know that the sound frequency range is 20 Hz to 20 KHz. If the multiple baseband sound signals of same frequency range (I.e. 20 Hz to 20 KHz) are combined into one signal and transmitted over a single communication channel without doing modulation, then all the signals get mixed together and the receiver cannot separate them from each other. We can easily overcome this problem by using the modulation technique.

By using modulation, the baseband sound signals of same frequency range (I.e. 20 Hz to 20 KHz) are shifted to different frequency ranges. Therefore, now each signal has its own frequency range within the total bandwidth.

After modulation, the multiple signals having different frequency ranges can be easily transmitted over a single communication channel without any mixing and at the receiver side, they can be easily separated.

2) Increase the range of communication

The energy of a wave depends upon its frequency. The greater the frequency of the wave, the greater the energy possessed by it. The baseband audio signals frequency is very low so they cannot be transmitted over large distances. On the other hand, the carrier signal has a high frequency or high energy. Therefore, the carrier signal can travel large distances if radiated directly into space.

The only practical solution to transmit the baseband signal to a large distance is by mixing the low energy baseband signal with the high energy carrier signal. When the low frequency or low energy baseband signal is mixed with the high frequency or high energy carrier signal, the resultant signal frequency will be shifted from low frequency to high frequency. Hence, it becomes possible to transmit information over large distances. Therefore, the range of communication is increased.

3) Wireless communication

In radio communication, the signal is radiated directly into space. The baseband signals have very low frequency range (I.e. 20 Hz to 20 KHz). So it is not possible to radiate baseband signals directly into space because of its poor signal strength. However, by using the modulation technique, the frequency of the baseband signal is shifted from low frequency to high frequency. Therefore, after modulation, the signal can be directly radiated into space.

4) Reduces the effect of noise

Noise is an unwanted signal that enters the communication system via the communication channel and interferes with the transmitted signal.

A message signal cannot travel for a long distance because of its low signal strength. Addition of external noise will further reduce the signal strength of a message signal. So in order to send the message signal to a long distance, we need to increase the signal strength of the message signal. This can be achieved by using a technique called modulation.

In modulation technique, a low energy or low frequency message signal is mixed with the high energy or high frequency carrier signal to produce a new high energy signal which carries information to a long distance without getting affected by the external noise.

5) Reduces height of antenna

When the transmission of a signal occurs over free space, the transmitting antenna radiates the signal out and receiving antenna receives it. In order to effectively transmit and receive the signal, the antenna height should be approximately equal to the wavelength of the signal to be transmitted.

Now,

The audio signal has a very low frequency (I.e. 20 Hz to 20 kHz) and longer wavelength, so if the signal is transmitted directly into space, the length of the transmitting antenna required would be extremely large.

For instance, to radiate an audio signal frequency of 20 kHz directly into space, we would need an antenna height of 15,000 meters.

The antenna of this height is practically impossible to construct.

On the other hand, if the audio signal (20 Hz) has been modulated by a carrier wave of 200 MHz. Then, we would need an antenna height of 1.5 meters.

The antenna of this height is easy to construct.

Types of Modulation

Basically, the modulation is classified into two types: analog modulation and digital modulation.

Analog modulation

In analog modulation, the analog signal (sinusoidal signal) is used as a carrier signal that modulates the analog message signal. In analog modulation, the characteristics (amplitude, frequency or phase) of the carrier signal is varied in accordance with the amplitude of the message signal.

Adjacent Channel 16 In 1 Analog Headend For Hotel Cable TV System

24 In 1 Analog Fixed Channel Modulator Headend

There are four basic types of analog modulation:

1) Amplitude modulation

2) Frequency modulation

3) Phase modulation

4) Analog pulse modulation

1) Amplitude modulation

Amplitude modulation is a type of modulation where the amplitude of the carrier signal is varied (changed) in accordance with the amplitude of the message signal while the frequency and phase of carrier signal remain constant.

The above figure shows the amplitude modulation.

The first figure shows the modulating signal or message signal which contains information, the second figure shows the high frequency carrier signal which contains no information and the last figure shows the resultant amplitude modulated signal.

From the above three figures, it can be observed that the amplitude of the carrier signal is varied in accordance with the instant amplitude of the message signal.

2) Frequency modulation

Frequency modulation is a type of modulation where the frequency of the carrier signal is varied (changed) in accordance with the amplitude of the message signal while the amplitude and phase of carrier signal remain constant.

The above figure shows the frequency modulation.

The first figure shows the modulating signal or message signal, the second figure shows the high frequency carrier signal which contains no information and the last figure shows the resultant frequency modulated signal.

From the above three figures, it can be observed that the frequency of the carrier signal is varied in accordance with the instant amplitude of the message signal.

3) Phase modulation

Phase modulation is a type of modulation where the phase of the carrier signal is varied (changed) in accordance with the amplitude of the message signal while the amplitude of carrier signal remains constant.

The above figure shows the phase modulation.

The first figure shows the modulating signal or message signal, the second figure shows the high frequency carrier signal which contains no information and the last figure shows the resultant phase modulated signal.

From the above three figures, it can be observed that the phase of the carrier signal is varied in accordance with the instant amplitude of the message signal.

In this type of modulation, when the phase is changed it also affects the frequency so this modulation also comes under frequency modulation.

The frequency and phase modulation comes under angle modulation. When the frequency or phase of the carrier signal is varied (changed) in accordance with the amplitude of the message signal, then it is called angle modulation.

4) Analog pulse modulation

In amplitude, frequency and phase modulation techniques, the carrier and message signals are continuous signals (sinusoidal signals). However, in analog pulse modulation, the carrier signal is a discontinuous signal (series of pulses) and message signal is a continuous signal (sinusoidal signal).

Analog pulse modulation is the process of changing the characteristics (pulse amplitude, pulse width or pulse position) of the carrier pulse, in accordance with the amplitude of the message signal.

The analog pulse modulation is again classified as,

1. Pulse amplitude modulation

2. Pulse width modulation

3. Pulse position modulation

Example:

The analog pulse modulation can be understood with a simple example. The below figure shows the pulse amplitude modulation.

The first figure shows the message signal (continuous signal), the second figure shows the carrier pulse train (series of pulses) which contains no information and the last figure shows the resultant pulse amplitude modulated (PAM) signal.

From the above three figures, it can be observed that the amplitude of the series of carrier pulses are varied (changed) in accordance with the instant amplitude of the message signal while the width and positions of the carrier pulses remain constant.

Analog modulation is more sensitive to noise. Noise is an unwanted signal that enters the communication system via the communication channel and interferes with the transmitted signal. The noise signal degrades the transmitted signal (signal containing information). Therefore, this drawback can be overcome by the digital modulation technique.

Digital modulation

The digital modulation technique is employed for efficient communication. The main advantage of the digital modulation over analog modulation include high noise immunity, available bandwidth, and permissible power. In digital modulation, the modulating signal or message signal is converted from analog to digital.

SKD3013 3 Channel HD Encode Modulator


SKD81 IP Set Top Box

Pulse Code Modulation (PCM)

In digital modulation, the modulation technique used is Pulse Code Modulation (PCM). The pulse code modulation is the method of converting an analog signal into a digital signal I.e. 1s and 0s. As the resultant signal is a coded pulse train, this is called as pulse code modulation.


Established in 2000, the Soukacatv.com main products are modulators both in analog and digital ones, amplifier and combiner. We are the very first one in manufacturing the headend system in China. Our 16 in 1 and 24 in 1 now are the most popular products all over the world.

For more, please access to https://www.soukacatv.com.


CONTACT US

Dingshengwei Electronics Co., Ltd

Company Address: Buliding A,the first industry park of Guanlong,Xili Town,Nanshan,Shenzhen,Guangdong,China

Tel : +86 0755 26909863

Fax : +86 0755 26984949

Phone: +86 13410066011

Email:[email protected]

Skype: soukaken



Source：physics-and-radio-electronics