Need of Modulation and Demodulation
What is Modulation
Modulation is defined as the process of varying some characteristics (amplitude, frequency or phase ) of a carrier wave in accordance to the amplitude of messaging signal or modulating signal, m(t). acircuit which provide modulation is called modulators.
The frequency of a Radio channel can be best explained as the frequency of a carrier wave. A carrier wave is pure constant frequency, bit same as sine wave. It doesn’t carry much information related to data or speech. The concepts of Amplitude Modulation, Modulation and Demodulation, along with their difference is explained below.
To include data or speech information, another wave is to be imposed known as input signal (modulating wave) above the carrier wave. This process is known as modulation. Modulation changes the shape of a carrier wave to encode the data information that we want to carry. Modulation is same as hiding the code in the carrier wave.
Fig:- Basic diagram of modulation
- Low frequency signal to a high frequency signal
- A baseband signal into bandpass signal
- A wideband signal into narrow- band signal.’
Need of modulation
- You may be ask, when the baseband signal can be transmitted directly why to use the modulation ? The answer is that the baseband transmission has many limitations which can be overcome using modulation . In the process of modulation, the baseband signal is translated i.e., shifted from low frequency to high frequency . This frequency shift is proportional to the frequency of carrier.
- In a carrier communication system, the baseband signal of a low-frequency spectrum is translated to a high frequency spectrum. This is achieved through modulation. The aim of this topic is to explore the reasons for using modulation. Modulation is defined as a process by virtue of which, some characteristic of a high frequency sinusoidal wave is varied in accordance with the instantaneous amplitude of the baseband signal.
- Two signals are involved in the modulation process. The baseband signal and the carrier signal. The baseband signal is to be transmitted to the receiver. The frequency of this signal is generally low. In the modulation process, this baseband signal is called the modulating signal.The waveform of this signal is unpredictable. For example, the waveform of a speech signal is random in nature and cannot be predicted. In this case, the speech signal is the modulating signal.
- The other signal involved with the modulation is a high frequency sinusoidal wave. This signal is called the carrier signal or carrier. The frequency of the carrier signal is always much higher than that of the baseband signal. After modulation, the baseband signal of low frequency is transferred to the high frequency carrier, which carries the information in the form of some variations. After the completion of the modulation process, some characteristic of the carrier is varied such that the resultant variations carry the information.
Why we require modulation?
- To improve the quality of reception
- To increase the range of communication
- To reduce the antenna height.
- For multiplexing and demultiplexing
- To reduce noise/interference.
- For narrow banding of signal:- since, usually for range 50Hz-10 kHz we require antenna having the ratio of highest to lowest frequency/ wavelength is 200, which is practically impossible. Modulation converts a wideband signal into a narrow-band signal whose ratio between highest frequency to lowest frequency is approximately one and single antenna will be sufficient to transmit the signal.
fh / fl » 1 ⇔ wideband signal, and
fh / fl ≈ 1 ⇔ narrow band signal
Therefore, fh / fl ≈ 1
Modulation is broadly divided in to following categories:
Baseband signal: is a signal which transmits without using modulation. The term baseband is used to designate the band of frequencies of the signal delivered by the source or input transducer. Voice, data and picture signals are baseband signals.
Bandpass signal : the signals which are modulated are called band pass signal. Its spectrum occupies the frequency spectrum from certain lowest frequency f1 to the highest frequency f2.
Baseband communication: Baseband signals are transmitted without modulation i.e. without any shift in the range of frequencies of the signal. As the baseband signals have low frequencies with low power, therefore, they cannot be transmitted over long distance.
Carrier communication: Carrier is usually a sinusoidal signal of high frequency. Carrier communication is the technique that uses modulation to shift the frequency spectrum of baseband signal from low to high frequency range.
Bandwidth of Modulated Signal
Let m(t) = message signal or baseband signal or modulating signal having spectrum shown below :
Before modulation, bandwidth of the message signal is fm
Carrier wave c(t) = Ac cos ?c t or
To modulate the message signal, the message signal is multiplied with high frequency
After modulation , bandwidth is 2fm.
Types of Modulation
We can modulate the information bearing signal into two types namely. These are called Modulation Techniques.
- Analog Modulation
- Digital Modulation
Analog modulation is the process of converting an analog input signal into a signal that is suitable for RF transmission.
Digital modulation is the process of converting a digital bistream into an analog signal suitable for RF transmission.
Modulation Index indicates the depth of modulation. As the amplitude of the modulating signal increases, modulation index increases. For amplitude modulation, the modulation index is given as
The Analog carrier signal is modulated by analog information signal so that information bearing analog signal can travel larger distance without the fear of loss due to absorption.
The Analog modulation is of two types:
- Amplitude Modulation
- Angle Modulation
The Angle modulation is further classified as Frequency modulation and Phase Modulation.
Amplitude Modulation: In this type of modulation the strength of the carrier signal is varied with the modulating signal.
Frequency Modulation: In this type of modulation the frequency of the carrier signal is varied with the modulating signal.
Phase Modulation: In this type of modulation the phase of the carrier signal is varied with the modulating signal. It is the variant of the frequency modulation.
The analog carrier signal is modulated by digital information signal. It is also considered as digital to analog conversion.
Digital modulation means an analog signal of carrier is converted by a digital data bit stream.
There are two types of bits in binary:
- Logic 0 (low)
- Logic 1 (High).
This method is use to convert digital signal to analog and the responding demodulating is applied to convert analog signal to digital signal. Here the analog signal bearing information is transmitted by the digital method.
There are four types of Digital modulation which are as below:
- Pulse code Modulation
- Differential Pulse code Modulation
- Delta Modulation
- Adaptive delta Modulation.
Pulse Modulation :
The Pulse wave modulation is a process of sampling of the continuous wave at periodic intervals and transmitting a very short pulse of radio frequency carrier for each sample, with the pulse characteristics being varied in some manner proportional to the signal amplitude at the sampling instant.
The Pulse wave modulation are of four types namely:
- Pulse amplitude Modulation
- Pulse position Modulation
- Pulse width Modulation
- Pulse code Modulation.
What is Demodulation
The process of recovering the original signal from the modulated wave is known as demodulation or detection. At the broadcasting station, modulation is done to transmit the audio signal over larger distances to a receiver. When the modulated wave is picked up by the radio receiver, it is necessary to recover the audio signal from it. This process is accomplished in the radio receiver and is called demodulation.
Need of demodulation
The wireless signal consists of radio frequency (high frequency) carrier wave modulated by audio frequency (low frequency). The diaphragm of a telephone receiver or a loud speaker cannot vibrate with high frequency. Moreover, this frequency is beyond the audible range of human ear. So, it is necessary to separate the audio frequencies from radio- frequency carrier waves.
Difference between Modulation and Demodulation
- Modulation is the process of imposing data information on the carrier, while demodulation is the recovery of original information at the distant end from the carrier.
- Modem is the device that performs both modulation and demodulation.
- Both processes try to achieve transfer information with the minimum distortion, minimum loss and efficient utilization of spectrum.
Even though there are different methods for modulation and demodulation processes, each has its own advantages and disadvantages. For example, AM is used in shortwave and radio wave broadcasting; FM is mostly used in high-frequency radio broadcasting, and pulse modulation is known for digital signal modulation.