Types of modulation , need for modulation or benefits of modulation , modulation process in communication :
what is MODULATION PROCESS
As discussed earlier, the purpose of a communication system is to deliver a message signal from an information source in recognizable form to a user
destination, with the source and the user being physically separated from each other.
To do this, the transmitter modifies the message signal into a form which is suitable for transmission over channel. This modification is achieved by means of a process known as modulation, which involves varying some parameter of a carrier wave in accordance with the message signal. Also the receiver recreates the original message signal from a degraded version of the transmitted signal after propagation through the channel.* This recreation is achieved by using a process called as demodulation. In fact, the demodulation is the reverse of the modulation process used in the transmitter. We can define modulation process in a more systematic form as under:
Modulation may be defined as the process by which some characteristic of a signal called carrier is varied in accordance with the instantaneous value of another signal called modulating signal. Signals containing information or intelligence are referred as modulating signals. This information bearing signal is also called baseband signal. The carrier frequency is greater than the modulating frequency. The signal resulting from the process of modulation is called modulated signal.
1.6.1. Types of Modulation
Modulation is basically of two types:
(i) Continuous Wave Modulation: When the carrier wave is continuous in nature, the modulation process is known as continuous wave (CW) modulation or analog modulation. Examples of continuous wave modulation are Amplitude Modulation and Angle Modulation. When the amplitude of the carrier is varied in accordance with the message signal, it is known as amplitude modulation (AM). Also, when the angle of the carrier is varied according to the instantaneous value of the modulating signal, it is called angle
modulation. Angle modulation may be further subdivided into Frequency modulation (FM) and Phase modulation (PM), in which the instantaneous frequency and phase of the carrier, respectively, are varied in accordance with the message signal.
(ii) Pulse Modulation: When the carrier wave is a pulse-type waveform, the modulation process is known as pulse modulation. In pulse modulation, the carrier consists of a periodic sequence of rectangular pulses. Pulse modulation can be of an analog or digital type.
In analog pulse modulation, the amplitude, duration or position of a pulse is varied in accordance with sample values of the message signal. The analog pulse modulation may be of following three types:
(i) pulse-amplitude modulation (PAM)
(ii) pulse-duration modulation (PDM).
(iii) pulse-position modulation (PPM).
On the other hand, the digital form of pulse modulation is known as pulse code modulation (PCM).
1.6.2. Need for Modulation or Benefits of Modulation
As discussed earlier, the message signal or baseband signal is used to modulate a high frequency carrier signal inside the transmitter. After modulation, the resulting modulated signal is transmitted with the help of an antenna which is connected at the output side of the transmitter. This modulated signal then travels down the channel to reach at the input of the receiver.
* The carrier performs certain functions in radio communication.
Now, one question can arise why we use modulation in communication system or what will happen if we transmit message signal or audio signal
without modulation. The answer is that the modulation serves several purpose in communication system as discussed below:
- Practicality of antenna
We know that in case when free space is used as a transmitting medium (i.e. channel), messages are transmitted and received with the help of antennas. For efficient radiation and reception the transmitting and receiving antennas must have lengths comparable to a quarter-wavelength of the frequency used. For example, in AM broadcast systems, the maximum audio frequency transmitted from a radio station is of the order of 5 kHz. If this message audio signal were to be transmitted without modulation, then the height of the antenna required for an effective radiation and reception will be 1/4th of the wavelength given as
Obviously, it will be totally impracticable to construct and install an antenna of such a height. However, this height of the antenna may be reduced by modulation technique and yet effective radiation and reception is achieved. In modulation process, low frequency or audio signal at radio stations are translated to higher frequency spectrum i.e., radio frequency range. These higher radio frequencies with the small wavelength act as carrier for the audio frequencies (i.e. modulating signal). Thus, the height of the antenna required is much reduced and becomes practical.
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| Devices called modems permit digital data to be transmitted over the analog telephone networks. |
As an example, if an audio frequency is translated to a radio frequency carrier of frequency 4 MHz, the antenna height required would be
x102 = 0.25 x 100 = 25 metres
This antenna height may be achieved practically.
- To remove interference
Another reason for not radiating modulating signal itself is that the frequency range of audio signal is from 20 Hz to 20 kHz. In radio-broadcasting, there are several radio stations. In case, there is no modulation, all these stations transmit audio or sound signals in the range of 20 Hz to 20 kHz. Due to this transmission over same range, the programmes of different stations will get mixed up.
Hence, in order to keep the various signals separate, it is necessary to translate or shift them to different portions of the electromagnetic spectrum. Thus each station is allocated a band of frequency. This also overcomes the drawback of poor radiation efficiency at low frequency.
As an example, in Amplitude Modulation radio-broadcast, the maximum modulating signal frequency permitted is 5 kHz. Amplitude Modulation requires a bandwidth of 10 kHz for each station or channel. Therefore, broadcast channels can be placed adjacent to each other, each channel occupying 10 kHz bandwidth. Hence, different stations may be allotted bandwidths say from 790 to 800 kHz, 800 to 810 kHz and so on. In radio receiver, a tuned circuit at the input selects the desired station and rejects all other stations.
- Reduction of noise
Noise is the major limitation of any communication. Although noise can not be eliminated completely, but with the help of several modulation schemes, the effect of noise can be minimized.