Modulation in physics is the process of changing one or more properties of a periodic waveform, the carrier signal, with a modulating signal that typically contains information that is to be transmitted from one place to another. There are three types of modulation namely:
- Frequency modulation
- Amplitude modulation
- Phase modulation
Until the process of superimposing a low frequency voice information component on a high frequency carrier signal was perfected,the most widely used form of communication was a system based on the transmission of a continuous wave signal.With this system,the signal was interrupted periodically (Morse code) to produce a coded message.The CW system required tremendous training and expertise on the part of the persons involved in transmitting and receiving the messages ,and therefore the field was limited to a few experts.
With the development of modulation ,a whole new era of communications evolved,the results of which can be seen all around us today.We will now examine the process of modulation in more detail.
Need for Modulation
There are two alternatives to the use of a modulated carrier for the transmission of messages in the radio channel.One could try to send the (modulating) signal itself,or else use an un modulated carrier.The impossibility of transmitting the signal itself will be demonstrated first.
Although the topic has not yet been discussed,several difficulties are involved in the propagation of electromagnetic waves at audio frequencies below 20 kHz.Foe efficient radiation and reception the transmitting and receiving antennas would have to have lengths comparable to a quarter wavelength of the frequency used.This is 75 meters at 1 MHz ,in the broadcast band,but at 15 KHz it has increased to 5000 m .A vertical antenna of this size is impracticable.
There is an even more important argument against transmitting signal frequencies directly,all sound is concentrated within the range from 20 Hz to 20kHz ,so that all signals from the different sources would be hopelessly and inseparably mixed up.In any city ,the broadcasting stations alone would completely blanket the “air” and yet they represent a very small proportion of the total number of transmitters in use.
In order to separate the various signals,it is necessary to convert them all to different portions of the electromagnetic spectrum.Each must be given its own frequency location.This also overcomes the difficulties of poor radiation at low frequencies and reduces interference.Once signals have been translated,a tuned circuit is employed in the front end of the receiver to make sure that the desired section of the spectrum is admitted and all the unwanted ones are rejected.The tuning of such a circuit is normally made variable and connected to the tuning control,so that the receiver can select any desired transmission within a predetermined range,such as the very high frequency (VHF) broadcast band used for frequency modulation(FM).
Although this separation of signals has removed a number of the difficulties encountered in the absence of modulation ,the fact still remains that un modulated carriers of various frequencies cannot,by themselves ,be used to transmit intelligence.An un modulated carrier has a constant amplitude ,a constant frequency and a constant phase relationship with respect to some reference .A message consists of ever varying quantities.Speech ,for instance ,is made up of rapid and unpredictable variations in amplitude (volume) and frequency (pitch).Since it is impossible to represent these two variables by a set of three constant parameters ,an un modulated carrier cannot be used to convey information.
In a continuous wave modulation system (amplitude or frequency modulation,but no not pulse modulation )one of the parameters of the carrier is varied by the massage.Therefore at any instant its deviation from the un modulated value (resting frequency) is proportional to the instantaneous amplitude of the modulating voltage,and the rate at which this deviation takes place is equal to the frequency of this signal.In this fashion,enough information about the instantaneous amplitude and frequency is transmitted to enable the receiver to recreate the original message.
Speech and music etc.are transmitted hundred of kilometers may by a radio transmitter. The scene in front of a television camera is also sent many kilometers away to viewers. In all these uses, the carrier of the program is a high frequency radio wave. The information i.e., light, sound or other data is pressed on the radio wave and is carried along with it to the destination.
Modulation is the process of combining the low-frequency signal with a high-frequency radio wave called carrier wave. The resultant wave is called a modulated carrier wave. The low-frequency signal is known as the modulation signal. Modulation achieved by changing the amplitude or the frequency of the carrier wave in accordance with the modulating signal.
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Types of modulation
According to modulation, there are of three different types namely:
- Amplitude modulation (AM).
- Frequency modulation (FM).
- Phase modulation (PM).
Besides these, there are also another two types of modulation which are :
- Pulse modulation.
- Digital modulation.
Now we learn in detail about modulation and its types.
What is Amplitude modulation?
In this process of modulation amplitude of carrier wave changes according to the signal that is the only the amplitude of carrier waves changes in amplitude modulation.
When there is no signal, the amplitude of carrier wave is equal to the modulated amplitude but when the signal is present the amplitude of carrier changes in accordance of 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.
Limitations of amplitude modulation:
- The amplitude modulation suffers from the following limitation
- The useful power contain in side bands and even at 100% modulation.The side bands contains only 33% of the total power and hence the modulation efficiency is poor.
- The reception in this modulation is noising.
- Due to poor efficiency the transmitter employing amplitude modulation as very poor range.
What is Frequency modulation?
The process by which the frequency of the carrier waves is changes according to the amplitude of the signal is known as frequency modulation.
Initially when the signal voltage zero at the frequency of the carrier does not changes so the modulation is zero.
During positive cycle of the signal frequency of the carrier increases so frequency of the carrier becomes maximum. During negative cycle the frequency of the carrier becomes minimum.
The range of frequency for Frequency modulation broad casting each from 88–108 MHz
In FM modulation the deviation of the carrier frequency from it’s centre frequency represent message.
If we during modulation the centre frequency drips the message will be destroyed.
Learn also: Difference between FM vs AM
What is Phase modulation?
In this process of the modulation phase of the carriers voltage is change accordance with the instant value of the modulating signal.Where the frequency and amplitude remain constant.
What is Pulse modulation?
The pulse code modulation is method that is convert an analog signal in to digital signal so that modified analog signal can be transmitted through the digital communication network.
The pulse code modulation done in three different steps:
Sampling is a process of measuring the amplitude of the continuous-time signals and converts in to direct signals. There are two type techniques used for sampling:
In quartization process an analog sample.with an amplitude at converted in to a digital sample with an amplitude.that takes one of the pacific definite quartization value.
The encodes the quantized sample each quantized sample is encoded as a bite work (watt).