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Difference between amplitude (AM) and frequency (FM) modulation


What is modulation?

Until the process of superimposing a low frequency (long wave) voice information component on a high frequency (short wave) carrier signal was perfected,the most widely used form of communication was a system based on the transmission of a continuous wave (CW) 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 modulated carrier wave. The low frequency signal is known as modulation signal. Modulation achieved by changing the amplitude or the frequency of the carrier wave in accordance with the modulating signal. Thus have two types of modulations which are

  • Amplitude Modulation (A.M)
  • Frequency Modulation (F.M)

Amplitude Modulation(AM)am modulation

This type of modulation the amplitude of the carrier wave is increased or diminished as the amplitude of the superposing modulating signal increases and decreases.

Represents a high frequency carrier wave of constant amplitude and frequency. Represents a (          ) or audio frequency signal of a sine waveform. The result obtained by modulating carrier wave with the modulating wave. The A.M. transmission frequencies range from 540 kHz to 1600 kHz.

Frequency Modulation (FM)frequency modulation

In this type of modulation the frequency of the carrier wave is increased or diminished as the modulating signal amplitude increases or decreases but the carrier wave amplitude remains constant. The frequency of the modulated carrier wave is highest (point H) when the signal amplitude is at its maximum positive value and is at its lowest frequency (point L) when signal amplitude has maximum negative. When the signal amplitude is zero, the carrier frequency is at its normal frequency f.

The F.M. transmission frequencies are much higher and range between 88 MHz to 108 MHz. The F.M. radio waves are affected less by electrical interference then A.M. radio waves and hence provide a higher quality transmission of sound. However, they have a shorter range than A.M. waves and are less able to travel around obstacles such as hills and larger buildings.

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