Difference between AM and FM

What is AM and FM?

FM (Frequency Modulation)

In FM, a radio wave is known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted.

The amplitude and phase remain constant.


AM (Amplitude Modulation)

AM is a technique used in electronic communication, most commonly for transmitting information through a radio carrier wave. 

It works by continuously changing the strength of the transmitted signal in relation to the information being sent.

For example, The changes in the strength of the signal may be utilized to specify the sounds to be reproduced by the speaker, or the light intensity of television pixels.
On this page, we will learn about the following:


  • Difference between AM and FM

  • Comparison between AM and FM

  • Difference between  FM and PM modulation in tabular form

  • Distinguish between amplitude modulation and frequency modulation

  • AM versus FM

  • Amplitude modulation

  • Frequency modulation

Difference Between AM and FM

S.No

Parameters

AM

FM

1.

Full form

Amplitude modulation

Frequency modulation

2.

Origin

AM method of audio transmission was successfully carried out in the mid-1870s.


FM radio was developed in the United States in the 1930s by Edwin Armstrong.


3.

Modulating differences

In AM, a radio wave is known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted. 


In FM, a radio wave is known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted.


4. 

Constant parameters


The frequency and phase remain the same.



The amplitude and phase remain the same.


5.

Quality


AM has poorer sound quality, and a lower bandwidth but is cheaper and can be transmitted over long distances as it has a lower bandwidth that is why it can hold more stations available in any frequency range.


FM is less affected by interference, but FM signals are impacted by physical barriers. They have a better sound quality due to higher bandwidth.

6.

Frequency range


AM radio ranges from 535 to 1700 kHz or up to 1200 bits per second.



FM radio ranges in a higher spectrum from 88.1 to 108.1MHz. or up to 1200 to 2400 bits per second.


7.

Bandwidth BW

BW  is much less than FM.

BW =  2 . fm


BW is large. Hence a wide channel is required.

BW = 2 x (δ + fm)


8.

Bandwidth requirements

Bandwidth is less than FM or PM and doesn’t depend upon the modulation index.


 Bandwidth requirement is twice the highest modulating frequency.


In AM radio broadcasting, if the modulating signal has a bandwidth of 15 kHz, then the bandwidth of an amplitude-modulated signal is 30 kHz. 


Bandwidth requirement is greater and depends upon the modulating.


.Bandwidth requirement is twice the sum of the modulating signal frequency and the frequency deviation.

 

Let’s say, if the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, the bandwidth required is 180kHz.


9.

No of Sidebands

The number of sidebands  are constant and equal to 2.


The number of sidebands having significant amplitude depends upon the modulation index


10.

Zero crossings in modulating signal

Equidistant

Not equidistant

11.

Complexity

AM transmitters and receivers are less complex than FM and PM, but synchronization is needed in the case of SSBSC carriers.


FM (or PM) transmitters are more complex than AM because the variation of modulating signal has to be converted and detected from the corresponding variation in frequencies. 


12.

Noise


AM receivers are very less susceptible to noise because noise affects the amplitude, which is where information is stored in AM signals.


FM receivers are better immune to noise and it is possible to decrease noise by further deviation.

13.

Efficiency

Power is wasted in transmitting the carrier.


All transmitted power is useful so that’s why FM is very efficient. 


14.

Application


MW (Medium wave), SW (short wave) band broadcasting, video transmission in T.V.


Broadcasting FM, audio transmission in T.V.



Difference Between AM, FM, and PM

S.No

Parameters

FM

AM

PM

1.

Definition

Frequency modulation is a technique of modulation, in which frequency of carrier varies in accordance with the amplitude of modulating signal. Keeping amplitude and phase constant.

The amplitude modulation is a technique of modulation in which the amplitude of the carrier wave varies in accordance with the amplitude of the modulating signal. Keeping frequency and phase constant.

The phase modulation is a technique of modulation in which phase of the carrier wave varies in accordance with the amplitude of the modulating signal. Keeping amplitude and frequency constant.

2.

Noise

Noise immunity of FM is superior to AM and PM

AM receivers are very susceptible to noise

Noise immunity better than AM but not FM

3.

Function

The frequency of the carrier wave deviates as per the voltage of the modulating signal input.

The amplitude of a carrier wave in AM diverges as per amplitude or voltage of modulating signal input

A phase of the carrier wave varies as per the voltage of modulating signal input.

4.

Constant parameter


The amplitude of the carrier wave is kept changeless.

The frequency of the carrier wave is kept invariable.

The amplitude of the carrier wave is kept changeless.

5.

Types

Digital FM types: FSK, GFSK, offset PSK

AM types: DSB-SC, SSB, VSB, etc.

Digital PM types: QPSK, BPSK, QAM (the combination of amplitude and phase, modulation).

6.

Waveforms





Summary

In AM radio, the amplitude, or overall strength, of the signal is varied to integrate the sound information.

In AM, a radio wave known as the "carrier" is modulated in amplitude by the signal that is to be transmitted. 


FAQ (Frequently Asked Questions)

1. What is the modulation?

Modulation is a technique in electronics that is used for impressing a piece of information such as voice, music, picture, or data on a radio-frequency carrier wave by changing one or more features of the wave in accordance with the intelligence signal. The most commonly altered features include amplitude, frequency, phase, pulse sequence, and pulse duration.

2. Write the advantages and disadvantages of FM.

Advantages

  • The noise can be reduced by increasing the deviation that’s why FM is more immune to noise.

  • Due to higher efficiency most, the power is utilized and the FM transmissions can be used for the stereo sound transmission due to a large number of sidebands.


Disadvantages

  • The bandwidth of FM is 10 times larger than the AM. Hence a wide channel would be required for FM transmission.

  • The transmitting and receiving equipment are very complicated in FM.