Amplitude Modulation and Demodulation: 7 Important Facts


  • What is Amplitude Modulation
  • Virtues, Limitations and Modifications of Amplitude Modulation
  • VSBSC Modulation
  • DSBSC Modulation
  • SSB Modulation
  • Advantages & disadvantages

What is Amplitude Modulation?

Define Amplitude Modulation:

“A modulation procedure where amplitude of carrier is varied with respect to the instantaneous value of modulating signal is called Amplitude Modulation”.

Amplitude Modulation
Amplitude Modulation, Image Credit – Almuhammedi at Arabic WikipediaAmplitude Modulation, marked as public domain, more details on Wikimedia Commons

From the context of communications, a main motivation for the modulation would be to ease transmission of this information-carrying signal over a communication channel or radio station via a prescribed pass-band. On this basis, we might classify continuous-wave modulation to two broad category: amplitude modulation and angle modulation. Both of these modulation differentiate themselves by providing absolutely distinctive spectral features and as a result distinct functional advantages. The classification is completed on the basis of if the amplitude of the sinusoidal carrier wave, or the frequency or phase the angle of the sinusoidal carrier wave, is varied in nature with the information signal.

Illustration of Amplitude Modulation 1
Illustration of AM, Image Credit – Ivan AkiraIllustration of Amplitude ModulationCC BY-SA 3.0

Concepts of Amplitude Modulation:

Consider a carrier signal is characterized by,

                                  C(t) = Ac cos (2πfct)

Here, AC is the carrier signal amplitude and fc is the carrier signal freq. The information or message signal is indicated by term m(t); An amplitude-modulated (AM) wave may thus be described as a function of time as follows:

                                          s(t) = Ac[1+Kam(t)]cos(2πf­ct)

Where Ka is a constant termed as the amplitude sensitivity. Characteristically, the carrier amplitude, message signal are stated in volts, and amplitude sensitivity is represented in volt-1

  1. The amplitude of |Kam(t)| is generally less than unity;

                       |Kam(t)|<1, for all t

  • The carrier freq. ( f­c ) is much higher than the maximum freq. element represented by W of the message signal m(t);

                                         fc >>W

  • For +ve freq., the maximum freq. of the Amplitude modulation wave is equal to (fc + W), and the lowest freq. element is equal to (fc – W). The difference between these two freq. terms as the transmission bandwidth (BT) of the amplitude modulation wave, which is precisely double the message signal bandwidth (W). So

                                     BT = 2W

Modulation Index of Amplitude Modulation:

Modulation index indicates how much modulated variable of the carrier signal fluctuates around its unmodulated level. In Amplitude modulation, this quantity also termed as modulation depth, specifies by exactly how much the modulated variable differs around its original level.

Mathematically modulation index is, ma, defined by,

2 3

     where, K = proportionality constant;

              Vm = amplitude of modulating signal;

              Vc = amplitude of carrier signal;

We know that,

               A = amplitude of modulated signal = Vc(1+masinωmt)

So,          Amax = Vc(1+ma) and Amin = Vc(1-ma)

Finally, modulation index,

3 2

What is VSB-SC Modulation?

Define Vestigial Side Band System Modulation in Amplitude Modulation:

Single sideband modulation works reasonably for an info signal with an energy gap centred around ‘0’ frequency. If more information is to be broadcasted in a given time then corresponding larger B.W. is required, for example: television

  • SSB can play important role in reducing the bandwidth
  • We can analyse the case of video transmission for television system
  • Bandwidth occupied by T.V. video signal minimum 4MHz. So, a transmitted B.W. of 9 MHz at least would be required. So SSB is used for saving the B.W.
  • While using SSB, care must be taken to see at the receiver end. No problem of demodulation arises. So the carrier passed undiminished or as it is.
  • As the phase response of the filter at the edges of the flat pass band is loud to have bad effect on video signals received in a T.V. receiver a part of the unwanted i.e., lower sideband also transmitted. The effect of this is to produce a vestigial transmission system also known as AGC. A typical frequency spectrum of this type is shown :
640px Ssb de
SSB, Image Credit – de:User:DB1BMNSsb-de, marked as public domain, more details on Wikimedia Commons
  • 1.25 MHz of the lower side band gets transmitted along with the USB so that the lowest frequencies of the required USB will not be distorted in their phase by the vestigial sideband filter as only 1.25 MHz of the LSB is transmitted; a saving of nearly 3 MHz of V.H.F spectrum is produced with every T.V channel. This makes it promising to allow multiple number of channel is in the same bandwidth.
  • In the above figure, it was observed that the receiver video amplifier frequency response the sound occupies a frequency band near the video amplifier frequency response. The sound occupies a frequency band near the video as it is required with the picture and in practice it is not possible to have separate receiver to receive the sound operating at distant frequency i.e., away from Video Frequency.
  • In television receiver attenuation is intentionally delivered for video frequency from 0 to 1.25MHz. the reason for this is the extra power is transmitted for this part of the information of the video signal as it is transmitted in both sideband this would have produced unnecessary emphasis in the video output of the receiver if the attenuation had been absent.

What is DSB-SC Modulation?

Define Double Side Band Modulation in Amplitude Modulation:

Fundamentally, the double side band suppressed carrier (DSB-SC) modulation comprises of the product of the message signal and the carrier wave as shown in the equation

                              s(t) = c(t)m(t)

                                     = Ac cos (2πfc t) m(t)

Consequently, the device utilized to produce the DSB-SC modulated signal is the denoted to as ‘product modulator’. It is also identified fact that not like AM, DSB-SC modulation is reduced to ‘0’ at whatever time the message signal is not present.

Thus, the apparatus used to create the DSB-SC controlled wave is termed as product modulator. In addition, we understand that unlike any amplitude modulation, DSB-SC modulation is decreased to zero if the message code is switched off.

Mostly, the signal goes into a phase change if the message signal is not zero. The packet of a DSB-SC controlled signal is so dissimilar from the message one, meaning that simple demodulation with the packet detection isn’t a feasible choice for DSB-SC modulation.

DSB-SC features:

  • Only two side-band with suppressed carrier is transmitted
  • With carrier suppressed power saving for m=1 is 66%
  • It requires lesser bandwidth
  • It has balanced modulation

What is SSB-SC Modulation?

Define Single Side Band (SSB-SC) Modulation:

In suppressing the carrier, DSB-SC modulation have a significant limit of Amplitude modulation when it is to this wastage of transmitted electricity. To look after another significant restriction of Amplitude modulation when it comes to station bandwidth, we will have to suppress one of both sidebands from the DSB-SC modulated wave. This adjustment of DSB-SC modulation is exactly what implemented in SSB modulation. In significance, SSB modulation be subject to entirely on the lower-sideband and upper-sideband to transmit the message transfer through communication channels based on which side-band is in fact is communicated.

Single Side Band can be represented mathematically as;

                    sssb (t) = s(t) . cos(2πf0t) – ŝ(t) . sin(2πf0t),

Where, s(t) is the message, ŝ(t) is its Hilbert Transform, and f­0 is the radio carrier frequency.

SSBSC Features:

A SSBSC has the following features:

  • Only one side band is transmitted
  • With one side boundary for m=1 it is 83.3%
  • Its bandwidth is least
  • This is a phase shift method modulator.

Comparison between DSB-SC and SSB-SC:

                  DSB-SC                  SSB-SC
Only two side-band with suppressed carrier is transmitted.   With carrier suppressed power saving for m=1 is 66%   It requires lesser bandwidth     It has balanced modulation    Only one side band is transmitted     With one side boundary for m=1 it is 83.3%   Its bandwidth is least     This is a phase shift method modulator.  

Advantages and disadvantages of Amplitude Modulation:


  • Small antenna size.
  • Long range communication.
  • Using repeater any distance communication is possible.
  • Noise can be eliminated.


  • Power requirement is high.

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