Difference Between Analog and Digital Modulation

Main Difference – Analog vs. Digital Modulation

Analog and digital modulation refers to how a carrier signal is modified in order to transmit information. The main difference between analog and digital modulation is that an analog modulated signal can take up any value (in a possible range) whereas a digitally modulated signal can only take up one of a discrete set of values.

What is Modulation

Suppose you want to broadcast your music so that people from kilometres around can hear it. Naively, you might simply turn up the volume. However, the sound would die off without travelling far away, and people who do not want to hear your music will be forced to listen, as well!

Think of instead “converting” your sound wave into an electromagnetic wave and transmitting your music that way. Now, people who want to listen can use a converter to convert the electromagnetic waves back into sound, and people who do not want to listen would not be disturbed. A problem occurs, however, when other people also start to broadcast their music. Their electromagnetic waves are going to interfere with yours and your listeners are going to end up with a jumble of sounds.

So how do radio stations do it? They each have a frequency, and they transmit their signals using that frequency. A person who wants to listen to a particular frequency should then “tune into” that frequency using their own radio. But now there is another problem. Humans can hear sounds over a large range of frequencies. How can the radio stations transmit all those different frequencies using just one frequency? The answer is: modulation.

A wave with the frequency of the radio station is called the radio station’s carrier signal. This is simply a sine wave with no interesting information in it. The information signal is the signal containing data that we want to transfer (e.g. music in case of a radio station). A radio station changes the properties of its carrier signal depending on the  information signal, and this process is called modulation. The modulated signal is broadcast, and the listeners’ radios should now demodulate the signal in order to extract the sound information from the received signal.

What is Analog Modulation

In analog modulation, the carrier signal is modulated in proportion to the information signal so that it can take any value (i.e. it is an analog signal). There are three main types of analog modulation:

Amplitude modulation (AM): Here, the amplitude of the carrier wave is changed, depending on the information signal:

Amplitude Modulation

As you can see, whenever the amplitude of the information signal changes, the amplitude of the modulated signal also changes along with it.

Frequency Modulation (FM): Here, the frequency of the carrier wave is changed according to the information signal.

Frequency Modulation

Note that in the case of radios there is a limit to how much the frequency can be changed by, so that one station’s signal can be kept separate from another station’s signal.

Phase Modulation (PM): Here, the phase of the carrier wave is altered according to he information signal:

Phase Modulation

What is Digital Modulation

In digital modulation, the information signal used is a digital one, i.e. it is a signal that can only take up specific values. Digital signals are usually represented in binary using a series of 0’s and 1’s. The larger the number of 0’s and 1’s used to represent the signal in a given time interval, the more is the number of values that the signal could take. For instance, in our example for a radio station, the original audio signal would need to be “chopped up” into a number of small time intervals, and for each time interval, an approximate “allowed” value for the signal would need to be selected. By chopping up the signal into very small time intervals and by using a large number of “allowed values” to represent data, the sound can be made to sound natural.

There are several different types of digital modulation as well.

In amplitude shift keying, the amplitude of the signal is modulated to represent the information. The simplest type of modulation is called on-off keying, where the carrier signal is turned on to represent a 1 and turned off to represent a 0.

In frequency shift keying, the frequency of the wave is modulated whereas, in phase shift keying, the phase of the wave is modulated. Quadrature amplitude modulation is a type of modulation where amplitude and phase are both modulated to, and because there are several different combinations, this type of modulation can represent many different values for the signal.

Unlike with analog modulation, in digital modulation the carrier wave is modified at specific time intervals. Since digital modulation can only transmit specified values, the information is technically not as perfect as the original version (people often refer to this as having “low fidelity” to the original signal). However, it is easier to isolate noise from digital signals. Multiplexing (sending several different signals using the same medium) is also easier when the modulation is digital.

Difference Between Analog and Digital Modulation

Allowed Values

Analog Modulation: An analog modulated signal can represent any value within a range.

Digital Modulation: A digitally modulated signal can only represent one of a set of discrete values.

Variation with Time

Analog Modulation: Analog modulation can produce a signal that carries continually changing information.

Digital Modulation: Digital modulation produces a signal whose value changes at specific intervals of time.

Separation of Noise

Analog Modulation: It is difficult to separate the signal from noise in analog modulation.

Digital Modulation: In digital modulation, the signal can be easily separated from noise.

Image Courtesy

“The illustration of amplitude modulation (AM) which depicts comparison between information signal, carrier signal, and AM signal.” by Ivan Akira (Own work) [], via

“The illustration of frequency modulation (FM) which depicts comparison between information signal, carrier signal, and FM signal.” by Ivan Akira (Own work) [], via

“The illustration of phase modulation (PM) which depicts comparison between information signal, carrier signal, and PM signal.” by Ivan Akira (Own work) [], via