Modulation (AM, FM)

Understanding Modulation

Modulation involves altering one or more properties of a high-frequency periodic waveform, in accordance to the original message signal.
Modulation is crucial in communication systems to send information over a distance, allowing the information to be encoded in a form that can be transmitted efficiently.
The three major types of modulation are Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM).

Amplitude Modulation (AM)

In Amplitude Modulation, the amplitude of a carrier wave is changed relative to the instantaneous amplitude of the modulating signal, while frequency remains constant.
The output signal of AM contains the original frequency and two sidebands each at the frequency of the carrier signal plus and minus the frequency of the input signal.
Upper and lower sidebands contain the same information; thus, one could be removed, resulting in single-sideband (SSB) modulation, to improve the efficiency of signal transmission.
AM is susceptible to noise, as noise can easily alter the amplitude of the signal.

Frequency Modulation (FM)

In Frequency Modulation, the frequency of the carrier wave is modified according to the instantaneous amplitude of the modulating signal, while the amplitude stays constant.
FM provides better sound quality than AM because changes in frequency do not get affected by the small-amplitude noise present in the environment.
However, FM requires higher bandwidth as compared to AM. This is because the bandwidth required in FM transmission is roughly equal to twice the frequency deviation.
FM is used predominantly for audio broadcasting, for instance, FM radio where signal-to-noise ratio and immunity to noise and interference are more important.

Other Important Concepts

In single-frequency modulation (SFM), frequency modulation (FM) is applied to a sinusoidal carrier wave. It is a subset of FM.
The device that alters the carrier wave is known as the modulator, while the device which extracts the original information signal is the demodulator or detector.
Modulation index is a value that describes the extent of the variation in a carrier signal according to the information being sent. It is different for different types of modulation.
In AM, the modulation index is the ratio of the peak amplitude of the modulating signal to the peak amplitude of the carrier signal, whereas in FM, it’s the ratio of the peak frequency deviation of the carrier signal to the peak frequency of the modulating signal.
The process of simultaneous transmission of multiple messages over a single channel is called Multiplexing. There are two primary types of multiplexing – Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM).
FDM uses different carrier frequencies for each user and all users can transmit simultaneously, whereas in TDM all users share the same frequency but transmit in different time slots.
Both types of multiplexing strategies can be used in conjunction with any type of analogue modulation (AM or FM) to increase the capacity of the communication channel.