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Title : Different amplitude modulation

(AM) and frequency modulation(FM). what

the importance of these modulation?

Abstract

AM (or Amplitude Modulation) and FM (or

Frequency Modulation) are ways of

broadcasting radio signals. Both transmit

the information in the form of

electromagnetic waves. AM works by

modulating (varying) the amplitude of the

signal or carrier transmitted according to the

information being sent, while the frequency

remains constant. This differs from FM

technology in which information (sound) is

encoded by varying the frequency of the

wave and the amplitude is kept constant.

Introduction

AM radio ranges from 535 to 1705

kilohertz, where as FM radio ranges in a

higher spectrum from 88 to 108 megahertz.

For AM radio, stations are possible every

10 kHz and FM stations are possible every

200 kHz. AM has poorer sound quality

compared to FM, but is cheaper and can be

transmitted over long distances. FM is less

prone to interference than AM. However,

FM signals are impacted by physical

barriers.

Comparison

The advantages of AM radio are that it is

relatively easy to detect with simple

equipment, even if the signal is not very

strong. The other advantage is that it has a

narrower bandwidth than FM, and wider

coverage compared with FM radio. The

major disadvantage of AM is that the signal

is affected by electrical storms and other

radio frequency interference. Also, although

the radio transmitters can transmit sound

waves of frequency up to 15 kHz, most

receivers are able to reproduce frequencies

only up to 5kHz or less. Wideband FM was

invented to specifically overcome the

interference disadvantage of AM radio.

A distinct advantage that FM has over AM

is that FM radio has better sound quality

than AM radio. The disadvantage of FM

signal is that it is more local and cannot be

transmitted over long distance. Thus, it may

take more FM radio stations to cover a large

area. Moreover, the presence of tall

buildings or land masses may limit the

coverage and quality of FM. Thirdly, FM

requires a fairly complicated receiver and

transmitter than AM signal.

AM was initially developed for telephone

communication. For radio communication,

a continuous wave radio signal called

double sideband amplitude modulation

(DSB-AM) was produced. A sideband is a

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band of frequencies higher (called upper

sideband) or lower (called lower sideband)

than the carrier frequencies which is a result

of modulation. All forms of modulations

produce sidebands. In DSB-AM the carrier

and both USB and LSB are present. The

power usage in this system proved

inefficient and led to the double-sideband

suppressed-carrier (DSBSC) signal in which

the carrier is removed. For greater

efficiency, single-sideband modulation was

developed and used in which only a single

sideband remained. For digital

communication, a simple form of AM

called continuous wave (CW) operation is

used in which the presence or absence of

carrier wave represents binary data. The

International Telecommunication Union

(ITU) designated different types of

amplitude modulation in 1982 which

include A3E, double sideband fullcarrier;

R3E, single-sideband reduced-carrier; H3E,

single-sideband full-carrier; J3E, single-

sideband suppressed-carrier; B8E,

independent-sideband emission; C3F,

vestigial-sideband and Lincompex, linked

compressor and expander.

By: Department of Psychology, University

of Minnesota, Minneapolis 55455.

Journal of the Acoustical Society of

America (1994)

Volume: 96, Issue: 2 Pt 1, Pages: 733-740

The encoding mechanisms for amplitude

modulation (AM) and frequency

modulation (FM) were investigated using

AM-FM discrimination tasks. In the first

experiment, AM and FM were set at equally

detectable levels within a trial, and

discrimination thresholds were obtained

adaptively in a 3IFC task. Here, AM-FMdiscrimination thresholds were considerably

larger than both AM and FM detection

thresholds. This is consistent with an

encoding system whereby AM and FM are

partially encoded by the same mechanism.

In the second experiment, performance on

AM-FM discrimination is measured with a

fixed-level procedure. Psychometric

functions obtained for a constant

modulation depth of AM were

nonmonotonic with FMs modulation index

beta and each displayed a single minimum.

The nonmonotonic nature of the functions is

consistent with a model in which FM is

encoded primarily with the same

mechanism that encodes AM but also with a

second mechanism, probably related to

changes in instantaneous frequency, that is

independent of the mechanism that extracts

AM. The fact that minima in the

discrimination psychometric functions

increase from d' = 0 as beta increases

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indicates that the information encoded by

the second mechanism becomes more

detectable with increasing beta.