ASSMENT6
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Transcript of ASSMENT6
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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.