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• amplitude modulation demodulation

Let us first define modulation before diving into the understanding of th definition of amplitude modulation and the disadvantages of amplitude modulation. Modulation is the process in which any one feature of the wave is changed. The changed wave is called the carrier wave. The signal with which the carrier wave’s property is changed is called modulating signal. This signal contains the actual information that has to be conveyed. So we can say that we are transmitting information with hole of another wave. This signal will contain the information. The wave is used because it can be transmitted over a physical channel.

Any property of the carrier wave can be changed or altered by the help of modulating signal. We classify the types of modulation on the basis of which property is altered.

Let us now talk about the definition of amplitude modulation. In this type of modulation the amplitude of the carrier wave is increased. Amplitude or strength is the same. It can lead to the formation of double sidebands which are mirror image of each other.

Disadvantages of amplitude modulation are many and they are stated below -
1. This type of transmission can be affected by natural sources of radiation. It can lead to electromagnetic interference. This may lead to disruption of the original signal or distortion.
2. This type of transmission is also affected by the man made noises or radiations like waves from other antennas or channels.
3. It cannot be used for transmitting music as done by FM.
4. It can only be used for sports talks etc.
5. Now we know that the carrier signal does not have any information that is transmitted and which is the entire reason for modulation. But in Amplitude modulation we have almost two third of the energy or power is in carrier waves. We can reduce this by cutting the carrier wave but then we have to have complex receiver equipment. Having such equipment is again costly. Hence serves no purpose.
6. In this type of modulation, for one transmitter we will have many corresponding receivers which further increase the cost.
7. It cannot be used for transmission of sensitive information like in army etc. where loss or disruption or interpretation during transmission is not an option.
8. All the above can lead to a bad overall and soppier transmission.
9. The bands in which this type of modulation is used is also low that means it is used in low frequency transmission.
10. It is again important to note that whatever we are doing in this type of transmission is the information present in sidebands. That is all the information is present in the sidebands. And all the power is used by the carrier waves.
All the above have affected the use of amplitude modulation a lot and even adversely. It has been limited to very primitive uses and is not use for Hi fi communication or transmissions. It has been suppressed in recent years due to the use of Frequency modulation.

From Wikipedia

Modulation

In electronics, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with respect to a modulating signal. This is done in a similar fashion as a musician may modulate a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequencysinusoid waveform is used as carrier signal, but a square wave pulse train may also occur.

In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passband signal, for example low-frequency audio signal into a radio-frequency signal (RF signal). In radio communications, cable TV systems or the public switched telephone network for instance, electrical signals can only be transferred over a limited passband frequency spectrum, with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband.

A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (modulatorâ€“demodulator).

Aim

The aim of digital modulation is to transfer a digital bit stream over an analog bandpasschannel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band.

The aim of analog modulation is to transfer an analogbaseband (or lowpass) signal, for example an audio signal or TV signal, over an analog bandpass channel, for example a limited radio frequency band or a cable TV network channel.

Analog and digital modulation facilitate frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate passband channels.

The aim of digital baseband modulation methods, also known as line coding, is to transfer a digital bit stream over a baseband channel, typically a non-filtered copper wire such as a serial bus or a wired local area network.

The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a phone call over a wideband baseband channel or, in some of the schemes, as a bit stream over another digital transmission system.

In music synthesizers, modulation may be used to synthesise waveforms with a desired overtone spectrum. In this case the carrier frequency is typically in the same order or much lower than the modulating waveform. See for example frequency modulation synthesis or ring modulation.

Analog modulation methods

In analog modulation, the modulation is applied continuously in response to the analog information signal.

Common analog modulation techniques are:

Phase modulation

Phase modulation (PM) is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.

Unlike its more popular counterpart, frequency modulation (FM), PM is not very widely used for radio transmissions. This is because it tends to require more complex receiving hardware and there can be ambiguity problems in determining whether, for example, the signal has changed phase by +180Â° or -180Â°. PM is used, however, in digital music synthesizers such as the Yamaha DX7, even though these instruments are usually referred to as "FM" synthesizers (both modulation types sound very similar, but PM is usually easier to implement in this area).

Theory

PM changes the phase angle of the complex envelope in direct proportion to the message signal.

Suppose that the signal to be sent (called the modulating or message signal) is m(t) and the carrier onto which the signal is to be modulated is

c(t) = A_c\sin\left(\omega_\mathrm{c}t + \phi_\mathrm{c}\right).

Annotated:

carrier(time) = (carrier amplitude)*sin(carrier frequency*time + phase shift)

This makes the modulated signal

y(t) = A_c\sin\left(\omega_\mathrm{c}t + m(t) + \phi_\mathrm{c}\right).

This shows how m(t) modulates the phase - the greater m(t) is at a point in time, the greater the phase shift of the modulated signal at that point. It can also be viewed as a change of the frequency of the carrier signal, and phase modulation can thus be considered a special case of FM in which the carrier frequency modulation is given by the time derivative of the phase modulation.

The spectral behaviour of phase modulation is difficult to derive, but the mathematics reveals that there are two regions of particular interest:

2\left(h + 1\right)f_\mathrm{M},
where f_\mathrm{M} = \omega_\mathrm{m}/2\pi and h is the modulation index defined below. This is also known as Carson's Rule for PM.

Modulation index

As with other modulation indices, this quantity indicates by how much the modulated variable varies around its unmodulated level. It relates to the variations in the phase of the carrier signal:

h\, = \Delta \theta\,,

where \Delta \theta is the peak phase deviation. Compare to the modulation index for frequency modulation.

Optical modulator

An optical modulator is a device which is used to modulate a beam of light. The beam may be carried over free space, or propagated through an optical waveguide. Depending on the parameter of a light beam which is manipulated, modulators may be categorized into amplitude modulators, phase modulators, polarization modulators etc. Often the easiest way to obtain modulation of intensity of a light beam, is to modulate the current driving the light source, e.g. a laser diode. This sort of modulation is called direct modulation, as opposed to the external modulation performed by a light modulator. For this reason light modulators are, e.g. in fiber optic communications, called external light modulators.

With laser diodes where narrow linewidth is required, direct modulation is avoided due to a high bandwidth "chirping" effect when applying and removing the current to the laser.

Classification of optical modulators

According to the properties of material that are used to modulate the light beam, modulators are divided into two groups: absorptive modulators and refractive modulators. In absorptive modulators absorption coefficient of the material is changed, in refractive modulators changed is refractive index of the material.

Absorption coefficient of the modulator's material can be manipulated by Franz-Keldysh effect, Quantum-confined Stark effect, excitonic absorption, or changes of free carrier concentration. Usually, if several such effects appear together, the modulator is called electro-absorptive modulator.

Refractive modulators most often make use of electro-optic effect, other modulators are made with acousto-optic effect or magneto-optic effect or take advantage of polarization changes in liquid crystals. The refractive modulators are named by the respective effect: i.e. electrooptic modulators, acousto-optic modulators etc. Immediate effect of refractive modulator operation is change of the phase of a light beam. This can be converted into amplitude modulation by interferometers or directional couplers.

Separate case of modulators are spatial light modulators (SLMs). The role of SLM is modification two dimensional distribution of amplitude and/or phase of an optical wave.

See:

Modulator, optical

Amplitude modulation - Wikipedia, the free encyclopedia

A suppressed-carrier amplitude modulation scheme is three times more .... An approach which marries the advantages of low-level modulation with the ...

Question:

Answers:Let's just talk about radio stations that use AM and FM. AM bounces off of the atmosphere and can be received world-wide when conditions are right. But AM broadcasts are a lower bandwidth and amplitude noise comes through. FM is strictly line of sight, so it's broadcast range on our round planet is limited. FM is a higher bandwidth, so music sounds better. FM is also not susceptible to amplitude noise. Actually, an FM transmission can contain many separate channels or side-band stations. So, for instance, the new Satellite Radio broadcasts many stations on the same FM signal.

Question:

Answers:Advantage FM: Immune to AM noise riding on it`s carrier frequency. Affected much less by static caused by adverse weather. Disadvantage Fm : Requires wider bandwidth for transmission

Question:What the advantages of analog modulation over digital modulation, and what are the disadvantages of anlog modulation?

Answers:Pulse Code, Pulse position, and Pulse width modulation are forms of digital modulation. By turning the carrier off and on in a pattern (a numerical code, the position of pulse within a certain time-frame, or the width of a pulse within a certain time-frame) one may convey information over the carrier. Advantages: - the modulation method is simple: switch the carrier on and off. - one can achieve close to the theoretical limit of information transfer within a given band. Disadvantages: - square waves and sharp "on"/"off" edges of the carrier produce harmonics which must be filtered out or interference with adjacent bands will result. - the receiver must have some form of synchronization with the transmitter to decode the digital information (synchronization wastes bandwidth because it is not 'information'). Analog modulation uses the analog information to modulate a carrier: either it's amplitude, frequency, phase, or a combination of the 3. Advantages: - Very simple demodulation techniques for AM and FM (1 or 2 diodes, capacitors, and a transformer (for FM)) - Linear amplifiers and modulators are used -- no digital circuits. Disadvantages: - Modulation methods require lots of power -- comparable with the carrier power. - Generally the information is not a dense as in digital modulation .

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