applications of am,ssb,vsb

7/27/2019 Applications of AM,SSB,VSB

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FREQUENCY-DIVISION MULTIPLEXING

Multiplexing is a technique whereby a number of independent signals can becombined into a composite signal suitable for transmission over a commonchannel.

This operation requires that the signals be kept apart so that they do notinterfere with each other, and thus they can be separated at the receiving end.This is accomplished by separating the signals either in frequency or in time.

The technique of separating the signals in frequency is referred to as frequency-division multiplexing (FDM) , whereas the technique of separating the signalsin time is called time-division multiplexing (TDM).

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A block diagram of an FDM system

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The incoming message signals are assumed to be of the low-pass type, buttheir spectra do not necessarily have nonzero values all the way down to zerofrequency.

Following each signal input, we have shown a low-pass filter, which isdesigned to remove high-frequency components that do not contributesignificantly to signal representation but are capable of disturbing other message signals that share the common channel.

These low-pass filters may be omitted only if the input signals are sufficiently band-limited initially.

The filtered signals are applied to modulators that shift the frequency ranges of the signals so as to occupy mutually exclusive frequency intervals.

The necessary carrier frequencies, to perform these frequency translations, areobtained from a carrier supply.

the most widely used method of modulation in frequency-divisionmultiplexing is single-sideband modulation , which requires a bandwidth that is

approximately equal to that of the original message signal. 3


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The band-pass filters following the modulators are used to restrict the band of each modulated wave to its prescribed range.

The resulting band-pass filter outputs are next combined in parallel to form theinput to the common channel.

At the receiving terminal, a bank of band-pass filters, with their inputsconnected in parallel, is used to separate the message signals on a frequency-occupancy basis.

Finally, the original message signals are recovered by individual demodulators.

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APPLICATION : RADIO BROADCASTING

In radio broadcasting, a central transmitter is used to radiate message signals

for reception at a large number of remote points. The message signals transmitted are usually intended for entertainment

purposes. There are three general types of radio broadcasting,

AM broadcasting , which uses standard amplitude modulation; FM broadcasting , which uses frequency modulation; and television broadcasting , which uses amplitude modulation of one carrier for picture

transmission and frequency modulation of a second carrier for sound transmission.

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AM RADIO The usual AM radio receiver is of the super heterodyne type, which is

represented schematically in Fig.. Basically, the receiver consists of a radio

frequency (RF) section, a mixer and local oscillator, an intermediate frequency(IF) section, and a demodulator. Typical frequency parameters of commercialAM radio are:

RF carrier range = 0.535-1.605 MHz

Mid-band frequency of IF section = 455 kHz IF bandwidth = 10 kHz

Antenna

IF Stage(intermediate frequency)

IF Amplifier & IF BPFX

Converter (Multiplier)

a (t ) b(t ) d (t )

c(t )

Envelope Detector

Diode, Capacitor,Resistor, & DC blocker

Audio Stage

Power amplifier

d (t ) e(t ) f (t ) g (t )

Ganged RFBPF and

Oscillator

RF Stage(radio frequency)

RF Amplifier & RF BPF

LocalOscillator

cos[( c+ IF)t ]

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The incoming amplitude modulated wave is picked up by the receiving antennaand amplified in the RF section, which is tuned to the carrier frequency of theincoming wave.

The combination of mixer and local oscillator (of adjustable frequency) provides a frequency conversion or heterodyning function, whereby theincoming signal is converted to a predetermined fixed intermediate frequency,usually lower than the signal frequency. This frequency conversion is achieved

without disturbing the relation of the sidebands to the carrier. The result of thisconversion is to produce an intermediate-frequency carrier defined by,

where f LO is the frequency of the local oscillator and f RF is the carrier frequency of

the incoming RF signal. We refer to f IF as the intermediate frequency (IF). because the signal is neither at the original input frequency nor at the final baseband frequency.

LO RF IF f f f

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TELEVISION Television (TV) refers to the transmission of pictures in motion by means of

electrical signals. To accomplish this transmission, each complete picture has

to be sequentially scanned. The scanning process is carried out in a TVcamera.

In a black-and-white TV, the camera contains optics designed to focus animage on a photocathode that consists of a large number of photo sensitive

elements. The charge pattern so generated on the photosensitive surface is scanned by an

electron beam, thereby producing an output current that varies temporally inaccordance with the way in which the brightness of the original picture varies

spatially from one point to another. The resulting output current is called thevideo signal.

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The type of scanning used in television is called a raster scan; It is somewhat analogous to the manner in which we read a printed paper in

that the scanning is performed from left to right on a line-by-line basis. In particular, a picture is divided into 525 lines that constitute a frame. Each

frame is decomposed into two interlaced fields, each one of which consists of 262.5 lines. For convenience of presentation, we will refer to the two fields as Iand II.

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The lines of field I are depicted as solid lines, and those of field II are depicteddashed lines.

The start and end of each field are also included in the figure. Field I is

scanned first. The scanning spot of the TV camera moves with constantvelocity across each line of the field from left to right.

When the end of a particular line is reached, the scanning spot quickly flies back in a horizontal direction) to the start of the next line down in the field.

This fly back is called the horizontal retrace. The scanning process describedhere is continued until the whole field has been accounted for. When thiscondition is reached, the scanning spot moves quickly (in a vertical direction)from the end of field I to the start of field II. This second fly back is lied thevertical retrace. Field II is treated in the same fashion as field I.

the time taken for each field to be scanned is 1/60 second. Correspondingly,the time taken for a frame or a complete picture to be scanned is 1/30 second.

With 525 lines in a frame, the line scanning frequency equals 6.75 kHz .11


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Thus, by flashing 30 still pictures per second on the display tube of the TV receiver,the human eye perceives them to be moving pictures. This effect is due to a

phenomenon known as the persistence of vision.

Video waveform for One full line of TV picture

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Video Bandwidth

The reproduction quality of a TV picture is limited by two basic factors: The number of lines available in a raster scan, which limits resolution

of the picture in the vertical direction. The channel bandwidth available for transmitting the video signal,

which limits resolution of the picture in the horizontal direction.

For each direction, resolution is expressed in terms of the maximumnumber of lines alternating between black and white that can beresolved in the TV image along the pertinent direction by a human

observer.

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Choice of M odulation

The type of modulation chosen to transmit the video signal is influenced by two

factors:

1. The video signal exhibits a large bandwidth and significant low-frequency

content. This suggests the use of vestigial sideband modulation.

2. The circuitry used for demodulation in the receiver should be simple and

therefore cheap. This suggests the use of envelope detection, which requires

the addition of a carrier to the VSB modulated wave.

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With regard to point 2, the use of envelope detection (applied to a VSBmodulated wave plus carrier) produces waveform distortion in the messagesignal recovered at the .detector output. The distortion is contributed by thequadrature component of the VSB wave.

The channel bandwidth used for NTSC TV broadcast is 6 MHz; This channel bandwidth not only accommodates the bandwidth requirement of the VSB-modulated video signal but also provides for the accompanying sound signalthat modulates a carrier of its own.

The values presented on the frequency axis in parts (a) and (b) of Fig. pertainto a specific TV channel. According to this figure, the picture carrier frequency is at 55.75 MHz, and the sound carrier frequency is at 59.75 MHz

Note, however, that the information content of the TV signal lies in abaseband spectrum extending from l.25 MHz below the picture carrier to 4.5MHz above it.

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Color TelevisionThe transmission of color in commercial TV broadcasting is based on the premise

that all colors found in nature can be approximated by mixing three additive

primary colors: red , green , and blue .These three primary colors are represented by the video signals m R(t), m G(t), andm B(t), respectively.

In the standard color-television system, the three signals that are transmitted havethe form,

m L(t) = 0.30 m R(t) + 0.59 m G(t) + 0.11 m B(t)m I (t) = 0.60 m R(t) - 0.28 m G(t) - 0.32 m B(t)mQ(t) = 0.21m R(t) - 0.52m G(t) + 0.31m B(t)

The signal m L(t) is called the luminance signal ,: when received on a conventionalmonochrome television receiver, it produces a black-and-white version of thecolor picture.

The signals m I (t) and mQ(t) are called the chrominance signals ; they indicate theway the color of the picture departs from shades of gray.

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m R(t) = m L(t) - O.96m I (t) + 0.62m Q(t)mG(t) = m L(t) - 0.28m I (t) - O.64m Q(t)m B(t) = m L(t) - 1.10m I (t) + 1.70m Q(t)

The luminance signal m L(t) is assigned the entire 4.2 MHz bandwidth. Owing tocertain properties of human vision, tests show that if the nominal bandwidths of the chrominance signals m I (t) and mQ(t) are 1.6 MHz and 0.6 MHz, respectively,then satisfactory color reproduction is possible.

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Fig. shows the baseband format of a split-luminance and split, chrominance(SLSC) type of transmission system that satisfies both of these requirements.It uses a 10-MHz baseband composite signal that can be transmitted as avestigial sideband modulated wave in a channel bandwidth of 12 MHz. Also,an NTSC receiver (tuned to the lower 6 MHz portion of the 12 MHzspectrum) will operate with the same quality achieved in a conventionalsystem.

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applications of am,ssb,vsb

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