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Transcript
RADAR AND TELEVISION ENGINEERING
No.1(i)why flicker is not removed by progressive scanning?
Ans .flicker can be removed by doubling the speed of progressive scanning,but doubling the speed
will double the band-width,which is not desirable.
(ii).why is retraced blanked?
Ans. retraced is blanked because if the retrace is visible on the screen,it will cause distortion in the
picture.
(iii).what is kell factor(k)?
Ans. The kell factor (k) or the resolution factor gives the effective number of horizontal scanning
lines can be scanned because of practical limitation.the value of kell factor lies between 0.65 to 0.75.
(iv).what is importance of back porch?
Ans. Back porch absorbs ringing oscillations due to fly-back. In the absence of the back porch,these
oscillations would have distorted the video signal.
(v).which principles are used for optical electrical conversion?
Ans. Two basic principles are used for optical electrical conversion.They are based on photo electric
effects. The two principles are: (a) photo emission. (b) photo conduction.
(vi). List 4 merits of digital TV receivers.
Ans.Reduced ghosts. Reduced of 50 HZ flicker. High resolution pictures. Slow motion action.
(vii).write short notes on video disc system.
Ans: The video disc is essential a gramophone record with pictures on it.The first video disc was
the laser optical and second video disc system is based on the capacitance principle
. (viii). Why do blind speeds occur?
Ans. Blind speeds occur because of the sampled nature of the pulse radar wave form.Thus it is
sampling that is the cause of ambiguities or aliasing in the measurement of the doopler frequency
. (ix). What do you mean by longitudinal video recording?
Ans: A method in which video signals are recorded on atleast several tracks along the length of the
tape.
(x).What are the two types of video disc system?
Ans. Laser or optical disc system. capacitance disc system.
No(.2) Write the vertical sync and blanking pulse standard.
Ans. A vertical sync waveform is inserted in the composite video signal at the end of each field of
312.5 lines .Each vertical sync consist of (a)pre-equalizing pulses(b)field sync pulses and (c) Field
Blanking Period (VB): During this period, video signal is suppressed and field retrace is completed
.In CCIR B-system, blanking period VB equals 20 H. Thus VB=20H=1280µS.Since there are two
fields per picture frame, the no of active lines is (625-40=585 lines. Field Sync Pulses: The vertical
pulses may be distinguished from the horizontal sync pulses by their larger duration, being about
2.5H long in CCIR B system .These serrations are so timed that the leading edges of horizontal sync
pulses existed. The extra half line pulses coinciding with horizontal sync pulses. It becomes
necessary at the middle of horizontal scanning lines in case of odd field and at the end of a
horizontal scanning line in case of an even field. EQUALISING PULSES Solves the shortcoming
occurring on account of half line discrepancy. Five narrow pulses of 2.5 line period are added on
either side of the vertical sync pulses known as pre-equalizing and post equalizing pulses The effect
of these pulses is to shift the half line discrepancy away from both the beginning and end of the
vertical sync pulses: PRE-EQUALISING PULSES: 2.3µS duration Result in the discharge of the
capacitor to zero voltage in both the fields POST-EQUALISING PULSES Necessary for a fast
discharge of the capacitor to ensure triggering of the vertical oscillator at proper time. With the
insertion of equalizing pulses:-the voltage rise and fall profile is the same for both the field
sequences. The vertical oscillator is triggered at the proper instants exactly at an interval of 1/50th of
a second.
No(.3.) What is the function of tv camera tube ?what are the major problem occuring during
the conversion of optical to elletrical signal in tv camera?
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Heart of a TV camera is a camera tube.
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Camera tube –converts optical information into electrical signal
Amplitude proportional to brightness.
Optical image is focused by a lens assembly to a rectangular glass face –plate.
Transparent conductive coating at the inner side of the glass face plate.
On which is laid a thin layer of photo conductive material having a very high resistance when
no light falls on it.
Resistance decreases when the intensity increases .
Ellectron beam –used to pick up the picture information available on the target plate in terms
of varying resistance.
Beam is formed by an electron gun
Deflection coils are fed separately from two oscillators-continuosly generates saw tooth
waveforms having different desired frequency
Uses magnetic deflection
Deflection by first coil-horizontal motion of beams and then brings quickly to left side to
commence the trace of next line.
Deflection by second coil-Vertical motion and its quick retrace back to the top to start the
process allover again.
Encounters different resistance across the target plate.
Result in a flow of current which varies in magnitude during scanning.
We get the true information of the scene
Scanning converts the inantenna formation existing in space and time coordinates into time
variation only –called a video signal.
Video signal is amplified-amplitude modulated with channel picture carrier frequency.
The major problems during this conversion are:
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Poor sensitivity.
Poor resolution.
Higher noise level .
Improper spectral response .
Instability.
Poor contrast range.
Difficulties in processing the output signal
camera pick up devices meet the above requirements.
Note that modern TV
No.(4)Derive a mathematical expression for radar equation.
The radar range equation relates the range of the radar to the characteristics of the transmitter,
receiver antenna ,target & the environment.
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It is used as a tool to help in specifying radar sub system specifications in the design phase
of a program.
The various factor that determine the range of a radar set is
Assume : Rt=Rr=R=Range (distance to target) PT = Transmitter power(peak power of radar
transmitter)watt. GT=Antenna power gain(w.r.t.isotropic)i.e max power gain of transmitting antenna
relative to isotropic radiators. S=Target cross section =RCS=Radar cross section m2. PR=Received
power,i.epower absorbed by receiving antenna relative to isotropic radiators. λ=Wave length
Ae=aperature area of receiving antennai.e effective antenna aperature. Ao=Capture area of receiving
antenna . _If the transmitter delivers PT watts into an isotropic Antenna,then the power
density(watt/m2)at a distance ‘R’ from the radar
is
PT/4πR2 watts/m2……………………………(1) Therefore,power density
from a directional antenna= PTGT/4πR2 watt/m2 ………………………..(2)
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The target intercepts a portion of the incident power which depends on its cross sectional
area(‘s’).
Therefore intercepted power at target is
=PTGTσ/4ΠR2*1/4ΠR2
=PTGTσ / (4ΠR2)2…………………………………………………………………….(4)
Power delivered to the receivers is
=PrGTσ/ (4ΠR2)2.Ae................................................. (5)
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Ae called power of the echo signal delivered to the receivers is
Ae= G.λ2/4π………………………………………………………… (6)
Also Ae= AA.na……………………………………………………… (7)
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As same antenna is used for reception & transmission then
G
R
= G (8)
T…
P =P σ.A *4π/16.π R λ =P σ.Ae /4π.R λ ……………………(9)
R
T.
e
2
2.
4.
2
T.
2
4.
2
Maxm Range, Rmax=(PT.AE2.σ/4π.λ2.Smin)1/4……………………………(10)
Rmax= (PTG2T.λ2σ/(4π)3.Smin)1/4 …………………………………….........(11)
RMAX= (PTGT.Gr λ2.
σ./(4π)3.Smin)1/4 ………………………………………..(12)
where GT=GR The equations (10)(11)(12)are different forms of radar range eqution.
No(.5) Explain how the image orthicon develops video signal when light from any scene is
focused on its face plate.
Ans.It was first produced in the year 1945 and because of its superior performance its soon
replaced all earlier tv camera tube like image dissector,iconoscope and orthicon.It has the
following qualities : (i)It has high photo graphic sensitivity . (ii)It provides a very dependable service.
(iii)It provides an excellent response over a wide. range of illumination level varying from bright
sunlight to dark shadow. As a result of this ,image orthicon remained in popular TV studio use for a
long period but it is now being replaced by plumb icon.
Principle of working : A lens system focuses light from the scene on to atranslucent photo
cathode . Electrons gate emitted from the various points on photocathode surface in proportion to
the illumination .These photo electrons travel to a thin glass target causing secondary emission
from it .These secondary electrons get collected by a fine mesh placed close to the glass target
plate as shown fig.This causes electron deficiency distribution or positive charge distribution on the
photo cathode. This positive charge distribution originally on the front surface of the target plate
leaks through thin plate to the back surface. A low velocity scanning beam from an electron at the
other end of the image orthicon tube scans the back surface of the target plate following a
predetermined scanning pattern . theelectroned is slowed down to a non zero velocity as it
approaches the target.From a dark element ,the scanning electron beam is returned unaffected
.However ,at light element(positively charge element),the scanning beam is deprived of some of its
electrons to netralise the positive charge so that the returning electrons to neutralize the positive
charge so that the returning electron beam contains lesser electron .This varying density returning
beam is fed to an electron multiplier.
No(.6). What is NTSC system and what are the limitation of NTSC?
NTSC system (NATIONAL TELEVISION SYSTEM COMMITTEE)
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NTSC system is compatible with 525 lineAmerican system.
In order to maintain compatibility two new colour difference signals are generated & they
arerepresented as I&Q instead of transmitting (R-Y)&(B-Y)directly.
The signal I&Q are prepared from (R-Y)&(B-Y) using the following relations e.g. Inphase
component,
I=0.74(R-Y)-0.27(B-Y)..........................(1) & Quadrature phase component, Q=0.48(R-Y)+0.41(BY).......................(2)
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In the NTSC system the luminance signal Y is used for brightness or monochrome
transmisssion.
Since eye is capable of resolving finer details in the region around I,it is allowed to have a
maximum bandwidth of 1.5 MHZ.The bandwidth of 'Q' signal is restricted to 0.5 MHZ.
The NTSC system has following limitations i.e
1. It is sensitive to transmission path difference which is introduce phase errors.These errors
change colors .
2. When over the program change over takesplace beetween local television network & video
tape recorders 'the phase error has introduced in chroma signal.
3. The phase angle of chrominance signal is also affected by levelof signal.
4. Cross talk between demodulator output causes distortion.