RADAR AND TELEVISION ENGINEERING

No.1(i)whyflicker 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 kellfactor(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 tofly-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 photoelectric 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 dooplerfrequency .(ix). What do you mean by longitudinal video recording?

Ans: A method in which video signals are recorded onatleast 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 PULSESSolves 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 fieldsPOST-EQUALISING PULSESNecessary 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 1/50thof 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?

  • Heart of a TV camera is a camera tube.
  • 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:

  1. Poor sensitivity.
  2. Poor resolution.
  3. Higher noise level .
  4. Improper spectral response .
  5. Instability.
  6. Poor contrast range.
  7. Difficulties in processing the output signal Note that modern TV camera pick up devices meet the above requirements.

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.

  • 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 PTwatts into an isotropic Antenna,then the power density(watt/m2)at a distance ‘R’ from the radar isPT/4πR2watts/m2……………………………(1) Therefore,power density from a directional antenna= PTGT/4πR2watt/m2………………………..(2)

  • 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)

  • Aecalled power of the echo signal delivered to the receivers is

Ae= G.λ2/4π………………………………………………………… (6)

Also Ae= AA.na……………………………………………………… (7)

  • As same antenna is used for reception & transmission then

GR = GT…(8)
PR=PT.σ.Ae2*4π/16.π2.R4.λ2=PT.σ.Ae2/4π.R4.λ2……………………(9)

MaxmRange, 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=GRThe 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 earliertv 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)

  • 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(B-Y)...... (2)

  • 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 takesplacebeetween 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.