CH-5 TELEVSION RECEIVERS Marks: 15(21)
Introduction: The aim of TV system is to extend the sense of sight beyond its natural limit and to transmit sound associated with scene. The picture signal is generated by a TV camera and sound signal by microphone. In India, picture signal is amplitude modulated and sound signal is frequency modulated. The two carrier frequencies are suitably spaced and their modulation products radiated through common antenna. Each TV station is allotted different carrier frequency to enable selection of desired station at receiving end.
Video Signal
TV picture and its Properties:
1. Aspect Ratio:
The frame adopted in all TV system in rectangular with width to height ratio i.e. aspect ratio = 4:3. The eyes can view with more ease and comfort when width of picture is more than its height. In motion pictures width to height ratio is also 4:3. This is important reason for adopting this shape and aspect ratio. It enables direct TV transmission of film programmes.
2. Scanning:
The process of converting optical image/information into corresponding electrical signal is called scanning. A TV camera is used to convert the optical information into a corresponding electrical signal. The amplitude of electrical signal varies in accordance with the variations of brightness.
The scanning process in TV is very much similar to reading the page of this book. The eye starts reading at the upper left-hand corner and travels to the right across the first line of words. At the end of the first line, it quickly returns to the left-hand side of the page and starts reading the second line and so on. This process is repeated for each line, till the end of the bottom line of the page is reached when eye returns to the top and starts the same process for the next page.
An optical image of the scene to be transmitted is focused by a lens assembly on the rectangular glass face-plate of the camera tube. The inner side of the glass face-plate has a transparent conductive coating on which is laid a very thin layer of photoconductive material. The photolayer has a very high resistance when no light falls on it, but decreases depending on the intensity of light falling on it. Thus depending on the light intensity variations in the focused optical image, the conductivity of each element of the photolayer changes accordingly. An electron beam is used to pick-up the picture information now available on the target plate in terms of varying resistance at each point. The beam is formed by an electron gun in the TV camera tube.
Two simultaneous motions are thus given to the beam, one from left to right across the target plate and the other from top to bottom thereby covering the entire area on which the electrical image of the picture is available. As the beam moves from element to element, it falls on different resistance across the target-plate The result is a flow of current which varies in magnitude as the elements are scanned. This current passes through a load resistance RL, connected to the conductive coating on one side and to a dc supply source on the other. A varying voltage appears across the resistance RL and is the video signal. It is used to modulate the picture carrier for transmission to distant places.
The scanning is done line by line horizontally from left to right at fast rate. The retrace of beam is very fast and retrace is blanked. During reproduction, an exactly similar scanning process is used at receiving end.
The beam scans the image horizontally. After each line scanned the beam scans second line slightly lower than previous one. In this way beam reaches to bottom of frame. In vertical scanning electron beam moves from top to bottom and during retrace it moves quickly from bottom to top. In scanning process 25 frames are scanned in one second. Each frame is divided into 625 lines. So frequency of horizontal scanning = 25x625 = 15625Hz. While vertical scanning frequency is 25Hz.
This scanning process is called progressive scanning.
2. Flicker:
Though scanning of 25 frames per second in TV pictures is enough to cause an illusion of continuity, they are not enough rapid to allow the brightness of one to blend smoothly into next when screen is blanked between successive frames. This results in flicker of light which is annoying the observer.
3. Interlaced scanning:
The repetition rate of 25 pictures per second in television creates the problem of flicker. This problem is solved by special method called interlaced scanning.
In interlaced scanning all the 625 lines are not scanned at a stretch but the scanning process is divided into two stages called fields. Each field will contain only half the total number of lines contained in one frame (i.e. 312.5 lines).
The scanning beam will scan only alternate lines. The scanning process starts at A when the first set of 312.5 lines is scanned sequentially in the first (odd) field. The scanning of odd lines will end at B. The beam suddenly flyback to point C and start scanning the even set of lines in the second (even) field. It will end at D. From here the beam will flyback to A and the scanning process will start all over again.
This makes the picture repetition rate double the frame repetition rate. It is 50 fields per second. The number of lines scanned per second remains the same. Thus, in interlaced scanning the problem of flicker is overcome without increasing the band width. The interlaced scanning used in India consists of 25 frames and 50 fields per second.
Synchronization:
For proper reproduction of the televised picture at the receiving end, it is essential that the scanning sequences at the transmitter (camera) and receiver are completely in step with each other.
In other words, the scanning of each line and each field should start as well as finish simultaneously at the transmitter and TV receiver.
The process by which the horizontal and vertical sweeps at the camera and TV receiver are kept in step with each other is known as synchronization.
This is achieved by sending sync signals from the transmitter. These timing signals are in the form of rectangular pulses used to control both the transmitter and receiver scanning.
Composite Video Signal:
Components Composite Video Signal:
Video/Picture signal: For maximum whiteness in picture the video signal level is fixed at 10 to 12.5% of maximum value of composite video signal. While black level corresponds to 72% called blanking level. The picture information varies from 10% to 75% of composite video signal. It depends on relative brightness of picture at any instant. The darker the picture the higher will be the voltage. Duration of picture information is 52μs.
Blanking pulses: At the end of each line or field, the scanning beam quickly returns to start the scanning of another line or field. The path followed by the returning electron beam is called retrace. The retrace is made invisible by the process known as Blanking.
The composite video signal contains these pulses to make the retrace lines invisible. It consists of horizontal (H) and vertical (V) blanking pulses to blank corresponding retrace intervals. The repetition rate of horizontal blanking pulses is therefore equal to the horizontal scanning freq. i.e. 15625Hz. Its width is 12μs. The freq. of vertical blanking pulses is same as vertical scanning freq. i.e. 50Hz.
Hori. Sync pulses: The interval for horizontal scanning lines is H= 64μs. Out of this the horizontal blanking period is 12μs. During this interval, hori. Sync pulse is inserted. It is used to synchronize the hori. scanning rate at receiver. Its duration is 4.7μs. During this period electron beam completes its back stroke (retrace) and arrives at extreme left end.
The hori blanking period is divided into three sections. These are front porch, hori sync and back porch.
Front porch: This is small period of 1.5μs inserted between the end of picture details and leading edge of hori sync pulse. This interval allows the receiver video ckt to settle down from whatever picture voltage level exists at the end of picture line to blanking level before sync pulse occurs.
Back porch: This small period of 5.8μs. It allows hori fly back to be completed and permits time for hori time base ckt to reverse direction of current for initiation of scanning of next line. It is also utilized in AGC ckt to produce an AGC voltage.
Vertical sync pulses: At end of each field of scanning, a V-sync pulse is inserted in composite video signal.
Field blanking period: The duration of field blanking period is equal to 20 line period i.e. 20x64 = 1280μs.
Field sync pulse: This pulse is of much longer duration in order to make distinguishable from H-sync pulse.
Equalizing pulses: After completing one field, five narrow pulses are added on either side of vertical sync pulses. These are known as pre-equalizing and post-equalizing pulses. Each set consists of five narrow pulses occupying 2.5H-period on either side of V-sync pulse.
Colour Television System
Introduction:
Colour television was first introduced in the United States in the year 1953. The PAL (Phase Alteration Line) colour system was approved for adoption in India on the 14th of July, 1981 and the first colour transmission was put on air on the 15th august, 1982.
Requirement of Colour Television:
The monochrome (Black & White) television was developed first and monochrome telecasting had started in many countries when colour television was being developed. This posed the main problem in the development of colour television as the colour system had to be compatible with the existing black and white systems.
Compatible means that, while a colour television receiver will reproduce colour picture from the colour transmission, the monochrome receiver should be able to reproduce black & white picture from the same signals without any modification to the receiver.
At same time the colour TV receiver should able to reproduce black & white picture from the normal monochrome transmission. This arises from the need that the existing monochrome receivers should not become useless on the introduction of colour.
To achieve compatibility, it is necessary that the colour signal be in two parts – one pertaining to the brightness part of the picture and the other to the colour part of the picture.
The signal pertaining to the brightness part of the picture corresponds to the video signal of the monochrome television. This signal by itself can reproduce black & white picture in monochrome as well as colour TV receiver.
The second part of the signal containing the colour information provides extra information required by the colour receivers. This part adds colour to the picture. Thus both the signals are used in colour receivers to reproduce colour pictures.
The second requirement of the compatibility is that it should be possible to accommodate the colour TV signals within the bandwidth allotted for monochrome TV signals. This requires that the total bandwidth of colour signals should not exceed the bandwidth of black & white TV signals.
These signals are processed as given below-
Signal pertaining to the brightness part of the picture is obtain from the three colour signals Red, Green & Blue by combining these in suitable proportions. This signal is transmitted as it is.
The colour signal and the brightness signals are combined to obtain two colour signals which contain the colour information of the picture and their bandwidth is restricted.
Then the colour signals are modulated on two parts of the carrier known as sub-carrier. These two parts are differ in phase by 90o and each one of these is modulated.
Colour fundamentals
Colour is the sensation which results from the reaction of certain portion of the human eye to electromagnetic radiation having a frequency range from about 1014 to 1015 Hz.
The actual colour perceived by the eye depends on the frequency (or wave-length) of the radiation.
The variety of colours which can be perceived by the human eye is very large.
We generally speak about the colours in the visible spectrum as being red, orange, yellow, blue and violet.
Actually the spectrum consists of many more colours. For example there are many varieties of shades of red, orange, yellow and green etc.
The white light consists of radiation of a variety of colours. That is, human eye sees white as a result of simultaneous stimulations by light radiations of certain intensity of these varieties.
It has, however, been found that the sensation of white light can be produced by simultaneous stimulations of the eye by light radiations corresponding to only three colours viz.- Red, Green and Blue. These are known as ‘Primary Colours’ and have been selected for colour television.
Additive mixing:
It is used in the colour TV for reproducing colours. In this type of mixing, light from two or more colours can create a sensation due to combined effect of these colours.
For example, white light can be produced by mixing 30% Red, 59% Green and 11% Blue. Similarly, Yellow, Magenta and Cyan can be produced by suitable mixing the primary colours as shown below.
Red, Green and Blue are also called Additive primaries because of producing a wide range of colour mixtures.
Red, Green and Blue colours produced by the Tri-colour phosphor screen are integrated by the human eye to reproduce all the natural colours of the televised scene.
Brightness- Brightness is the amount of light intensity as perceived by the human eye. In colour TV the brightness is termed as Luminance.
In black and white pictures the areas which emit more light (i.e. which are whiter) have more brightness than the dark areas. The brightness reaction of the human eye will not be the same for the different colours. The eye’s response is maximum to green and yellow and is progressively less for red and blue. Thus yellow, yellow-green and green are the brightest.
Hue- Hue indicates the special colour of the received light. Thus the colour of any object is distinguished by its colour or hue. For example the green grass has the green hue and the red tomatoes have red hue. Different hues results from different wavelengths of spectral radiation.
Saturation- The saturation of a colour is a measure of its dilution with white light. A colour which is fully saturated (100 percent saturated) is a pure colour and has no white light mixed with it. If it is mixed with progressively increasing proportions of white light, the saturation is decreased. As the portion of white light will increased in red, the shade will become less red and with certain proportion of white light it will become pink. In this case the basic colour remains the same. As such hue remains the same while saturation decreases.
Chrominance is the term used to describe all colour information except brightness. It is a combination of hue and saturation in a colour.
The different colours only represent wave of different frequencies or wavelengths in the visible electromagnetic spectrum. When treated as an electromagnetic wave, a colour will have a frequency and amplitude. The frequency corresponds to hue and the amplitude corresponds to brightness.
Block Diagram of Colour TV Transmitter:
Working:
Three colour signals i.e. signals corresponding to the Red, Green and Blue components of the picture are obtained by a colour camera. These signals are known as R,G and B signals.
The luminance or ‘Y’ signal is obtain by combining the colour signals in the matrix in the proportions given below :
Y = 0.3R+ 0.59G+0.11B
The luminance signal contains the brightness information and by itself it can reproduce black and white pictures.
Colour difference signals are then obtain by combining the colour signals with the luminance signal. Since the entire colour information can be contain in two colour difference signals and Luminance signals, only two colour difference signals i.e. R-Y and B-Y signals are obtained. These can be obtained by subtracting the luminance signals from the colour signals. These signals form the chroma signal and contain the colour information.
The R-Y and B-Y signals are weighted to obtain U and V signals. The weighting factors are –
U = 0.493 (R-Y) and
V = 0.877 (B-Y).
The weighting is done just to avoid over modulation of the carrier and except for weighting the U and V signal carry the same colour information as combined in U and R-Y and B-Y signals. The U and V signals form the final colour difference signals. The section which performs the function of combining the signals and weighting these is known as ‘matrix section’.
The luminance signal is transmitted as it is and provides a high definition black and white picture. The chroma signals which are transmitted as some what low definition signals to add colour to picture. Because eye has limited resolving power of colour.
5. The luminance and chroma signals have finally to be transmitted by modulating these on the vision (picture) carrier.
Finally the modulated chroma signals and Luminance signals are Amplitude modulated with video (picture) carrier and then frequency modulated sound signals are added and transmitted through the transmitting antenna.
Colour TV Receiver
Simplified Block Diagram of Colour TV Receiver:
Block diagram of colour TV receiver:
Working:
Tuner: Colour TV receivers employ electronic tuners where channel tuning is done by varactor diode and band selection by switching diodes. The lower VHF band (band-I) covers frequency range of 41 to 68 MHz and VHF band (band-III) has frequency range of 174 to 230 MHz. This wide range of frequency channels require band switching in VHF tuners. To avoid mistuning of receiver, an arrangement called (Automatic Fine Tuning) AFT is used. Tuner consists of RF amplifier, mixer and local oscillator. Heterodyning action takes place in mixer. As a result IF signals namely picture IF (PIF=38.9MHz), sound IF (SlF=33.4MHz) and colour IF (CIF=34.47MHz) are produced.
Video IF amplifier: It receives Ifs from tuner amplify them. It provides high gain.
Sound signal separation: A separate detector is used to develop SIF signal. In this detector, PIF and SIF signals heterodyne to produce intercarrier SIF signal at 5.5MHz. 5.5MHz trap circuit permits to pass only this signal to SIF amplifier and FM detector circuit.
Audio o/p stage: It consists of driver and power amplifier to amplify the AF signal. Volume and tone controls are provided in this amplifier.
Video detector: This demodulates the amplitude modulated picture IF to recover video signal.
Video amplifier: It is necessary to raise the level of detected video signal before feeding to various sections of receiver. This is done by video amplifier circuit. The o/p of amplifier, composite video signal branches off into following circuits
(i) chroma detector (ii) AGC circuit (iii)sync separator.
Chroma (section) decoder: It contains all the circuits necessary for amplification and detection of modulated chroma signal from o/p of video amplifier. The main function of chroma decoder is to recover U and V colour difference signals which are later combined with Y-signal to obtain R, G and B video signals. Chroma detector has to perform the following functions-
(i) chroma signal separation and amplification
(ii) separation of U& V signals
(iii) demodulation of U &V signals to recover colour difference signals.
All modern receivers employ an IC in chroma section to carry out all the above functions. These signals are amplified by drive amplifiers then feed to cathodes of colour guns of picture tube.
Sync separator: Sync pulses are derived from the video signal in sync separator ckt by clipping about 70% picture signal portion. So, only sync pulses are present in the signal o/p.
Vertical oscillator: The oscillator oscillates at frequency below 50 Hz. The oscillator generates sawtooth drive voltage for o/p amplifier.
Vertical o/p amplifier: It sends sawtooth currents through vertical deflection coils to deflect beam vertically. This stage also supplies the blanking pulse to picture tube to blank the vertical retrace.
AFC: Automatic frequency control corrects the frequency of horizontal oscillator. So that horizontal oscillator can oscillate at 15625 Hz.
Horizontal oscillator: it generates trapezoid shaped drive voltage required for horizontal line o/p stage to feed saw tooth currents into horizontal deflection coils via horizontal o/p transformer.
The large flyback pulses during horizontal retrace can be stepped up by extra high tension (EHT) winding horizontal o/p transformer and rectified by rectifier to obtain supply for picture tube.
Power supply: The power supply in colour TV receiver normally provides DC voltage of about 115V to 160V. As the performance of TV receivers depends largely on the supply voltage, the supply voltage has to be stabilized to maintain it very close to the normal voltage.
Two types of voltage regulated supplies are normally used in colour TV receiver-
Transistor regulated power supplies and
Switching mode power supplies (SMPS).
Colour TV (CCIR-B) standards used in India:
Standards of the colour TV system adopted in our country (CCIR-B) are given below:
Nos. of lines per frame 625
Frame frequency 25 per sec. (25Hz)
Field frequency 50 per sec. (50Hz)
Line interlace 2:1
Picture aspect ratio 4:3
Channel separation 7.0 MHz
Picture carrier modulation AM
Polarity of Transmission Negative
Separation of sound carrier to vision carrier 5.5 MHz
Sound modulation FM
Line sweep frequency 15625 Hz
Chroma sub-carrier 4.43 MHz
Functional controls (operating controls) of colour TV receiver:
ON/OFF: Press to give power supply to TV set.
Channel up/down button: Press to select channel. On screen indicator displays channel of operation.
Volume up/down button: Press to change volume level.
Channel preset: Press to tune channels auto/manual. Screen displays tuning menu. Press `VOLUMIE+ or VOLUME ─ button to Search subsequent channel.
Brightness control: It adjusts the illumination on screen.
Contrast control: It controls the sharpness of picture on screen.
Colour control: It controls intensity or shade of colours in pictures from light shades to strong colours.
Menu: When pressed the screen displays menu. It allows setting clock, alarm, clock timer etc.
Power indicator: LED indicates power ON/OFF.
Remote sensor: It senses the signals of remote control unit. In modem colors TV receivers, most of functions are controlled through the provided ‘Remote Control’ transmitter.
Servicing of TV receiver
Safety precautions in TV servicing:
A contact with AC line can be fatal. Line connected receivers must have insulation so that no chassis point is available to the user. After servicing, all insulators, bushes, knobs etc must be replaced to their original position. ’
Voltage in the receiver, such as EHT can also dangerous. The service man should stand or sit on an insulated surface and use only one hand when probing receiver. The interlock and the back cover of receiver must be replaced properly.
Technician should be very careful not to introduce a fire hazard in the process of repairing TV receivers. The parts replaced must have correct or higher rating to avoid overheating.
If picture tube envelope is damaged, the glass may shatter violently and its pieces ily great distance with force. It can cause serious inury to any part of body.
For proper operation, the ground terminal of instrument is always connected to ground of receiver being serviced.
All connections with test leads to high voltage points must be made after disconnecting the receiver from AC mains.
High voltage capacitors must be discharged before connecting to test leads.
Only shielded wires and probes should be used. Fingers should not be touched to tips of probe when probe is in high voltage circuit.
Power source should have a suitable fuse to interrupt supply in case of a short circuit in line cord. .
Voltages in picture tube and high voltage rectifier must be maintained at designed values. These circuits may produce X-radiation which is hazardous.
Common faults in TV receiver with remedy for it:
Fault (1): No raster, no picture and no sound (Dead receiver)
Causes: 1. Defective power supply.
2. Defective mains cord.
3. Fuse blow, defective rectifier-filter
4. Defective voltage regulator.
Remedies: l. Check power supply circuit.
2. Replace defective mains cord.
3. Replace blown fuse.
4. Check voltage regulator circuit.
Fault (2): No meter, no picture but sound normal
Causes: l. Defective EHT transformer. .
2. Open boosted condenser,
3. Defective picture tube circuits, filament circuit, picture tube socket.
4. Defective video amplifier. l
Remedies: l. Check EHT rectifier circuit and winding.
2. Check ckt connection of boosted power supply.
3. Check and replace defective components of picture tube ckt.
4. Test the output of video amplifier.
Fault (3): Raster and picture normal out no sound.
Causes:1. Defective FM detector.
2. No o/p at AF stage. I
3. Defective sound IC.
4. Defective loud speaker.
Remedies: l. Check o/p of FM detector and replace defective components.
2. Check AF stage.
3. Check sound IC.
4. Check the loudspeaker and replace defective loudspeaker by new one.
Fault (4): Raster normal but no picture and no sound.
Causes: 1. Broken antenna. ..
2. Broken/defective feeder wire.
3. Defective tuner, video IF amplifier.
4. Improper AGC circuit and detector circuit o/p.
Remedies: l. Repair broken antenna or feeder.
2. Check tuner ckt and video IF amp, replace defective components.
3. Check operation of AGC and detector ckt.
Fault (5): Only horizontal bright line on screen.
Causes: 1. Open or short vertical deflection coil.
2. Dry soldering in vertical stage.
3. Defective vertical sweep.
Remedies: l. Check and remove defect in vertical deflection coil.
2. Check soldering in vertical stage,
3. Check operation of sweep circuit.
Fault (6): Only vertical bright line on screen.
Causes: l. Defective horizontal deflection coil.
2. Weak horizontal o/p stage.
3. Dry soldering in horizontal stage.
Remedies: l. Check and remove the defect in horizontal deflection coil.
2. Check horizontal o/p stage.
Fault (7): Picture width is less.
Causes: l. Defective horizontal oscillator.
2. Reduced H .T. voltage.
3. Defective horizontal deflection coil and booster diode.
Remedies: l. Test operation of horizontal oscillator.
2. Test the H .T. voltage.
3. Check and replace defective horizontal deflection coil and booster diode
Fault (8): Picture height is less.
Causes: l. Defective vertical oscillator.
2. Defective vertical o/p transformer.
3. Defective vertical deflection coil.
4. Faulty VDR.
Remedies: l. Replace defective transistor oscillator.
2. Check open or short ckt in vertical o/p transformer.
3. Check and replace defective vertical deflection coil.
4. Replace faulty VDR.
Fault (9): Picture rolling, vertically.
Causes: l. Vertical hold control may be defective.
2. Defective vertical oscillator.
Remedies: l. Check operation of vertical hold control.
2. Check operation sync separator.
3. Check frequency of vertical oscillator, it may be high or low
Fault (l0): Ghosts on picture.
Causes: l. Antenna direction is not correct. a
2. Missing reflector.
3. Defective feeder wire.
Remedies: l. Adjust proper direction of the antenna.
2. Check feeder wire.
Fault (11): No colour.
Causes: l. Defective chroma amplifier.
2. Defective colour killer.
3. Faulty sub-carrier oscillator.
Remedies: 1. Replace defective components of chroma amplifier.
2. Check operation of colour killer and sub-carrier oscillator.
Fault (12): Weak colour.
Cause: Defective chroma band pass filter.
Remedy: Adjust tuning of band pass amplifier.
Fault (13): One colour missing.
Causes: l. Defective chroma demodulator.
2. Defective electron gun.
Remedies: 1. Check the defective chroma demodulator.
2. Test the defective electron gun.
Fault (14): Abnormal intense of colours.
Cause: Defective automatic colour control.
Remedy: Replace defective components in automatic colour control.
Questions
Marks wise Questions: 1M(3), 3M(3), 4M(3), 6M(1)
1. Fill in the blanks.
For sound wave transmission in TV --------- is used.
(a) Amplitude modulation (b) Frequency modulation (c) phase modulation (d) pulse modulation. -
In India ---------- lines per field is used for TV scanning.
(a)312.5 (b) 812 (c) 525 (d) 625.
In India ---------- lines per frame is used for TV scanning.
(a)312.5 (b) 812 (c) 525 (d) 625.
The separation between the vision carrier and for the sound carrier India is --------.
(a)1.25 MHz (b) 5.5 MHz (c) 6.75 MHz (d) 100kHz
Aspect Ratio for a television picture is -----------. [O-09. M-13]
(a)4:3 (b) 3:4 (c) 3:2 (d) 2: 3.
In CCIRB standard the line frequency is ------------. [M-09]
(a)18750 lines per second (b) 17750 lines per second
(c) 15625 lines per second (d) 15850 lines per second.
In television, one picture frame contains ----------- horizontal lines. [O-11]
(a)625 (b) 525 (c) 455 (d) 312.5
Peak white level in video signal transmission is limited to ---------.
(a)5% (b) 10% (c) 15% (d) 20%.
The frequency of colour burst is ---------------.
(a) 5.5 MHz (b)7 MHz (c) 4.43 MHz (d) 38.9MHz
For picture wave transmission in TV -------- is used.
(a) phase modulation (b) frequency modulation
(c) amplitude modulation (d) pulse modulation.
In CCIR-B standard the field frequency is -----------.
(a)50KHz (b) 50MHz (c) 50Hz (d) 7MHz
.If horizontal line appears on the screen, the fault is in -----------section.
(a)Horizontal (b) Vertical (c) Tuner (d) video
---------- type of colour mixing is used in colour TV.
(a) Positive (b) Negative (c) Additive (d) Subtractive
In TV communication, BW of each channel is -------- MHz [M-10,M-12]
2. Match the following
1 A B
(a) AFC (1) 4.43MHz
(b) Chroma section (2) TV reception
(c) EHT voltage (3) 18kV/24kV
(d) Colour Sub-carrier (4) Burst signal
2 A B
(a) SIF[M-11] (1) 15625Hz
(b) Channel BW [M-09] (2) 38.9MHz
(c) Line frequency (3) 50Hz
(d) Field frequency [O-09] (4) 33.4MHz
(e)PIF (5) 7MHz
(f) Synch pulse (6) Composite video signal [M-13]
3. State True and False.
AM is used for transmitting sound signals and FM is used for transmitting video signals in colour TV receiver. [O-09]
Red, Green and Yellow are the primary colours in colour TV receiver. [O-11]
A colour killer stage is required in a colour TV receiver.
The horizontal sync frequency is 50Hz.
For giving Cathode voltage EHT is used in TV receiver.
The aspect ratio is 3:4 [M-12]
The frequency of sub-carrier burst is 5.5MHz.
In TV transmission, sound and picture both are amplitude modulated. [M-09]
In colour TV, picture tube contains three electron guns. [O-09]
Resolution of picture tube is expressed in pixels. [M-10]
In T.V. transmission, F.M. is used for picture transmission. [M-11]
In T.V. Transmission, picture is amplitude modulated. [O-11]
Colour TV picture tube has RBG electron gun. [M-13]
4. Answer the following.
What is composite video signal? Explain it with wave form. [M-09, M-11, O-11]
Write the different components of composite video signal. [M-13]
Explain aspect ratio and scanning for T. V. receiver. [M-11, O-11, M-12]
Explain the function of sync separator in TV receiver.
Which type of modulation is used for audio & video transmission? Explain in brief any one of them.
Name the picture tube controls in TV receiver? Explain the function of each.
Draw the block diagram of colour TV receiver and write function of any two stages.
Write short note on 1)colour burst 2) colour killer 3) colour matrix
Define the terms 1) luminance 2) chrominance 3) colour difference signals.
What is scanning? Explain it with diagram.
What is flicker?
Explain interlaced scanning with diagram. [O-09, M-10, O-11]
Enlist the common faults occur in colour TV receiver [M-10]
Locate the faulty stage in colour TV receiver for following faults.
i) Only horizontal line ii) Black and white picture, no colour
iii) Weak picture, no sound iv) only vertical line on screen v) No sound, picture OK. vi)One colour missing vi) vertical rolling vii) No, raster, No picture, No sound
viii) Picture Normal, No sound ix) Normal Sound & raster, weak picture ix) No raster, no picture but sound OK [M-09, O-09, M-11, O-11, M-12, M-13]
What are the standards (CCIR-B) used in India for TV system.
In which stage of a TV receiver are the following controls located.
Brightness control. ii) Contrast control iii)Volume control iv) Channel selector
v) Vertical linearity control vi) V hold Control vii) Width control viii) Colour control
Draw the block-diagram of colour TV receiver & write its working principle. [M -11, 12]
Explain function of TV transmitter with block diagram. [O-09, O-11, M-12, M-13]
Explain different front panel controls of colour `TV receiver. [M-09, O-09, M-12]
State and explain different safety precautions in TV servicing. [M-10]
Write use of synchronization and blanking pulses in composite video signal. [M-09]
Write down the functions of any three operating controls in TV receiver.
List any six properties of TV picture.
Ans: Aspect ratio, scanning, picture resolution, brightness, contrast, image continuity, flicker, bandwidth
