Electronics and Communication Engineering Seventh Semester EC 2034-TELEVISION AND VIDEO ENGINEERING IMPORTANT QUESTIONS AND ANSWER 2MARKS PREPARED BY: VIGNESHWARAN.S AP/ECE 1. What is vertical and horizontal scanning? The capability of resolving picture details in the vertical direction is called vertical resolution & The ability of the system to resolve maximum number of picture elements along the scanning lines determines horizontal resolution. 2.
Differentiate monochrome and colour camera tube?
In black and white camera only one pickup tube is needed but three such tubes or its equivalent is necessary in color cameras to develop separate signals for red, green and blue information present in the scene. 3. Define Focal length. The focal length of the lens is the distance between the centre of the lens and the image formed by it of an object at infinity. 4.
Define Field of view.
This describe the width and height of the scene viewed and is determined by the focal length, lens to object distance and the width and height of the scanned area Field of view = scanned width x (lens to object distance/focal length) 5.
What do you refer by persistence of eye?
The persistence of eye refers to the storage capability of the human eye. It is 60ms 6. Define kell factor and number of scanning lines.
The pixels are distributed
randomly in a picture. So the spot of scanning beam at some places may cover
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two pixels partly. This will reduce resolution subjective tests made on several people showed that reduction in general is form 28% to 32% (average value=30%).Hence while calculating practical resolution the theoretical resolution should be multiplied by a factor of 0.7. 7. Define Aspect ratio and image continuity. Aspect ratio can be defined as the ratio of width to height of the picture frame. For television, it is standardized as 4:3. If each frame is displayed in 25frames/second due to our persistence of vision we will see it continuous 8. What kind of modulation is used for video signal?Why? The type of modulation in which the modulated video signal amplitude decreases with the increase in brightness of the picture is called as negative modulation. Here the peak white signal corresponds to near nea r zero level.The blanking level is at 75% modulation level Less disturbances due to noise pulses. Less transmitting power is required to transmit transmit because most of the picture information is white. Less transmitting power in transmitter. Gain of the receiver circuit is automatically adjusted using AGC circuit 9. List the contents of composite video signal . Composite video signal consists of a camera signal, blanking pulses and synchronizing pulses. 10. Compare image orthicon and vidicon camera tubes. S.No
IMAGE ORTHICON Works on photoconduction principle
VIDICON CAMERA TUBES Works on photo emission principle
Consists of bismuth silver cesium Antimony trisulphide(a compound semiconductor) Resolution is poor
Resolution is better than image orthicon
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11. Define pedestal level and pedestal height. In positive polarity, the pedestal level is represented by zero IRE ,This is a fixed level and is treated as reference .IRE values of all other levels are indicated with respect to reference level .In negative modulation ,it is represented by75% of the carrier signal. The pedestal level is also called blanking level. Pedestal height is the distance between the pedestal level and the average value axis of the video signal . 12. Why is vertical scanning very slow in comparison with horizontal scanning. To cover large number of pixels in scanning. 13. Define Active lines. All the scanning lines are not visible on the screen due to blanking during the retrace period.The actual lines which reproduce pixels are called active lines.The number of active lines (N A ) is equal to the total number of scanning lines per frame(NT) minus the line lost in vertical blanking(N L),as expressed by N A = NT- NL 14. What is dark current? In camera tubes the target used is semiconductor material it is sensitive to light in absence of light it should hold no charge but due to room temperature there may be charge on the target which pocess current on the output circuit this is known as dark current. current . 15. Determine height and width of a TV screen of 30cm size? SOLUTION 5x=30cm,therefore x=6cm Hence,height=6*3=18cm and Width=6*4=24cm 16. What do you understand by TV standards? Why are these standards different in different countries? TV standards consist of a set of specification for transmission of TV programmes.Broadly speaking these specifications pertain to the scanning process.The reason for this being different is electric main frequency of the country .
17. Describe scanning and justify interlaced scanning. Scanning is the process of converting optical image into electrical signal. When the scanning beam reaches the bottom of the picture frame, it quickly returns to the top to scan those lines that were missed in the previous scanning. Hence the total number of lines are divided into two groups called fields. Each field is scanned alternately. This way of scanning is called interlaced scanning.
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PART B ANSWERS 18. Neatly draw and explain a vidicon camera tube cross section. CONSTRUCTION
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It is a small size camera tube operates on the principle of photo conduction. Here the conduction of target plate increases with the intensity of light falling on it. Vidicon camera tube has the following three sections Photo conductive target plate Scanning section and Electron gun section TARGET SECTION The target consist of thin photo-conductive layer madeup of either antimony or selenium compounds. On the inner surface of the face plate a transparent, conductive film is coated and on this the photo-conductive material is deposited. The conductive coating is also called as signal electrode. SCANNING SECTION The scanning section is used to move the electron beam horizontally (left to right) and vertically (T-B) to scan the entire target plate. Horizontal and vertical deflection coils are used for this purpose. Electron beam approaches the target plate with low velocity and due to low velocity, secondary emission are avoided. ELECTRON GUN SECTION Heater, cathode, control grid, focussing coils and accelerating anode forms the electron gun section. Cathode: used to emit electron
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Control grid: used to control the electrons coming out of the cathode Focussing coils: used to focus the electron beam to a sharp point on the target plate ELECTRIC CIRCUIT:
ADVANTAGES OF VIDICON Resolution is high Long life and small size By varying the target voltage, it can be operated at different levels of sensitivity. Gamma cancellation circuit is not necessary. DISADVANTAGES OF VIDICON High dark current Poor sensitivity High image lag
Scanning
NOTE:This NOTE: This diagram is common to all camera tubes
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19. Explain in detail about image orthicon camera tube.
(8)
Developed in 1945 by radio corporation of america. The main transducer in image orthicon camera tube is photocathode. It consists of coating of bismuth-silver-cesium compound on the inside of the glass face plate of the tube. When light from the scene is focused on the photo cathode, electrons are emitted from it. The energy of the electrons emitted from different points of the cathode depends on the intensity of light(energy of the photons) Being a conductor, the photocathode cannot store charge. Design of cathode is such that electron comes out in parallel beams. The electrons are accelerated towards the target, strike a n-type silicon plate placed parallel to the cathode. They cause emission of secondary electron from the target plate. The number of secondary electrons emitted from the target depends on the energy of the striking electrons. Loss of secondary electrons from the n-type semiconductor makes positively charged. The positively charge at any point on the semiconductor depends on the number of electrons lost from that point. Different points of the target possess different positive charges proportional to the intensity of light of the corresponding points in the scene. Hence a charge image of the picture is formed on the surface of the target. On the other end electron gun scans the target plate and the return beam is collected by dynode for amplification.
20.Draw the composite composite video signal and explain its components components in detail.
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(12)
CVS CONSISTS OF Camera signal: corresponding to picture or scene to be transmitted. Blanking pulses: to make the horizontal and vertical retrace invisible. Synchronizing pulses: to synchronize the transmitter and receiver scanning system Information about colour signal and some sample of colour sub-carrier frequency also be send if we are using colour TV transmission VIDEO SIGNAL DIMENSIONS Video signal amplitude is separated into 2 sections: Camera signal: occupies 75% Sync. Pulses: occupies 25% The level of video signal obtained for maximum white light is called as peak white level. This level exist between 10 to 12.5% Black level: which is at 73% corresponds to dark picture. The blanking pulses are added at 75% level and called as blanking level. Black level and blanking level is closed to each other practically they merge with each other. The difference between black level and blanking level is called pedestal height Useful video signal information lies between 10% to 75% level. Since the modulation used is – ve ve modulation, if the picture is dark then the amplitude of video signal is high. The region between 0% to 10% is called as whiter than white region. The signals in those region create more bright spots on the screen and it will create disturbance to eye of the viewer. To reduce noise effect, usually the whiter than white regions are not used The average background brightness of the scene is represented by the DC level. Since the DC level for different lines are at different level, background brightness are different depending on the scene to be transmitted. The distance between DC level and blanking level is called as pedestal height. It is used as a measure of average brightness of the scene. It is not possible to follow the changes that takes place in background brightness if the DC levels are not properly maintained
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DC COMPONENT The average background brightness of the scene is represented by the DC level. Since the DC level for different lines are at different level, background brightness are different depending on the scene to be transmitted. The distance between DC level and blanking level is called as pedestal height. It is used as a measure of average brightness of the scene. It is not possible to follow the changes that takes place in background brightness if the DC levels are not properly maintained DC INSERTION The video control operator, who observe the scene to be telecast directly at the studio knows the background brightness and it is his responsibility to properly set (insert) the DC level with the video signal. This process is called as DC insertion. BLANKING PULSES To make the retrace invisible, blanking pulses are added at 75% level. Since this is above black level, no picture is seen on the screen.
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Synchronization pulses are also added in this blanking region to achieve the synchronization It is not possible to use blanking pulses as synchronization pulse coz some time extreme black signal may rise above the blacking level. If this happens then it may false trigger the synchronization oscillator VIDEO SIGNAL TO SYNC. PULSE RATIO RATIO The picture signal amplitude to sync. Pulse amplitude ratio is 4:3. This has been found satisfactory due to the following reasons Let picture signal amplitude is increased and sync. Pulse amplitude is decreased: S/N ratio of Rx. Signal falls (ie., noise level increases) below a limit, then amplitude of sync pulse is not sufficient to keep the synchronization properly. However picture signal can able to produce acceptable picture Let sync. Pulse amplitude is increased and picture signal amplitude is decreased: S/N ratio falls below the limit, then amplitude of sync. Pulse is sufficient to synchronize and lock the picture frame. But video signal amplitude is insufficient to produce a good acceptable picture on the screen 21.Discuss the merits and demerits of positive and negative modulation and Justify the choice of negative modulation in most TV systems.
Transmitting power is less in negative modulation hence it is the best choice for video modulation 22.Describe the construction and working of monochrome picture tube. (13) The picture tube is very similar to the cathode-ray tube used in an oscilloscope. The glass envelope contains an electron Gun structure that produces a beam of electrons aimed at the fluorescent screen. When the electron beam strikes the screen, light is emitted.
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A monochrome picture tube has one electron gun and a continuous cont inuous phosphor coating that produces a picture in black and white. For colour picture tubes the screen is formed of three different phosphors and there are three electron beams, one for each colour phosphor. The three colours are red, green and blue produced by three phosphors combined to produce different colours.
MONOCHROME PICTURE TUBE
S,VIGNESHWARAN
6
ELECTROSTATIC FOCUSSING The electric field due to the positive potential at the accelerating grid extends through the opening the of the control grid right to the cathode surface. The orientation of this field is such that besides accelerating electrons down the tube. It also brings all the electrons in the stream into a tiny spot called the cross over. This is known as the first electrostatic lens action. The electrode voltages are so chosen or the electric field is so varied that the second point where all the electrons get focused is the screen of the picture tube. Electrostatic focusing is preferred over magnetic focusing because it is not affected very much by changes in the line voltage. BEAM VELOCITY In order to give the electrons stream sufficient velocity to reach the screen material with proper energy to cause it to fluoresce, a second anode is included within the tube. This is a conductive coating with colloidal graphite on the inside of the wide bell of the tube.
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This coating called aquadag usually extends from almost half way into the narrow neck to within 3 cm of the fluorescent screen. DEFLECTION YOKE It may be noted that a perpendicular displacement results because the magnetic field due to each coil reacts with the magnetic field of the electron beam to produce a force that deflects the electrons at right angles to both the beam axis and the deflection field.
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BEAM DEFLECTION As already stated the electron beam must attain a very high velocity to deliver enough energy to the atoms of the phosphor coating. Because of this the electrons of the beam remain under the influence of the deflecting field for a very short time. This necessitates application of high deflecting fields to achieve the desired deflection. It is very difficult to generate such high voltages at the deflection frequencies. On the other hand with magnetic deflection it is a large current that would be necessary to achieve the same deflection. Since it is more convenient to generate genera te large currents than high voltages. All picture tubes employ electromagnetic deflection. With electrostatic deflection the beam electros gain energy. Thus, larger deflection angles tend to defocus the beam. The deflection plates need to be placed further apart as a deflection angle is made larger. Thus requiring higher voltages to produce the same deflection fields. Magnetic deflection is free from both these shortcomings and much larger deflection angles can be achieved without defocusing or nonlinearities with these consequent saving in tube length and cabinet size. PICTURE TUBE The beam is deflected by a pair of deflecting coils mounted on the neck of the picture tube in the same way and rate as the beam scans the target in the camera tube. The amplitudes of the currents in the horizontal and vertical deflecting coils are so adjusted that the entire screen, called raster, gets illuminated because of the fast rate of scanning. 23.What is the purpose of thin aluminium coating over fluroscent screen in a picture tube? (3) To reflect light back towards the front and to stop heavy ions from going to the screen,while allowing high velocity light electron to pass 24. Explain the following (i)Solid state image scanner. (8) These scanners operates on the principle of charge coupled devices (CCD).
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ARRANGEMENTS OF PHOTO DIODE,GA DIODE,G ATES AND SHIFT REGISTERS IN A CCD
Large number of CCD array are formed together to construct the image plate. It can store and transfer analog signals . Aspect ratio 4:3 is followed in the above arrangement. In actual set up there may be 600 diodes in each rows and 450 diodes in each column in 1/3 inch size sensor. There are as many photodiodes as there are the number of pixels that is one photo diode for one pixel. Horizontal and vertical scanning are done by timing it is accurately controlled by clock and multivibrator circuit. TREATING PHOTO DIODE AS CAPACITOR
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ADVANTAGES No need for electron gun and electron beam. No need for high voltage. No need for vacuum envelope. Light weight, small size, portable. High resolution.
24.(ii)Plumbicon camera tube.
(8)
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PLUMBICON CIRCUIT
25 Draw the horizontal and vertical sync pulse and mark their time duration. Explain their application in scanning process in detail (16) HORIZONTAL SYN DETAILS
Out of 64 μs line period, the blanking period occupies 12 μs and useful information occupies 52 μs. Within the blanking period sync. Pulses are also added.
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Leading edge of these sync pulses are used to trigger the deflection oscillator circuit Horizontal blanking period consists of following 4 main regions: Front porch Line (Horizontal) sync pulse Back porch and Colour burst signal FRONT PORCH It is a brief cushioning period of 1.5 μs length. It is inserted b/w the end of picture information and leading edge of the sync pulse Pulling on whites: the video signal for peak white information exists in the whiter than white region and nearly touches the zero level. LINE SYNC PULSES At the end of front porch, sync pulses necessary for horizontal synchronization are added. It having a period of 4.7 μs (see fig) These pulses are separated at the receiver and after differentiation used for triggering the horizontal deflection oscillator BACK PORCH The back porch is having a period of 5.8 μs. It gives necessary time for the retrace (fly back) to be completed. During this period, the reversal of current direction of the horizontal time base circuit takes place. Also at the receiver side, this level is used for the production pro duction of AGC voltage in the AGC circuit COLOUR BURST SIGNAL If we are using colour signal, then some samples of these colour subcarrier frequency called colour burst signal is added for each line and send along this back porch S. No.
period
Time (μs)
1
Horizontal blanking pulse
12
2
Horizontal sync pulse
4.7
3
Front porch
1.5
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4
Back porch
5.8
5
Visible time period
52
6
Total time period
64
VERTICAL SYNC DETAILS Vertical sync pulses (VSP) are added at the end of each field. Construction of VSP is tedious than horizontal sync pulses. The requirements to satisfy by VSP train are: It should provide vertical blanking interval so that vertical retrace is not visible on screen. It should provide VSP for proper triggering of vertical oscillator During the vertical blanking and sync pulse period, horizontal scanning and triggering of horizontal oscillator should not affected.
Vertical retrace has 20 horizontal lines. It has a period of 1280 μs or 1.28 ms (20 x 64 μs).
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To blank the vertical retrace, we have to add such a long pulse having the duration of 1.28 ms. If we are using such a long pulse then large amount of power is wasted. So we are using a pulse duration of 2.5 line period as VSP SERRATED VERTICAL PULSE From the figure of vertical sync pulse train and sync pulse generation circuit, the following problems are to be considered for future modifications Horizontal sync pulses are available during both vertical trace and retrace period. But during the vertical sync period (2.5 line period) horizontal sync pulse are not available. Due to this the horizontal oscillator may come out of synchronization during vertical sync period At the end of first field, the leading edge of vertical sync pulse came in the midway of a line (line 313). This may false trigger the horizontal oscillator. To achieve the horizontal sync during the vertical sync period, 5 narrow slots of 4.7 μs duration is inserted (see fig). The leading edges of these pulses are used to trigger the horizontal oscillator The vertical slots of 4.7 μs duration that are inserted in the vertical sync period to achieve horizontal sync are called as serrated pulses. This process is called notching or serration of field sync pulse 26. Discuss the advantage of AM for video signal and explain VSB detail. (16) The AM is suitable for long transmission and conserve bandwidth so VSBAM is used in TV transmission VESTIGIAL SIDEBAND TRANSMISSION TRANSMISSION The maximum video frequency obtained in the 625 line system is 5 Mhz. If we are using double side band AM modulation, the video signal bandwidth becomes 10 Mhz. Due to practical difficulty of filter design, on both side of the channel we have an attenuation slope of 0.5 Mhz. To avoid interference with adjacent channel a guard band of 0.25 Mhz should be allowed. So the channel bandwidth is 11.25 Mhz If we are using such a long bandwidth channels, then within the given range of frequencies, the number of channels used are reduced. So we have to reduce the bandwidth Since the upper and lower side bands have the same information, one of the side band may be completely removed and we can use single sideband system. Due to this upto 5 Mhz channel bandwidth is reduced. The lower video frequency components are near the carrier frequency. Using filter circuits they cannot be completely attenuated. Also the fine details of picture are in low frequency range only.
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So if completely eliminating one side band, phase distortion may occur. Coz of this problem, some part (vestige) of lower side band, upto 0.75 Mhz is retained and others are attenuated. This method of transmitting a full upper side band and vestige of lower side band is called as vestigial side band transmission Channel bandwidth = Attenuation slope + vestigial part (VLSB) + USB + attenuation slope + guard band = (0.5 + 0.75 + 5 +0.5 + 0.25) = 7 Mhz So, if we are using VSB modulation, then channel bandwidth is reduced from 11.25 Mhz to 7 Mhz and bandwidth saving upto 4.25 Mhz is achieved
VSB TRANSMISSION
VESTIGIAL SIDEBAND RECEPTION In VSB modulation, low frequencies upto 0.75 Mhz are available on both the sideband. But the signal in the frequency range of 0.75 Mhz to 5 Mhz are available in one band only. Due to this, the gain of voltage level at the receivers, RF and IF should be properly controlled.
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The o/p voltage obtained for low frequency (0 to 0.75 Mhz) signal are twice, than the high frequency (0.75 to 5 Mhz) signals. To correct this problem the response curve of IF amplifier should be properly designed ADVANTAGES Channel bandwidth is reduced Reduction in transmitted power DISADVANTAGES Some powers are wasted in VSB filters S/N ratio is reduced upto 6 db, due to IF picture carrier placed at 50% gain at the receiver Chances for amplitude and phase distortion. The tuning at the receiver is more critical
27
What is interlaced scanning?How does it reduce flicker and conserve
Bandwith?Explain
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INTERLACED SCANNING
In interlaced scanning each frame is scanned in two sequence and each sequence is called field.this cause two fields /frame and 50fields/second and this eliminate flicker in progressive scanning and also conserve bandwidth
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SCANNING PERIOD SAWTOOTH CURRENT FOR HORIZONTAL DEFLECTION
SAWTOOTH CURRENT FOR VERTICAL DEFLECTION
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28.Discuss the importance of scanning and synchronization pulse?
(6)
Scanning is the important process carried out in a television system in order to obtain continuous frames and provides motion of picture. The scene is scanned both in the horizontal and vertical directions simultaneously in a rabid rate. As a result sufficient number of complete picture of frames per second is obtained to give the illusion of continuous motion. Synchronization pulse synchronizes the scanning in camera at receiver and scanning in picture tube at receiver providing no delay. 29. Show the standard channel characteristics of CCIR B adopted in India. (16)
30. Draw a block diagram diagram of a monochrome TV receiver and explain the function of each block? (16) The block schematic diagram below of a typical monochrome TV receiver. As shown there, the receiver has been divided into several main sections depending on their functions and are discussed below.
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MONOCHROME TV RECEIVER
IMPORTANT SECTIONS IN THE RECEIVER ARE: Antenna section RF tuner section Video signal and AGC section Audio signal section Sync separator section ANTENNA SECTION Mounted top of the building TUNED ANTENNAS: to ANTENNAS: to receive good signal strength FOLDED DIPOLE TYPE ANTENNAS: ANTENNAS: good reception A folded dipole with one reflector and more directors called as yagi-uda antenna RF TUNER SECTION RF tuner: RF amp., mixer and local osc. Collectively called RF tuner To amplify the weak signal RF amp. is used Amplified signal is given given as one i/p to mixer. Other i/p of mixer is from L.O o/p of mixer is IF of audio and video signal
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L.O produces freq. equal to channel picture freq. of +38.9 Mhz or channel audio freq. of +33.4 Mhz SO THE O/P OF MIXER FOR ANY CHANNEL SELECTION Picture IF = 38.9 Mhz and Sound IF = 33.4 Mhz VIDEO SIGNAL AND AGC SECTION Video section: (IF section: (IF amplifiers, video detector, video amplifier and picture tube The signal amplitude obtained at the o/p of mixer is very small so 2 or 3 stages of IF signal amplification is necessary. After amplification they are given to the video detector Video detector: demodulation detector: demodulation takes place and CVS is obtained , VIDEO SIGNAL AND AGC SECTION To produce picture with good contrast about 80 to 100 volts are needed. But the o/p of video detector is only about 2 volt peak to peak. Therefore a video amplifier is used at the o/p of video detector. To adjust the contrast b/w black and white parts of the picture, a contrast control is provided at the front panel. The amplified signal is given to cathode or control grid depending upon the picture tube Signals received from long distance transmitter are very weak so, maximum gain is required and the signals received from near by transmitters are strong and reduced gain for amplifier is needed. But it is not possible to change the gain by selecting different channels. So an automatic gain control (AGC) circuit is used to automatically control the gain of the amplifiers AUDIO SIGNAL SECTION Combination of IF amplifier, FM demodulator, audio amplifier and loud speaker. Using 5.5 Mhz sound trap circuit, the audio signal is separated from the video detector section. The IF (5.5 Mhz) Mhz) of audio signals are amplified and fed fed to FM discriminator discriminator for demodulation. Demodulation signals are given to audio amplifier and finally loud speaker for the reproduction of sound SYNC SEPARATOR SECTION Combination of sync separator, integrator and differentiator. The o/p of video detector (CVS) is given i/p to the sync separator. This separates both horizontal and vertical sync pulses. Integrator(LPF) is used to separate the vertical sync pulses having field freq. of 50 Hz for triggering vertical oscillator.
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Differentiator(HPF) is used to separate the horz. Sync pulses having line freq. of 15625 Hz for triggering horz. Oscillator HORIZONTAL AND VERTICAL OSCILLATOR SECTION Combination of vertical oscillator, o/p amplifier and vertical deflection coil. Vert. osc is synchronised by the trigger pulses from the integrator. Then the saw tooth signal is given to deflection coil, after proper amplification by vert. o/p amplifier Horz. osc is synchronised by the trigger pulses from the differentiator. Then the saw tooth signal (15625 Hz) is given to deflection coil, after proper amplification by horz. o/p amplifier. To maintain the freq. at 15625 Hz, an automatic freq. control (AFC) circuit is used EXTRA HIGH TENSION (EHT) High voltages needed by final anode are developed. Voltages of 15 to 20 KV are developed by an auto transformer. After rectification, the high voltages are applied to final anode and aquadag coating. NOTE: Aquadag NOTE: Aquadag is a trade name for a graphite based coating commonly found in cathode ray tubes.The coating can be on the inside of the tube, where it helps to maintain a constant electrical field near the screen.
31. Draw a block diagram diagram of a monochrome TV receiver and explain the function of each block?
(16)
In a TV Transmitter, we have to transmit both video and audio signal. So we have 2 types of transmitter within the TV main transmitter. They are Visual transmitter Aural transmitter VISUAL TRANSMITTER TRANSMITTER is used for transmitting video signals and aural transmitter is used for transmitting audio or sound signal. o/p of these two transmitters are combined before transmitting through common antenna. The combining network is called diplexer. Before transmission, the video signal is amplitude modulated (AM) and audio signal is frequency modulated (FM). After modulation and proper power amplification they are fed to a common antenna through the diplexer. Initially a camera is used to convert optical image into electrical signal. After adding sync pulses and blanking pulses we get composite video signal which is to be AM modulated
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BLOCK DIAGRAM OF MONOCHROME TV RECEIVER
Similarly, a microphone (MIC) is used to convert audio signal into electrical signal. After pre-emphasis and proper amplification we have to FM modulate the audio signal IN THE SIMPLIFIED BLOCK DIAGRAM OF MONOCHROME TELEVISION TRANSMITTER, THE MAIN SECTIONS ARE GIVEN BELOW: Video and audio processing unit Video signal modulation unit and Audio signal modulation unit VIDEO AND AUDIO PROCESSING UNIT In video processing unit, the camera o/p signal having 1 volt p-p amplitude is converted into standard form by eliminating hum and noise. If necessary, linearity correction can also be done. This correction is done during the back porch by clamping the video signal. Controlling facilities for linearity correction, gain, black setup, sync level, picture to sync ratio, sync stretching are also available in this unit
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Sync pulses and blanking pulses are added to the camera signal to get CVS. Several cameras are used to telecast the scene from different angles. In audio processing unit we have microphone, which convert the audio signals into electrical signal. Here pre-emphasis circuit is used, which will give necessary frequency compensation for the audio signal. To pickup audio signals from different locations associated with the program, large number of microphones are provided VIDEO SIGNAL MODULATION UNIT OR PICTURE SIGNAL MODULATION OR VISUAL EXCITER Amplified CVS signals are fed to the modulation unit through the distributor and switcher. The distributer is used to receive the signals from other studios also. CVS and picture carrier signal are fed as i/p to the modulator unit. Crystal controlled oscillator is used for generating the picture carrier signal VISUAL EXCITER After AM modulation, it is power amplified and fed as i/p to the modulator unit. Crystal controlled oscillator is used for generating the picture carrier signal. After AM modulation, it is power amplified and fed to the combining network. To save the power, VSB technique is used in TV transmitters. Before combining with the audio signal, the video signals are passed through the VSB filters . AUDIO SIGNAL MODULATION OR AURAL AURAL EXCITER FM modulation is used for audio signals in TV transmitter. This frequency modulator is a crystal controlled one. Here the FM modulation of audio signal takes place with a carrier signal from crystal oscillator. To have an automatic frequency control (AFC) facility, phase detector and CPF stages are used. If there is any change in frequency of carrier signal, then phase detector will produce a dc correcting voltage. This dc voltage is used to automatically correct any change in carrier frequency AURAL EXCITER To increase the power level, audio power amplifiers are used. To up convert the frequency level, frequency multipliers are used. Turnstile antenna array is used to broadcast the TV signals from the combining network. The combining network is used to combine the audio and video signal, without any interference between them
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