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In the lime monochrome system, for successful interlaced scanning, the lines of each frame or picture are divided into sets of To achieve this the horizontal sweep oscillator is made to work at a frequency of Hz Note that since the beam is now deflected from top to bottom in half the time and the horizontal oscillator is still operating at Hz, only half the total lines, i.
Since the first field ends in a half line and the second field commences at middle of the line on the top of the target plate or screen see Fig. In all then, the beam scans lines Therefore, with interlaced scanning the flicker effect is eliminated without increasing the speed of scanning, which in turn does not need any increase in the channel bandwidth. It may be noted that the frame repetition rate of 25 rather than 24 as used in motion pictures was chosen to make the field frequency equal to the power line frequency of 50 Hz.
This helps in reducing the undesired effects of hum due to pickup from the mains, because then such effects in the picture stay still, instead of drifting up or down on the screen. Scanning periods. The waveshapes of both horizontal and vertical sweep currents are shown in Fig. As shown there the retrace times involved both horizontal and vertical are due to physical limitations of practical scanning systems and are not utilized for transmitting or receiving any video signal.
The nominal duration of the horizontal line as shown in Fig. The beam returns during this short interval to the extreme left side of the frame to start tracing the next line. Similarly with the field frequency set at 50 Hz, the nominal duration of the vertical trace see Fig.
Out of this period of 20 ms, Since the horizontal and vertical sweep oscillators operate continuously to achieve the fast sequence of interlaced scanning, 20 horizontal lines.
Thus 40 scanning H 64 s K. Scanning sequence. The complete geometry of the standard interlaced scanning pattern is illustrated in Fig. Note that the lines are numbered in the sequence in which these are actually scanned. During the first vertical trace actually The beam starts at A, and sweeps across the frame with uniform velocity to cover all the picture elements in one horizontal line.
At the end of this trace the beam then retraces rapidly to the left side of the frame as shown by the dashed line in the illustration to begin the next horizontal line. Note that the horizontal lines slope downwards in the direction of scanning because the vertical deflecting current simultaneously produces a vertical scanning motion, which is very slow.
The slope of the horizontal trace from left to right is greater than during retrace from right to left. The reason is that the faster retrace does not allow the beam so much time to be deflected vertically. After line one, the beam is at the left side ready to scan line 3, omitting the second line. However, as mentioned earlier it is convenient to number the lines as they are scanned and so the next scanned line skipping one line, is numbered two and not three.
This process continues till the last line gets scanned half when the vertical motion reaches the bottom of the raster or frame. As explained earlier skipping of lines is accomplished by doubling the vertical scanning frequency from the frame or picture repetition rate of 25 to the field frequency of 50 Hz.
With the field frequency of 50 Hz the height of the raster is so set that Now the retrace starts and takes a period equal to 20 horizontal line periods to reach the top marked C.
These 20 lines are known as inactive lines, as the scanning beam is cut-off during this period. Thus the second field starts at the middle of the raster and the first line scanned is the 2nd half of line number The scanning of second field, starting at the middle of the raster automatically enables the beam to scan the alternative lines left unscanned during the first field.
The vertical scanning motion otherwise is exactly the same as in the previous field giving all the horizontal lines the same slope downwards in the direction of scanning. As a result The inactive vertical retrace again begins and brings the beam back to the top at point A in a period during which 20 blanked horizontal lines to get scanned. Back at point A, the scanning beam has just completed two fields or one frame and is ready to start the third field covering the same area no.
This process of scanning fields is continued at a fast rate of 50 times a second, which not only creates an illusion of continuity but also solves the problem of flicker satisfactorily. The ability of the image reproducing system to represent the fine structure of an object is known as its resolving power or resolution.
It is necessary to consider this aspect separately in the vertical and horizontal planes of the picture. Vertical resolution. The extent to which the scanning system is capable of resolving picture details in the vertical direction is referred to as its vertical resolution.
It has already been explained that the vertical resolution is a function of the scanning lines into which the picture is divided in the vertical plane. Alternate black and white lines of resolution. Horizontal resolution. The capability of the system to resolve maximum number of picture elements along the scanning lines determines horizontal resolution.
This can be evaluated by considering a vertical bar pattern as shown in Fig.
As explained earlier, it ultimately depends on the random distribution of black and white areas in the picture. Thus for equal vertical and horizontal resolution, the same resolution factor may be used while determining the effective number of distinct picture elements in a horizontal line.
Therefore, the effective number of alternate black and white segments in one horizontal line for equal vertical and horizontal resolution are: This is shown along the bar pattern drawn in Fig. Thus the time duration th of one square wave cycle is equal to. Since the consideration of both vertical and horizontal resolutions is based on identical black and white bars in the horizontal and vertical planes of the picture frame, it amounts to considering a chessboard pattern as the most stringent case and is illustrated in Fig.
Here each alternate black and white square element takes the place of bars for determining the capability of the scanning system to reproduce the fine structure of the object being televised. The actual size of each square element in the chess pattern is very small and is equal to thickness of the scanning beam.
It would be instructive to know as an illustration that the size of such a square element on the screen of a 51 cm picture tube is about 0. Since the spacing of these small elements in the above consideration corresponds to the limiting resolution of the eye, it will distinguish only the alternate light and dark areas but not the shape of the variations along the scanning line.
Thus the eye will fail to distinguish the difference between a square wave of brightness variation and a sine wave of brightness variation in the reproduced picture. Therefore, if the amplifier for the square-wave signal is capable of reproducing a sine-wave of frequency equal to the repetition frequency of the rectangular wave, it is satisfactory for the purpose of TV signals.
It may be mentioned that even otherwise. Another justification for restricting the bandwidth up to 5 MHz is that in practice it is rare when alternate picture elements are black and white throughout the picture width and height, and a bandwidth up to 5 MHz has been found to be quite adequate to produce most details of the scene being televised.
Therefore, the highest approximate modulating frequency fh that the line television system must be capable of handling for successful transmission and reception of picture details is. In the second line widely used television system, where the active number of lines is and the duration of one active line is 57 s, the highest modulating frequency fh 4 MHz.
This explains the allocation of 6 MHz as the channel bandwidth in USA and other countries employing the line system in comparison to a channel bandwidth allocation of 7 MHz in countries that have adopted the line system. Similarly in the French TV system where the highest modulating frequency comes to Colour resolution and bandwidth.
As explained above a bandwidth of 5 MHz 4 MHz in the American system is needed for transmission of maximum horizontal detail in monochrome. However, this bandwidth is not necessary for the colour video signals. The reason is that the human eyes colour response changes with the size of the object. For very small objects the eye can perceive only the brightness rather than the colours in the scene. Perception of colours by the eye is limited to objects which result in a video frequency output up to about 1.
Thus the colour information needs much less bandwidth than monochrome details and can be easily accommodated in the channel bandwidth allotted for monochrome transmission. Low-frequency requirements. The analysis of the signals produced by the bar pattern gives no information regarding the low-frequency requirement of a video amplifier used to handle such signals. This requirement may be determined from consideration of a pattern shown in Fig. The signal output during vertical excursions of the beam would be a square wave see Fig.
It is apparent then, that any amplifier capable of reproducing this waveform would be required to have good square-wave response at 50 Hz. Any degradation in response as shown in Fig. In order to have satisfactory square-wave response at field frequency, an amplifier must have good sine-wave response with negligible phase distortion down to a much lower frequency than the field frequency.
In addition, to correct phase and amplitude response at the field frequency, it is necessary to preserve the dc component of the brightness signal. Thus a good frequency response from dc to about 5 MHz becomes necessary for true reproduction of the brightness variations and find details of any scene. Influence of number of lines on bandwidth.
As the number of lines employed in a television picture is increased, the bandwidth necessary for a given quality of definition also increases. This is due to the fact that increasing the number of lines per picture decreases the time duration of each line. This means that the spot travels across the screen at a higher velocity and results in increase of the highest modulating frequency. For example doubling the number of lines per frame would very much improve the vertical resolution, infact it would get doubled but would need increasing the bandwidth in the same ratio.
If now, it is required to increase the horizontal resolution so that it again equals the vertical resolution it would be necessary to scan double the number of alternate black and white signal elements in a line, and this would necessitate multiplying the original highest video frequency by a factor of four.
The conclusion is that, if the number of lines employed in a television system is increased, it is necessary to increase the video frequency bandwidth in direct proportion to the increase in number of lines to maintain the same degree of vertical definition as before , and in order to increase horizontal definition in the same proportion as the increase in vertical resolution the video frequency bandwidth must increase as the square of the increase in number of lines.
Effect of interlaced scanning on bandwidth. As already explained, interlaced scanning reduces flicker.
However, scanning 50 complete frames of lines in a progressive manner would also eliminate flicker in the picture but this would need double the scanning speed which in turn would double the video frequencies corresponding to the picture elements in a line.
This would necessitate double the channel bandwidth of that required with interlaced scanning. It should be noted that by employing interlaced scanning, the basic concept of interchangeability of time and bandwidth is not violated, because more time in allowed for transmission and this results in decrease of bandwidth needed for each TV channel. Thus interlaced scanning reduces flicker and conserves bandwidth. Effect of field frequency on bandwidth.
Text..Monochrome & Colour Television - R.R.Gulati.pdf
With increase in field frequency the time available for each field decreases and this results in a proportionate decrease of the active line period. Hence, bandwidth increases in direct proportion to the increase in the field frequency. Bandwidth requirement for transmission of synchronising pulses.
The equalizing pulses to be discussed later have a pulse width of 2. The highest sinusoidal frequency which must lie in the pass band of the system for effective transmission of these pulses is given by the expression:. Interlace error. As explained earlier interlaced scanning provides a means of decreasing the effect of flicker in the TV picture without increasing the system bandwidth. The selection of 2: Here, by selecting an odd number of lines, the symmetry in frame blanking pulses is achieved and this enables perfect interlaced scanning.
Any error in scanning timings and sequence would leave a large number of picture elements unresolved and thus the quality of the reproduced picture gets impaired. For convenience of explanation the retrace time has been assumed to be zero. Interlace error occurs due to the time difference in starting the second field.
For perfect interlace the second field should start from point b see Fig. If it starts early or late interlace error will be there. For a 16 s delay in the start of the second field Fig. Then the percentage interlace error. For a delay of 32 s the two fields will overlap Fig. The above examples demonstrate that incorrect start of any field produces vertical displacement between the lines of the two fields.
This brings these lines closer leaving gaps. The result is a deterioration of the pictures vertical resolution because certain areas do not get scanned at all.
For perfect interlaced scanning it is essential that the starting points at the top of the frame is separated exactly one half line between first and second fields. To achieve this it is necessary to feed two regularly spaced synchronising pulses to the field time base during each frame period. One of these pulses must arrive in the middle of a line and the next at the end of a line.
This is shown in Fig. Thus the vertical time base must be triggered 50 times per second in the manner explained above. For half line separation between the two fields only the topmost and the extreme bottom lines are then half lines whereas the remaining lines are all full lines. If there are x number of full lines per field, where x may be even or odd, the total number of full lines per frame is then 2x, an even number.
To this, when the two half lines get added the total number of lines per frame becomes odd. Thus for interlaced scanning the total number of lines in any TV system must be odd. With an even number of lines the two fields are bound to fall on each other and interlaced scanning would not take place. Further for correct interlacing it becomes necessary that at the transmitter automatic frequency control must be utilized to maintain a horizontal scanning frequency that is exactly This is accomplished by generating a stable frequency at Hz by crystal controlled oscillator circuits.
A frequency doubling circuit produces a frequency of Hz, which is utilized to control the correct generation of equalizing and vertical sync pulses. Four frequency division circuits each with a ratio of 5: Thus all the required frequencies are derived from a common stable source and they automatically remain interlocked in the correct ratios. To achieve this, i. The frame frequency of satisfies all the above requirements.
Similarly lines in the American system and lines in the French system also meet these requirements. Comparison of various TV systems. Picture and sound signal standards for the principal monochrome television systems are given at the end of chapter 4. Obviously, here 0. So the British system is marginally better than the European system.
The French TV system employs lines with a video bandwidth of This system therefore has both much improved vertical resolution and a better horizontal resolution.
Thus, the line frequency in this system is , which compares very closely to our system where the line frequency is However, the American system employs a bandwidth of 4 MHz which suggests that the horizontal resolution of this system is less than all other systems in use. It must be noted that the number of lines employed by a given TV system is not in itself, a guide to the quality of resolution available from the system. It is true that greater the number of lines the better the vertical resolution, but an assessment of the horizontal resolution, i.
In addition to proper bandwidth required to produce the details allowed by the scanning system at the transmitting end and the picture tube at the receiving end, the signal-transmission system should have proper transfer characteristics to preserve same brightness gradation as the eye would perceive when viewing the scene directly. Any non-linearity in the pick-up and picture tube should also be corrected by providing inverse nonlinearities in the channel circuitry to obtain overall linear characteristics.
Note that the sensation in the eye to detect changes or brightness is logarithmic in nature and this must be taken into account while designing the overall channel. Various other factors that influence the tonal quality of the reproduced picture are: This is the difference in intensity between black and white parts of the picture over and above the brightness level.
The ratio of maximum to minimum brightness relative to the original picture is called contrast ratio. In broad daylight the variations in brightness are very wide with ratio as high as Ratio of brightness variations in the reproduced picture on the screen of the picture tube, to the brightness variations in the original scene is known as Gamma of the picture.
Its value is close to 0. In studios, under controlled conditions of light, the variations are less wide than outside and so the brightness variations that can be reproduced by the picture tube are not very much different than that of the scene. Realism is still maintained because the viewer does not actually see the scene being televised.
Another factor which makes stringent demands from the system unnecessary is the fact that our eye can accommodate not more than Too bright a representation of the bright areas in a picture would make.
This is true at all levels of light intensity with brightness variations in relative ratios of When a TV receiver is off, there is no beam impinging on the fluorescent screen of the picture tube and no light gets emitted.
Then with normal light in the room the screen appears as dull white. But when the receiver is no, and a TV programme is being received the bright portions of the scene appear quite bright because the corresponding amplitude of the video signal makes the control-grid of the picture tube much less negative and the consequent increased beam current causes more light on the screen. However, for a very dark portion of the scene the corresponding video signal makes the grid highly negative with respect to the cathode and thus cuts-off the beam current and no light is emitted on the corresponding portions on the screen.
These areas appear to the eye as dark in comparison with the high light areas of the screen, whereas the same area in the absence of beam current when the set was off appeared close to a white shade. This as explained earlier is due to the logarithmic response of the human eye and its inability to accommodate light intensity variations greater than The viewing distance from the screen of the TV receiver should not be so large that the eye cannot resolve details of the picture.
The distance should also not be so small that picture elements become separately visible. The above conditions are met when the vertical picture size subtends an angle of approximately 15 at the eye.
The distance also depends on habit, varies from person to person, and lies between 3 to 8 times the picture height. Most people prefer a distance close to five times the picture height.
While viewing TV, a small light should be kept on in the room to reduce contrast. This does not strain the eyes and there is less fatigue. How is the illusion of continuity created in television pictures? Why has the frame reception rate been chosen to be 25 and not 24 as in motion pictures? What do you understand by interlaced scanning? Show that it reduces flicker and conserve bandwidth. What do you understand by active and blanking periods in horizontal and vertical scanning?
Give the periods of nominal, active and retrace intervals of horizontal and vertical scanning as used in the line system. How many horizontal lines get traced during each vertical retrace? What is the active number of lines that are actually used for picture information pick up and reception? Draw a picture frame chart showing the total number of active and inactive lines during each field and establish the need for terminating the first field in a half line and the beginning the second at the middle of a line at the top.
Justify the choice of lines for TV transmission. Why is the total number of lines kept odd in all television systems? What is the significance of choosing the number of lines as and not or ? What do you understand by resolution or Kell-factor? How does it affect the vertical resolution of a television picture? Show that the vertical resolution increases with increase in number of scanning lines.
What is meant by equal vertical and horizontal resolution? Derive an expression for the highest modulating frequency in a television system and show that it is nearly 5 MHz.
Show that if the number of lines employed in a TV system is increased then the highest video frequency must increase as the square of the increase in number of lines for equal improvement in vertical and horizontal resolution.
Show that the B TV system is only marginally superior to the line American system. What do you understand by interlace error and how does it affect the quality of the picture? Calculate the percentage interlace error when the second field is delayed by 8 s. Retrace time may be assumed to be negligible. In the British lines system the resolution factor employed is 0. All other scanning details remaining the same, calculate the highest modulating frequency used in the British system. Explain the need for providing very good low frequency response and phase characteristics in amplifiers used in any TV link, for proper reproduction of brightness variations.
The relevant data for a closed circuit TV system is given below. Calculate the highest modulating frequency that will be generated while scanning the most stringent case of alternate black and white dots for equal vertical and horizontal resolution. Explain the meaning of terms-tonal gradation, contrast, contrast ratio and gamma of the picture. When a TV receiver is off, no electron beam strikes the picture tube screen and the screen face looks a dull white. With the set on and a black and white picture showing on the screen, no electron beam impinges on the darker area of the reproduced picture.
But these areas now appear quite black instead of the dull white of the switched-off set. Explain the reason for this difference in appearance. A horizontal synchronizing sync pulse is needed at the end of each active line period whereas a vertical sync pulse is required after each field is scanned.
The amplitude of both horizontal and vertical sync pulses is kept the same to obtain higher efficiency of picture signal transmission but their duration width is chosen to be different for separating them at the receiver. Since sync pulses are needed consecutively and not simultaneously with the picture signal, these are sent on a time division basis and thus form a part of the composite video signal.
Figure 3. As illustrated there, the video signal is constrained to vary between certain amplitude limits. The level of the video signal when the picture detail being transmitted corresponds to the maximum whiteness to be handled, is referred to as peak-white level.
This is fixed at 10 to The sync pulses are added at 75 percent level called the blanking level. The difference between the black level and blanking level is known as the Pedestal. However, in actual practice, these two levels, being very close, tend to merge with each other as shown in the figure. Thus the picture information may vary between 10 percent to about 75 percent of the composite video signal depending on the relative brightness of the picture at any instant.
The darker the picture the higher will be the voltage within those limits. Note that the lowest 10 percent of the voltage range whiter than white range is not used to minimize noise effects. This also ensures enough margin for excessive bright spots to be accommodated without causing amplitude distortion at the modulator. At the receiver the picture tube is biased to ensure that a received video voltage corresponding to about 10 percent modulation yields complete whiteness at that particular point on the screen, and an analogous arrangement is made for the black level.
Besides this, the television receivers are provided with brightness and contrast controls to enable the viewer to make final adjustments as he thinks fit. In addition to continuous amplitude variations for individual picture elements, the video signal has an average value or dc component In the absence of dc component the receiver cannot follow changes in brightness, as the ac camera signal, say for grey picture elements on a black background will then be the same as a signal for white area on a grey back-ground.
In Fig. It may be noted that the break shown in the illustration after each line signal is to emphasize that dc component of the video signal is the average value for complete frames rather than lines since the background information of the picture indicates the brightness of the scene. Thus Fig. Pedestal height. As noted in Fig. This indicates average brightness since it measures how much the average value differs from the black level.
Even when the signal loses its dc value when passed through a capacitor-coupled circuit the distance between the pedestal and the dc level stays the same and thus it is convenient to use the pedestal level as the reference level to indicate average brightness of the scene. Setting the pedestal level. The output signal from the TV camera is of very small amplitude and is passed through several stages of ac coupled high gain amplifiers before being coupled to a control amplifier.
Here sync pulses and blanking pulses are added and then clipped at the correct level to form the pedestals. Since the pedestal height determines the average brightness of the scene, any smaller value than the correct one will make the scene darker while a larger pedestal height will result in higher average brightness.
The video control operator who observes the scene at the studio sets the level for the desired brightness in the reproduced picture which. This is known as dc insertion because this amounts to adding a dc component to the ac signal. Once the dc insertion has been acomplished the pedestal level becomes the black reference and the pedestal height indicates correct relative brightness for the reproduced picture.
However, the dc level inserted in the control amplifier is usually lost in succeeding stages because of capacitive coupling, but still the correct dc component can be reinserted when necessary because the pedestal height remains the same. The blanking pulses. The composite video signal contains blanking pulses to make the retrace lines invisible by raising the signal amplitude slightly above the black level 75 per cent during the time the scanning circuits produce retraces.
As illustrated in Fig. The repetition rate of horizontal blanking pulses is therefore equal to the line scanning frequency of Hz. Similarly the frequency of the vertical blanking pulses is equal to the field-scanning frequency of 50 Hz. It may be noted that though the level of the blanking pulses is distinctly above the picture signal information, these are not used as sync pulses.
The reason is that any occasional signal corresponding to any extreme black portion in the picture may rise above the blanking level and might conceivably interfere with the synchronization of the scanning generators. Therefore, the sync pulses, specially designed for triggering the sweep oscillators are placed in the upper 25 per cent 75 per cent to per cent of the carrier amplitude of the video signal, and are transmitted along with the picture signal.
Sync pulse and video signal amplitude ratio. The overall arrangement of combining the picture signal and sync pulses may be thought of as a kind of voltage division multiplexing where about 65 per cent of the carrier amplitude is occupied by the video signal and the upper. Thus, as shown in Fig. This ratio has been found most satisfactory because if the picture signal amplitude is increased at the expense of sync pulses, then when the signal to noise ratio of the received signal falls, a point is reached when the sync pulse amplitude becomes insufficient to keep the picture locked even though the picture voltage is still of adequate amplitude to yield an acceptable picture.
On the other hand if sync pulse height is increased at the expense of the picture detail, then under similar conditions the raster remains locked but the picture content is of too low an amplitude to set up a worthwhile picture.
This represents the most efficient use of the television system. The horizontal blanking period and sync pulse details are illustrated in Fig. The interval between horizontal scanning lines is indicated by H.
As explained earlier, out of a total line.
Front porch blanked Back porch blanked Picture space on the raster. During this interval a line synchronizing pulse is inserted. The pulses corresponding to the differentiated leading edges of the sync pulses are actually used to synchronize the horizontal scanning oscillator.
This is the reason why in Fig. The line blanking period is divided into three sections. These are the front porch, the line sync pulse and the back porch. The time intervals allowed to each part are summarized below and their location and effect on the raster is illustrated in Fig. Details of Horizontal Scanning Period. Front porch. This is a brief cushioning period of 1. This interval allows the receiver video circuit to settle down from whatever picture voltage level exists at the end of the picture line to the blanking level before the sync pulse occurs.
Thus sync circuits at the receiver are isolated from the influence of end of the line picture details. The most stringent demand is made on the video circuits when peak white detail occurs at the end of a line. Despite the existence of the front porch when the line ends in an extreme white detail, and the signal amplitude touches almost zero level, the video voltage level fails to decay to the blanking level before the leading-edge of the line sync pulse occurs.
This results in late triggering of the time base circuit thus upsetting the horz line sync circuit. As a result the spot beam is late in arriving at the left of the screen and picture information on the next line is displaced to the left. This effect is known as pulling-on-whites.
Line sync pulse. After the front proch of blanking, horizontal retrace is produced when the sync pulse starts. The flyback is definitely blanked out because the sync level is blacker than black.
Line sync pulses are separated at the receiver and utilized to keep the receiver line time base in precise synchronism with the distant transmitter. The nominal time duration for the line sync pulses is 4. During this period the beam on the raster almost completes its back stroke retrace and arrives at the extreme left end of the raster.
Back porch. This period of 5. It also permits time for the horizontal time-base circuit to reverse direction of current for the initiation of the scanning of next line. Infact, the relative timings are so set that small black bars see Fig.
These blanked bars at the sides have no effect on the picture details reproduced during the active line period. At the receiver this level which is independent of the picture details is utilized in the AGC automatic gain control circuits to develop true AGC voltage proportional to the signal strength picked up at the antenna. The vertical sync pulse train added after each field is somewhat complex in nature. The reason for this stems from the fact that it has to meet several exacting requirements.
Therefore, in order to fully appreciate the various constituents of the pulse train, the vertical sync details are explored step by step while explaining the need for its various components. The basic vertical sync added at the end of both even add odd fields is shown in Fig. Its width has to be kept much larger than the horizontal sync pulse, in order to derive a suitable field sync pulse at the receiver to trigger the field sweep oscillator.
The standards specify that the vertical sync period should be 2. If the width is less than this, it becomes difficult to distinguish between horizontal and vertical pulses at the receiver.
End of second even field H. Note, the widths of horizontal blanking intervals and sync pulses are exaggerated. This is known as colour burst and is located at the back porch of the horizontal blanking pedestal. If the width is greater than this, the transmitter must operate at peak power for an unnecessarily long interval of time. In the line system 2. Thus a vertical sync pulse commences at the end of 1st half of th line end of first field and terminates at the end fo th line.
Similarly after an exact interval of 20 ms one field period the next sync pulse occupies line numbers 1st, 2nd and 1st half of third, just after the second field is over. Note that the beginning of these pulses has been aligned in the figure to signify that these must occur after the end of vertical stroke of the beam in each field, i. This alignment of vertical sync pulses, one at the end of a half-line period and the other after a full line period see Fig.
However, a detailed examination of the pulse trains in the two fields would show that horizontal sync pulses continue to occur exactly at 64 s intervals except during the vertical sync pulse periods throughout the scanning period from frame to frame and the apparent shift of 32 s is only due to the alignment of vertical sync instances in the figure.
As already mentioned the horizontal sync information is extracted from the sync pulse train by differentiation, i. Indeed pulses corresponding to the differentiated leading edges of sync pulses are used to synchronise the horizontal scanning oscillator. The process of deriving these pulses is illustrated in Fig. Furthermore, receivers often use monostable multivibrators to generate horizontal scan, and so a pulse is required to initiate each and every cycle of the horizontal oscillator in the receiver.
Sync pulses. This brings out the first and most obvious shortcoming of the waveforms shown in Fig. The horizontal sync pulses are available both during the active and blanked line periods but there are no sync pulses leading edges available during the 2.
Thus the horizontal sweep oscillator that operates at Hz, would tend to step out of synchronism during each vertical sync period. The situation after an odd field is even worse. Consequently, looking further along this waveform, we see that the leading edge of the vertical sync pulse comes at the wrong time to provide synchronization for the horizontal oscillator. Therefore, it becomes necessary to cut slots in the vertical sync pulse at half-line-intervals to provide horizontal sync pulses at the correct instances both after even and odd fields.
The technique is to take the video signal amplitude back to the blanking level. The waveform is then returned back to the maximum level at the moment the line sweep circuit needs synchronization. Thus five narrow slots of 4. The trailing but rising edges of these pulses are actually used to trigger the horizontal oscillator. The resulting waveforms together with line numbers and the differentiated output of both the field trains is illustrated in Fig.
This insertion of short pulses is known as notching or serration of the broad field pulses. Note that though the vertical pulse has been broken to yield horizontal sync pulses, the effect on the vertical pulse is substantially unchanged. It still remains above the blanking voltage level all of the time it is acting. The pulse width is still much wider than the horizontal pulse width and thus can be easily separated at the receiver.
E book of Monochrome and colour Television by r r Gulati
Returning to Fig. Time-constant of the differentiating circuit is so chosen, that by the time a trailing edge arrives, the pulse due to the leading edge has just about decayed.
The negative-going triggering pulses may be removed with a diode since only the positive going pulses are effective in locking the horizontal oscillator. End of 2nd field Note, the differentiated pulses bearing line numbers are the only ones needed at the end of each field. However, the pulses actually utilized are the ones that occur sequentially at 64 s intervals. Such pulses are marked with line numbers for both the fields. Note that during the intervals of serrated vertical pulse trains, alternate vertical spikes are utilized.
The pulses not used in one field are the ones utilized during the second field. This happens because of the half-line difference at the commencement of each field and the fact that notched vertical sync pulses occur at intervals of 32 s and not 64 s as required by the horizontal sweep oscillator.
The pulses that come at a time when they cannot trigger the oscillator are ignored. Thus the requirement of keeping the horizontal sweep circuit locked despite insertion of vertical sync pulses is realized. Now we turn to the second shortcoming of the waveform of Fig. First it must be mentioned that synchronization of the vertical sweep oscillator in the receiver is obtained from vertical sync pulses by integration. The integrating circuit may equally be looked upon as a lowpass filter, with a cuit-off frequency such that the horizontal sync pulses produce very little output, while the vertical pulses have a frequency that falls in the pass-band of the filter.
The voltage built across the capacitor of the low-pass filter integrating circuit corresponding to the sync pulse trains of both the fields is shown in Fig. Note that each horizontal pulse causes a slight rise in voltage across the capacitor but this is reduced to zero by the time the next pulse arrives. This is so, because the charging period for the capacitor is only 4. Hence there is no residual voltage across the vertical filter L. Once the broad serrated vertical pulse arrives the voltage across the output of the filter starts increasing.
However, the built up voltage differs for each field. The reason is not difficult to find. At the beginning of the first field odd field the last horz sync pulse corresponding to the beginning of th line is separated from the 1st vertical pulse by full one line and any voltage developed across the filter will have enough time to return to zero before the arrival of the first vertical pulse, and thus the filter output voltage builds up from zero in response to the five successive broad vertical sync pulses.
The voltage builds up because the capacitor has more time to charge and only 4. The situation, however, is not the same for the beginning of the 2nd even field.
Here the last horizontal pulse corresponding to the beginning of th line is separated from the first vertical pulse by only half-a-line. The voltage developed across the vertical filter will thus not have enough time to reach zero before the arrival of the first vertical pulse, which means that the voltage build-up does not start from zero, as in the case of the 1st field.
The residual voltage on account of the half line discrepancy gets added to the voltage developed on account of the broad vertical pulses and thus the voltage developed across the output filter is some what higher at each instant as compared to the voltage developed at the beginning of the first-field.
This is shown in dotted chain line in Fig. The vertical oscillator trigger potential level marked as trigger level in the diagram Fig. Note that this inequlity in potential levels for the two fields continues during the period of discharge of the capacitor once the vertical sync pulses are over and the horizontal sync pulses take-over. Though the actual time difference is quite short it does prove sufficient to upset the desired interlacing sequence. End of 2nd field.
Note the above sync pulses have purposely been drawn without equalizing pulses. Equalizing pulses. To take care of this drawback which occurs on account of the halfline discrepancy five narrow pulses are added on either side of the vertical sync pulses. These are known as pre-equalizing and post-equalizing pulses. Each set consists of five narrow pulses occupying 2. Pre-equalizing and postequalizing pulse details with line numbers occupied by them in each field are given in Fig. The effect of these pulses is to shift the half-line discrepancy away both from the beginning and end of vertical sync pulses.
Pre-equalizing pulses being of 2. Post-equalizing pulses are necessary for a fast discharge of the capacitor to ensure triggering of the vertical oscillator at proper time. If the decay of voltage across the capacitor is slow as would happen in the absence of post-equalizing pulses, the oscillator may trigger at the trailing edge which may be far-away from the leading edge and this could lead to an error in triggering.
Thus with the insertion of narrow pre and post equalizing pulses, the voltage rise and fall profile is essentially the same for both the field sequences see Fig. This problem could possibly also be solved by using an integrating circuit with a much larger time constant, to ensure that the capacitor remains virtually uncharged by the horizontal pulses. However, this would have the effect of significantly reducing the integrator output for vertical pulses so that a vertical sync amplifier would have to be used.
In a broadcasting situation, there are thousands of receivers for every transmitter. Consequently it is much more efficient and economical to cure this problem in one transmitter than in thousands of receivers. This, as explained above, is achieved by the use of pre and post equalizing pulses. The complete pulse trains for both the fields incorporating equalizing pulses are shown in Fig.
From the comparison of the horizontal and vertical output pulse forms shown in Figs. The scale chosen exaggerates the extent of the vertical pulses. The voltage build-up period is only s and so far as the vertical synchronizing oscillator is concerned this pulse occurs rapidly and represents a sudden change in voltage which decays very fast.
The polarity of the pulses as obtained at the outputs of their respective fields may not be suitable for direct application in the controlled synchronizing oscillator and might need inversion depending on the type of oscillator used. This aspect will be fully developed in the chapter devoted to vertical and horizontal oscillators. First Field odd field Line numbers: Approximate location of line numbers.
The serrated vertical sync pulse forces the vertical deflection circuity to start the flyback. However, the flyback generally does not begin with the start of vertical sync because the sync pulse must build up a minimum voltage across the capacitor to trigger the scanning oscillator. If it is assumed that vertical flyback starts with the leading edge of the fourth serration, a time of 1. Also five equalizing pulses occur before vertical sync pulse train starts.
Then four lines 2. A typical vertical retrace time is five lines. These lines provide the sweep oscillator enough time to adjust to a linear rise for uniform pick-up and reproduction of the picture. By serrating the vertical sync pulses and the providing pre- and post-equalizing pulses the following basic requirements necessary for successful interlaced scanning are ensured.
In the line American TV system where the total number of lines scanned per second is , the sync pulse details are as under: Details of Horz Blanking Period.
Sketch composite video signal waveform for at least three three successive lines and indicate: Why is the combining of picture signal and sync pulses called a voltage division multiplex? Sketch the details of horizontal blanking and sync pulses. Label on it i front porch, ii horizontal sync pulse, iii back porch and iv active line periods.
Why are the front porch and back porch intervals provided before and after the horizontal sync pulse? Explain why the blanking pulses are not used as sync pulses. Enumerate the basic requriments that must be satisfied by the pulse train added after each field. Why is it necessary to serrate the broad vertical sync pulse? Sketch the pulse trains that follow after the second and first field of active scanning. Why are the vertical sync pulses notched at 32 s interval and not at 64 s interval to provide horizontal sync pulses?
Explain how the horizontal and vertical sync pulses are separated and shaped at the receiver. For a time constant of 5 s for the differentiating circuit, and s for the integrating circuit, plot the output waveforms from both the circuits for the entire vertical period. Calculate the error in timing for successive vertical fields in the absence of equalizing pulses. Sketch the complete pulse trains that follow at the end of both odd and even fields.
Fully label them and explain how the half line discrepancy is removed by insertion of pre-equalizing pulses. Justify the need for pre and post equalizing pulses. Why it is necessary to keep their duration equal to the half-line period? Justify the need for a blanking period corresponding to 20 complete lines after each active field of scanning. Why does the vertical retrace not begin with the incoming of the first serrated vertical sync pulse? Sketch the complete pulse trains that follow at the end of odd and even fields in the line television system.
Justify the need for six instead of five pre and post equalizing pulses. Show by any suitable means approximate correspondence between line numbers and the location of the electron beam on the screen, both for odd and even fields. R line, the picture signal is amplitude modulated and sound signal frequency modulated before transmission. The channel bandwidth is determined by the highest video frequency required for proper picture reception and the maximum sound carrier frequency deviation permitted in a TV system.
Need for modulation. The need for modulation stems from the fact that it is impossible to transmit a signal by itself. The greatest difficulty in the use of unmodulated wave is the need for long antennas for efficient radiation and reception. For example, a quarter-wavelength antenna for the transmitting frequency of 15 kHz would be meters long. A vertical antenna of this size is unthinkable and in fact impracticable. Another important reason for not transmitting signal frequencies directly is that both picture and sound signals from different stations are concentrated within the same range of frequencies.
Therefore, radiation from different stations would be hopelessly and inextricably mixed up and it would be impossible to separate one from the other at the receiving end. Thus in order to be able to separate the intelligence from different stations, it is necessary to translate them all to different portions of the electromagnetic spectrum depending on the carrier frequency assigned to each station.
This also overcomes the difficulties of poor radiation at low frequencies.
Once signals are translated before transmission, a tuned circuit provided in the RF section of the receiver can be used to select the desired station. As an illustration, an amplitude modulated signal is shown in Fig. Note that the camera signal is actually complex in nature but a single modulating frequency has been chosen for convenience of analysis. The equation of the modulated wave is: On substituting the value of A we get: Equation 4.
However, if the modulating signal consists of more than a single frequency, as it would be for a video signal, the equation can be extended to include the sum and difference of the carrier and all frequency components of the modulating signal.
Therefore if the modulated wave is to be transmitted without distortion by this method, the transmission channel must be atleast of width 2fm centred on fc. The actual band space allocated to the television channel would have to be still greater, because with practical filter characteristics it is not possible to terminate the bandwidth of a signal abruptly at the edges of the sidebands. Therefore, an attenuation slope of 0. This adds 1 MHz to the required total band space.
In addition to this, each television channel has its associated FM frequency modulated sound signal, the carrier frequency of which is situated just outside the upper limit of 5. This, together with a small guard band, adds another 0.
P is picture carrier and S is sound carrier. Such a bandwidth is too large, and if used, would limit the number of channels in a given high frequency spectrum allocated for TV transmission. Therefore, to ensure spectrum conservation, some saving in the bandwidth allotted to each channel is desirable. Single sideband transmission SSB.
A careful look at eqn. Therefore, though superfluous from the point of view of transmission of intelligence, the carrier frequency is radiated along with the sideband components in all radio-broadcast and TV systems. Such an arrangement results in simpler transmitting equipment and needs a very simple and inexpensive diode detector at the receiver for recovering the modulation components without undue distortion. From eqn. Also, any change in the frequency of the modulating signal results in identical changes 2 in the band spread of the two sidebands.
It is seen, therefore, that all the information can be conveyed by the use of one sideband only and this results in a saving of 5 MHz per channel. It may, however, be noted that the magnitude of the detected signal in the receiver will be just half of that obtained when both the sidebands are transmitted. This is no serious drawback because the IF intermediate frequency amplifier stages of the receiver provide enough gain to develop reasonable amplitude of the video signal at the output of video detector.
These components give rise to sidebands very close to the carrier frequency which are difficult to remove by physically realizable filters. Thus it is not possible to go to the extreme and fully suppress one complete sideband in the case of television signals.
The low video frequencies contain the most important information of the picture and any effort to completely suppress the lower sideband would result in objectionable phase distortion at these frequencies. This distortion will be seen by the eye as smear in the reproduced picture.
Therefore, as a compromise, only a part of the lower sideband, is suppressed, and the radiated signal then consists of a full upper sideband together with the carrier, and the vestige remaining part of the partially suppressed lower sideband. This pattern of transmission of the modulated signal is known as vestigial sideband or A5C transmission.
In the line system, frquencies up to 0.The equation of the modulated wave is: The viewing distance from the screen of the TV receiver should not be so large that the eye cannot resolve details of the picture. The deciding factor for adoption was compatibility with the already existing monochrome system.
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. Coverage Most programmes are produced live in the studio but recorded on video tape at a convenient time to be broadcast later. Since the consideration of both vertical and horizontal resolutions is based on identical black and white bars in the horizontal and vertical planes of the picture frame, it amounts to considering a chessboard pattern as the most stringent case and is illustrated in Fig.
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