1.Introduction to Video Signals

2. Contents Picture transmission TV transmitter TV receiver Synchronization Receiver controls Geometric form and aspect ratio Image continuity No. of scanning lines Interlaced scanning 3. Contents Resolution Brightness Contrast Video signal dimensions Horizontal sync composition Vertical sync details Function of vertical pulse train Scanning sequence details Perception of brightness and colour Additive and subtractive colour mixing 4. Contents Video signals for colour transmission Luminance signal (Y) Compatibility Colour-difference signals Encoding of colour difference signals Formation of chrominance signal 5. Picture transmission Picture information is optical in nature and may be thought of as 2-D arrangement of pixels Thus it is needed to pick up each pixels simultaneously for transmission It is practically not possible Thus Scanning is done for each row of pixels After scanning, the optical information is converted to electrical information This is done at a very fast rate and hence the eye cannot detect a change 6. Camera tube-2D 7. Camera tube-3D 8. Camera tube 9. Scanning 10. Colour camera tube 11. Television Transmitter 12. Television Receiver 13. Elements of picture tube 14. Colour receiver 15. Synchronization It is essential that the same co-ordinates be scanned at any instant both at the camera tube target plate and at the raster of picture tube otherwise, the picture details would split and get distorted To ensure perfect synchronization between the scene being televised and the picture produced on the raster, synchronizing pulses are transmitted during the retrace i.e., fly-back intervals of horizontal and vertical motions of the camera scanning beam Thus, in addition to carrying picture details, the radiated signal at the transmitter also contains synchronizing pulses 16. Synchronization These pulses which are distinct for horizontal and vertical motion control, are processed at the receiver and fed to the picture tube sweep circuitry Thus ensuring that the receiver picture tube beam is in step with the transmitter camera tube beam In a colour television system additional sync pulses called colour burst are transmitted along with horizontal sync pulses These are separated at the input of chroma section and used to synchronize the colour demodulator carrier generator 17. Receiver controls Most black and white receivers have on their front panel (i ) channel selector ( ii ) fine tuning ( iii ) brightness ( iv ) contrast ( v) horizontal hold (vi ) volume controls besides an ON-OFF switch Some receivers also provide a tone control 18. Receiver controls The channel selector switch is used for selecting the desired channel The fine tuning control is provided for obtaining best picture details in the selected channel The brightness control varies the beam intensity of the picture tube and is set for optimum average brightness of the picture The contrast control has actually gained control of the video amplifier This can be varied to obtain desired contrast between white and black contents of the reproduced picture 19. Receiver controls The hold control is used to get a steady picture in case it rolls up or down The volume and tone controls form part of the audio amplifier in sound section, and are used for setting volume and tonal quality of the sound output from the loudspeaker In colour receivers there is an additional control called colour or saturation control It is used to vary the intensity or amount of colours in the reproduced picture 20. Receiver controls In modern colour receivers that employ integrated circuits in most sections of the receiver, the hold control is not necessary and hence usually not provided 21. Aspect ratio The frame adopted in all television systems is rectangular with width/height ratio In human affairs most of the motion occurs in the horizontal plane and so a larger width is desirable It is not necessary that the size of the picture produced on the receiver screen be same as that being televised but it is essential that the aspect ratio of the two be same This is achieved by setting the magnitudes of the current in the deflection coils to correct values to both at the TV camera and receiving picture tube 22. Image continuity While televising picture elements of the frame by means of the scanning process, it is necessary to present the picture to the eye in such a way that an illusion of continuity is created To achieve this, advantage is taken of persistence of vision The sensation produced when nerves of the eyes retina are stimulated by incident light does not cease immediately after the light is removed but persists for about 1/16th of a second 23. Image continuity So when the picture elements are scanned rapidly enough, they appear to the eye as a complete picture unit In present day motion pictures 24/25 still pictures of the scene are taken per second and later projected on the screen at the same rate Image continuity can be achieved through scanning 24. Horizontal scanning 25. Horizontal scanning The linear rise of current in the horizontal deflection coils deflects the beam across the screen with a continuous, uniform motion for the trace from left to right At the peak of the rise, the sawtooth wave reverses direction and decreases rapidly to its initial value This fast reversal produces the retrace or flyback 26. Horizontal scanning 27. Vertical scanning 28. Vertical scanning 29. Vertical scanning The sawtooth current in the vertical deflection coils moves the electron beam from top to bottom of the raster at a uniform speed while the electron beam is being deflected horizontally Both during horizontal retrace and vertical retrace intervals the scanning beams at the camera tube and picture tube are blanked and no picture information is either picked up or reproduced 30. No. of scanning lines The ability of the scanning beam to allow reproduction of electrical signals and the capability of the human eye to resolve the scene distinctly depends on the total number of lines employed for scanning More number of scanning lines gives space to cover more optical information to be converted into digital 625 line are selected as standard for India(CCIR standard) The number of scanning line is limited by below factors Large bandwidth requirement Small possible thickness of scanning beam Capability of the human eye to resolve the scene 31. Interlaced scanning Although the rate of 24 pictures per second in motion pictures is enough to cause an illusion of continuity, they are not rapid enough to allow the brightness of one picture or frame to blend smoothly into the next through the time when the screen is blanked between successive frames This results in a definite flicker of light that is very annoying to the observer when the screen is made alternately bright and dark 32. Interlaced scanning This problem is solved in motion pictures by showing each picture twice, so that 48 views of the scene are shown per second although there are still the same 24 picture frames per second As a result of the increased blanking rate, flicker is eliminated In television pictures an effective rate of 50 vertical scans per second is utilized to reduce flicker 33. Interlaced scanning This is accomplished by increasing the downward rate of travel of the scanning electron beam, so that every alternate line gets scanned instead of every successive line Then, when the 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 Thus the total number of lines are divided into two groups called fields Each field is scanned alternately 34. Interlaced scanning 35. Interlaced scanning In the 625 lime monochrome system, for successful interlaced scanning, the 625 lines of each frame or picture are divided into sets of 312.5 lines Each set is scanned alternately to cover the entire picture area To achieve this the horizontal sweep oscillator is made to work at a frequency of 15625 Hz (312.5 50 = 15625) To scan the same number of lines per frame (15625/25 = 625 lines) 36. Interlaced scanning Horizontal scanning periods 37. Interlaced scanning Vertical scanning periods 38. Interlaced scanning Scanning sequence 39. Interlaced scanning 40. Picture Resolution The ability of the image reproducing system to represent the fine structure of an object is known as its resolving power or resolution 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 Vertical resolution Vr can be represented as Vr = Na x k Here Vr = vertical resolution as number of line Na = Active number of lines in the system k = KELL factor or resolution factor 41. Picture Resolution Assuming a reasonable value of k = 0.69, Vr = 585 0.69 = 400 lines 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 N = Na aspect ratio k = 585 4/3 0.69 = 533 42. Picture Resolution A method to determine horizontal resolution 43. Picture Resolution Since along one line there are 533/2 = 267 complete cyclic changes, 267 complete square wave cycles get generated during the time the beam takes to travel along the width of the pattern Thus the time duration th of one square wave cycle is equal to The frequency of the square wave will be 44. Picture Resolution A method to study both horizontal and vertical resolution 45. Brightness Brightness is the overall or average intensity of illumination It determines background light level in the reproduced picture The brightness of a picture, can be varied to obtain optimum average illumination of the scene 46. Contrast Contrast is the difference in light intensity between black and white parts of picture over and above the average brightness level A picture with more contrast has bright white and dark black intensity levels Too much contrast makes the picture difficult and painful to see for a long time Too little contrast in picture makes it looking washed away 47. Viewing distance 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 While viewing TV, a small light should be kept ON in the room to reduce overall contrast This prevents strain to the eyes and there is less fatigue 48. Optimum viewing angle 49. Luminance This is the amount of light perceived by the eye regardless of the colour In monochrome television, more lighted parts of images have more luminance In colour TV, colours also have their own intensities which can be resolved by luminance on a monochrome TV 50. HUE This is the predominant spectral colour of the received light The colour of any object is distinguished by its hue Different hues result from different wavelengths of spectral radiation 51. Example of hue 52. Saturation This is the spectral purity of the colour light Single hue colours alone occur rarely in nature Thus saturation may be taken as an indication of how little the colour is diluted by white A fully saturated colour has no white Hue and saturation, when put togather, known as chrominance 53. Improvement of saturation Original image Image with 50 % increase of saturation 54. Composite Video Signal A composite video signal consists of: Camera signal - corresponding to the desired picture information Blanking pulses to make the retrace invisible Synchronizing pulses to synchronize the transmitter and receiver scanning horizontal sync pulse vertical sync pulse their amplitudes are kept same but their duration are different needed consecutively and not simultaneously with the picture signal so sent on a time division basis 55. Composite Video Signal 56. Composite Video Signal Video signal varies between certain limits Peak white level: 10 to 12.5% Black level : 72% Blanking level : Sync pulses added - 75% level Pedestal : difference between black level and blanking level tend to merge Pedestal height : distance between the pedestal level and the dc level indicates the average brightness Picture information : 10% - 75% 57. DC component of the video signal Average value or dc component corresponding to the average brightness of the scene Average brightness can change only from frame to frame and not from line to line Low pedestal height scene darker Larger pedestal height higher average brightness 58. Pedestal height The pedestal height is the distance between the pedestal level and the average value (dc level) axis of the video signal 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 59. Blanking pulses Make the retrace lines invisible by raising the signal amplitude slightly above the black level (75%) Repetition rate of horizontal blanking pulse = scanning freq = 15625Hz Freq of vertical blanking pulse = field scanning freq. = 50 Hz 60. Interlaced scanning : Revisited 61. Interlaced scanning : Revisited Horizontal scanning periods 62. Interlaced scanning : Revisited Vertical scanning periods 63. Interlaced scanning : Revisited Scanning sequence 64. Horizontal Sync Composition 65. Horizontal Sync Composition 66. Horizontal Sync Composition Front porch: This is a brief cushioning period of 1.5 s inserted between the end of the picture detail for that line and the leading edge of the line sync pulse 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 67. Horizontal Sync Composition Line sync pulse: After the front porch 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 4.7 s. During this period the beam on the raster almost completes its back stroke (retrace) and arrives at the extreme left end of the raster 68. Horizontal Sync Composition Back porch: This period of 5.8 s at the blanking level allows plenty of time for line flyback to be completed It also permits time for the horizontal time-base circuit to reverse direction of current for the initiation of the scanning of next line The back porch also provides the necessary amplitude equal to the blanking level and enables to preserve the dc content of the picture information at the transmitter 69. Vertical Sync Details 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 In the 625 line system 2.5 line period (2.5 64 = 160 s) has been allotted for the vertical sync pulses Thus a vertical sync pulse commences at the end of 1st half of 313th line (end of first field) and terminates at the end for 315th line 70. Vertical Sync Details 71. Vertical Sync Details 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 The horizontal sync information is extracted from the sync pulse train by differentiation, i.e., by passing the pulse train through a high-pass filter 72. Vertical Sync Details Indeed pulses corresponding to the differentiated leading edges of sync pulses are used to synchronizes the horizontal scanning oscillator But there is a problem of skipping of horizontal pulses 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 2.5 line vertical sync period This will lead the horizontal oscillator out of sync 73. Vertical Sync Details 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 4.7 s before the line pulses are needed The waveform is then returned back to the maximum level at the moment the line sweep circuit needs synchronization 74. Vertical Sync Details Thus five narrow slots of 4.7 s width get formed in each vertical sync pulse at intervals of 32 s The trailing but rising edges of these pulses are actually used to trigger the horizontal oscillator However, the pulses actually utilized are the ones that occur sequentially at 64 intervals Such pulses are marked with line numbers for both the fields 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. 75. Vertical Sync Details 76. Vertical Sync Details 77. Vertical Sync Details Synchronization of the vertical sweep oscillator in the receiver is obtained from vertical sync pulses by integration For scanning of 1st field, voltage builds up because capacitor has more time to charge and 4.7 us time to discharge When the second field comes, the vertical pulse comes at half line period of horizontal pulse Due to this the voltage built up due to charging of horizontal pulse does not comes to zeros when the vertical pulse occurs This introduces voltage difference between both vertical pulses 78. Vertical sync problem 79. Vertical Sync problem solution 80. Functions of vertical pulse train A suitable field sync pulse is derived for triggering the field oscillator The line oscillator continues to receive triggering pulses at correct intervals while the process of initiation and completion of the field time-base stroke is going on It becomes possible to insert vertical sync pulses at the end of a line after the 2nd field and at the middle of a line at the end of the 1st field without causing any interlace error The vertical sync build up at the receiver has precisely the same shape and timing on odd and even fields 81. Scanning sequence details 82. Scanning sequence details 83. Scanning sequence details 84. Perception of brightness and colour All objects that we observe are focused sharply by the lens system of the eye on its retina The retina which is located at the back side of the eye has light sensitive organs which measure the visual sensations The retina is connected with the optic nerve which conducts the light stimuli as sensed by the organs to the optical centre of the brain 85. Perception of brightness and colour The light sensitive organs are of two typesrods and cones The rods provide brightness sensation and thus perceive objects only in various shades of grey from black to white The cones that are sensitive to colour are broadly in three different group One set of cones detects the presence of blue colour in the object focused on the retina, the second set perceives red colour and the third is sensitive to the green range 86. Additive colour mixing In additive mixing which forms the basis of colour television, light from two or more colours obtained either from independent sources or through filters can create a combined sensation of a different colour Thus different colours are created by mixing pure colours and not by subtracting parts from white The impression of white light can also be created by choosing suitable intensities of these colours Red, green and blue are called primary colours. These are used as basic colours in television 87. Additive colour mixing By pairwise additive mixing of the primary colours the following complementary colours are produced: Red + Green = Yellow Red + Blue = Magenta (purplish red shade) Blue + Green = Cyan (greenish blue shade 88. Perception of eye for different colours 89. Additive colour mixing 90. Additive colour mixing 91. Additive colour mixing The brightness (luminance) impression created by the combined light source is numerically equal to the sum of the brightnesses (luminances) of the three primaries This property of the eye of producing a response which depends on the algebraic sum of the red, green and blue inputs is known as Grassmans Law 100% White = 30% Red + 59% Green + 11% Blue 92. Video signals for colours Colour voltage amplitudes: 93. Video signals for colours Desaturated colours: Any colour is said to be desaturated when mixed with white In a colour camera output signal, Red colour is desaturated to a small amount, then the Vg and Vb have lower values But as desaturation of red increases, Vg and Vb values are increased For 100% desaturation Vr = Vg =Vb 94. Video signals for colours Colour video Frequencies: When the scene is not dominated by one or few colours the information to be transmitted occupies more frequency spectrum It is discovered that colour frequencies need 1.5 Mhz band in order to transmit finest details of a scene The luminance signal frequency range is up to 5 Mhz 95. Luminance signal Y Luminance refers to the brightness of scene It is formed by adding the three camera outputs in the ratio, Y = 0.3 R + 0.59 G + 0.11 B These percentages correspond to the relative brightness of the three primary colours Therefore a scene reproduced in black and white by the Y signal looks the same as when it is televised in monochrome 96. Luminance signal Y 97. Generation of luminance signal Y 98. Compatibility It is necessary that a colour TV should produce black and white picture and a black and white TV should be able to process colour signal to extract the black white scene information This feature is known as compatibility of video signal Here we can not transmit Vr, Vg, Vb separately because of limitation of 5.5 bandwidth To solve this problem colour difference signals are used, which can be accommodated in 5.5 Mhz band 99. Colour difference signal Colour difference voltages are derived by subtracting the luminance voltage from the colour voltages Only (R Y) and (B Y) are produced It is only necessary to transmit two of the three colour difference signals since the third may be derived from the other two The circuit for getting colour difference signals is as follows 100. Colour difference signal 101. Colour difference signal Here by definition we have Y = 0.3R + 0.59G + 0.11B Therefore, (R Y) = 0.7R 0.59G 0.11B (B Y) = 0.89B 0.59G 0.3R. The colour difference signals equal zero when white or grey shades are being transmitted On peak whites let R = G = B = 1 volt Then Y = 0.59G + 0.3R + 0.11B = 0.59 + 0.3 + 0.11 = 1 (volt) (R Y) = 1 1 = 0, volt and (B Y) = 1 1 = 0 volt 102. Colour difference signal On any grey shade let R = G = B = v volts (v < 1) Then Y = 0.59v + 0.3v + 0.11v = v (R Y) = v v = 0 volt and (B Y) = v v = 0 volt Thus it is seen that colour difference signals during the white or grey content of a colour scene of during the monochrome transmission completely disappear and this is an aid to compatibility in colour TV systems 103. Colour difference signal Consider we have a desaturated magenta(Purple) colour to transmit Suppose R = 0.7, G = 0.2 and B = 0.6 volts The white content is represented by equal quantities of the three primaries and the actual amount must be indicated by the smallest voltage of the three, that is, by the magnitude of G Thus white is due to 0.2 R, 0.2 G and 0.2 B. The remaining, 0.5 R and 0.4 B together represent the magenta hue 104. Colour difference signal (i) The luminance signal Y = 0.3 R + 0.59 G + 0.11 B Substituting the values of R, G, and B we get Y = 0.3 (0.7) + 0.59 (0.2) + 0.11(0.6) = 0.394 (volts) (ii) The colour difference signals are: (R Y) = 0.7 0.394 = + 0.306 (volts) (B Y) = 0.6 0.394 = + 0.206 (volts) (iii) Reception at the colour receiverAt the receiver after demodulation, the signals, Y, (B Y) and (R Y), become available 105. Colour difference signal Then by a process of matrixing the voltages B and R are obtained as: R = (R Y) + Y = 0.306 + 0.394 = 0.7 V B = (B Y) + Y = 0.206 + 0.394 = 0.6 V (G Y) matrixThe missing signal (G Y) that is not transmitted can be recovered by using a suitable matrix based on the explanation given below: Y = 0.3 R + 0.59G + 0.11B also (0.3 + 0.59 + 0.11)Y = 0.3R + 0.59G + 0.11B 106. Colour difference signal Rearranging the above expression we get: 0.59(G Y) = 0.3 (R Y) 0.11 (B Y) Substituting the values of (R Y) and (B Y) (G Y) = (0.51 0.306) 0.186(0.206) = 0.15606 0.038216 = 0.194 G = (G Y) + Y = 0.194 + 0.394 = 0.2 107. Colour difference signal Unsuitability of (G Y) Signal for Transmission: The proportion of G in Y is relatively large(59%) in most cases, the amplitude of (G Y) is small The smaller amplitude together with the need for gain in the matrix would make S/N ratio problems more difficult then when (R Y) and (B Y) are chosen for transmission 108. Encoding of colour difference signals The problem of transmitting (B-Y) and (R-Y) video signals simultaneously with one carrier frequency is solved by creating two carrier frequencies from the same colour subcarrier without any change in its numerical value Two separate modulators are used, one for the (B-Y) and the other for the (R-Y) signal However, the carrier frequency fed to one modulator is given a relative phase shift of 90 with respect to the other before applying it to the modulator 109. Encoding of colour difference signals 110. Encoding of colour difference signals 111. The horizontal scanning frequency of camera beam is 15625 Hz Therefore, the video frequencies generated on scanning any scene are multiples of this frequency So video information can be shown as below Encoding of colour difference signals 112. Encoding of colour difference signals Here the chrominance signal is fitted into the gaps of Y signal frequencies 113. Encoding of colour difference signals 114. Encoding of colour difference signals The transmitted signal does not contain the subcarrier frequency but it is necessary to generate it in the receiver with correct frequency and phase relationship for proper detection of the colour sidebands To ensure this, a short sample of the colour subcarrier oscillator (8 to 11 cycles) called the colour burst is sent to the receiver along with sync signals This is located in the back porch of the horizontal blanking pedestal 115. Encoding of colour difference signals 116. Formation of chrominance signal The chroma signal has magnitude and phase angle as shown below 117. Formation of chrominance signal Consider that we need to transmit red colour For a pure red, R = 1v, G = 0v, B = 0v We know that (R Y) = 0.7R 0.59G 0.11B (B Y) = 0.89B 0.59G 0.3R. Putting values for Red colour we have, (R Y) = 0.7(1) 0.59(0) 0.11(0) = 0.7v (B Y) = 0.89(0) 0.59(0) 0.3(1) = -0.3v 118. Formation of chrominance signal )Y)-(RY)-((B 22 76.0)(0.7)((-0.3) 22 )( )( tan 1 YB YR 104 )3.0( )7.0( tan 1 Magnitude of chroma signal can be found as below: And the phase of chroma signal with respect to (B-Y) can found as below Thus for pure red colour , the chroma signal falls in second quadrant 119. Formation of chrominance signal For cyan(Blue +Green) R = 0v, G = 1v, B = 1v Putting values for cyan we have, (R Y) = 0.7(0) 0.59(1) 0.11(1) = -0.7v (B Y) = 0.89(1) 0.59(1) 0.3(0) = 0.3v Magnitude of chroma signal can be found as below: And the phase of chroma signal with respect to (B-Y) can found as below )Y)-(RY)-((B 22 76.0)(-0.7)((0.3) 22 104180284 )3.0( )7.0( tan 1 )( )( tan 1 YB YR 120. Formation of chrominance signal From previous analysis we can say that as cyan is a complementary colour of red it has the same magnitude but exactly opposite angle In a natural scene we have many combination of colours in a single horizontal line of an image or video Therefore, for a natural scene the chroma signal has different magnitude and phase angle for each horizontal line The chroma signal decides the hue and saturation of a colour picture