Colout TV Fundamentals

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Transcript of Colout TV Fundamentals

  • COLOUR TV FUNDAMENTALS

  • Chromaticity diagram

  • Colour camera

  • Colour signal generation

  • Electrical output of camera tube is not linear function of light input.

    Light output of picture tube is also not a linear function of electrical input.

    Exponent of transfer function called Gamma.

    Gamma Correction

  • = 1

    System is linear.

    Overall = 1.2 gives slightly more

    pleasing picture.

    B/W PT - = 2

    Colour PT - = 2.2

    Hence gamma of transmission

    chain should be 0.5.

    Gamma correction

  • Gamma correction

  • Colour Picture Tube

  • Delta Gun Colour Picture Tube

    Gun-in-Line or Precision-in-line Colour Picture Tube

    Single gun or Trintron Colour Picture Tube

    Colour Picture Tube

  • Radio Corporation of America RCA

    3 separate guns for 3 colours.

    Guns equally spaced at 120 ,tilted towards axis.

    Screen coated with 3 different phosphors for R, G, B.

    Triad contains 1 dot of each colour.

    3,33,000 triads.

    Eye integrates 3 colour information to give a sensation of combined hue.

    Shadow mask.

    One hole per triad.

    All stray electrons (80%) get collected by mask.

    Hence need higher EHT.

    Delta Gun Colour Picture Tube

  • Delta Gun Colour Picture Tube

    Gun viewed from base

    120 120

    120

    Electron

    Gun

    Tube

    Neck

  • Delta Gun Colour Picture Tube

    Electron Guns with mask and screen

  • Delta Gun

    Colour

    Picture

    Tube

  • Convergence is difficult.

    Very elaborate arrangements to overcome it.

    Focus not sharp over entire screen.

    As focus and convergence planes for three guns separated by 120 are not same.

    Mask passes only 20% of electrons.

    Delta Gun Colour Picture Tube

    Disadvantages

  • Precision-in-line Colour Picture Tube

  • 3 Guns in line simplifies convergence adjustment.

    Colour phosphors in form of vertical stripes.

    Distance same as in Delta gun.

    More efficient.

    Larger % of electrons pass through mask.

    Fewer convergence adjustments.

    Most widely used.

    Precision-in-line Colour Picture Tube

  • By Sony Corporation, Japan

    Single gun having three cathodes.

    Simplified construction as only one electron assembly.

    Three phosphor triads arranged in vertical strips.

    Each strip is few thousandth of a cm.

    A metal aperture grill has one slot each triad.

    Grill has greater electron transparency.

    Three beams appear to emerge from same point.

    TRINTRON Colour Picture Tube

  • TRINTRON Colour Picture Tube

  • TRINTRON Colour Picture Tube

  • Each beam must fall at center of corresponding dot.

    Irrespective of position of triad on screen.

    Circular magnets called purity magnets on neck of yoke does the alignment.

    If all tabs moved together, they change direction of field.

    If tabs are separated, magnetic field reduces.

    Purity and Convergence

  • Purity

  • 2, 4, 6 pole magnets to achieve collective or individual beam deflection.

    Error may also occur if yoke is not positioned properly.

    Purity

  • Technique that all electrons hit the same part of the screen to produce 3 coincidental rasters.

    Errors

    Non-coincidental convergence plane.

    Non-uniformity of deflection field.

    Flat surface of picture tube screen.

    Convergence

  • Convergence

  • Correction:

    Static and dynamic convergence.

    Static convergence by permanent magnet.

    Once correctly set, brings the beam into convergence in the central area of the screen

    Dynamic convergence converges beam over rest of the screen.

    Dynamic convergence achieved by additional winding in series with yoke coil.

    Achieved by continuously varying magnetic field.

    Instantaneous strength depends upon position of the spot on the screen.

    Convergence

  • Static pincushion may cause purity problem.

    Dynamic pincushion correction.

    This stretches the horizontal width and vertical size of raster at edges.

    Pincushion correction

  • Due to Earth magnetic field and other nearby magnetic fields, mask and mounting frame may get magnetized.

    Will cause purity error causing in colour patches.

    Can be repaired by influencing the screen by an alternating magnetic field which gradually reduces to zero.

    Degaussing

  • Automatic degaussing

  • At switch ON, thermistor is cold and has high resistance.

    More AC passes through coil and varistor, causing alternating magnetic field.

    Thermistor gradually heats up, reducing its resistance.

    More current flows through thermistor.

    Lesser current through coil and varistor.

    Heats thermistor further

    This continues till

    current through thermistor becomes maximum.

    Current through varistor and coil gradually reduces to zero.

    Components chosen to give initial surge of 4ampere to D.Coil.

    Final current less than 25mA in less than a second.

    Automatic degaussing

  • R G B must combine to give to give white.

    3 phosphors have different efficiencies.

    3 guns may not have identical emission and cutoff point (Ip/Vgk).

    Tint appears instead of pure grey shades for monochrome transmission.

    Suitable adjustment required to produce correct monochrome information with no colour tint for all settings of contrast control.

    Grey Scale Tracking

  • Appearance of tint instead of pure grey shades in areas of low brightness.

    Necessary to bring cutoff points in coincidence.

    Achieved by making screen grid voltage (1st anode) different for each cathode.

    Potentiometers are normally provided in DC voltage supply to 3 screen grids.

    Grey Scale Tracking Adjustment on low light

  • All levels of light must be correctly reproduced.

    Need to Compensate for slightly different slopes and also for substantially different phosphor efficiencies.

    Achieved by the video signal Y drive to the 3 guns.

    Red has lower efficiency, maximum video signal Y fed to red cathode.

    By potentiometer B and G inputs are varied to produce optimum reproduction of high lights.

    Grey Scale Tracking Adjustment on high light

  • 3 Systems---

    American NTSC(National Television System Committee)

    German PAL( Phase Alteration by Line)

    French SECAM(Sequential couleures a memoire)

    All systems are good.

    India adopted PAL for 625 line CCIR-B standard for B/W.

    NTSC and PAL being similar are dealt together.

    Colour signal transmission and reception

  • Should accommodate hue and saturation in same band of 7MHz.

    Colour signal should not disturb B/W information and vice versa.

    Done by Frequency interleaving.

    Colour signal transmission

  • Should accommodate hue and saturation in same band of 7MHz.

    Colour signal should not disturb B/W information and vice versa.

    Done by Frequency interleaving.

    Colour signal transmission

  • Video signal band bears a definite relation with scanning frequencies.

    Energy content of video signal is contained in individual energy bundles.

    Bundles occur at harmonics of line frequency.15625Hz

    Components of each bundle separated by multiple of field frequency. 50, 100, 150

    Each bundle has peak at exact line harmonic gradually reducing in amplitude on either side.

    Amplitudes of bundles reduce towards higher harmonics of line frequency.

    Frequency interleaving

  • Energy bundle

    Amplitude

    fH = 15625Hz

  • Vertical sidebands contain lesser energy than Horizontal because of lower rate of scanning frequencies.

    Overall Energy contents decreases to very small value beyond 3.5MHz from picture carrier.

    Part of B/W BW unused at spacing between the bundles.

    Used for colour energy bundles.

    Energy bundle

  • Colour subcarrier so chosen that side band energy frequencies exactly fall between harmonics of line frequencies.

    Hence colour subcarrier chosen to be odd multiple of half of line frequency.

    To avoid any cross talk between B/W and colour info, higher portion of B/W BW is chosen to place coloue BW.

    Hence Colour subcarrier is 567 times half line frequency.

    PAL = 567x 15625/2 = 4.43MHz

    NTSC = 455x 15750/2 = 3.58MHz

    Colour Energy bundle

  • Interleaving of colour signal

  • Interleaving of Energy bundles

  • Study show that eye can perceive colour in object areas of 1/25th of screen width or more.

    For smaller areas, Eye can only perceive its brightness.

    Between 0 0.5MHz, all colours can be seen.

    0.5 1.5MHz, only two primary colours seen. G, B.

    Eye can not distinguish purple and green-yellow hue.

    For smaller objects, colour not visible, only shades of grey.

    Hence colour B/W = 1.5MHz = 3MHz (DSB-SC)

    Band width of Colour Signal

  • (B-Y), (R-Y).

    Carrier = 4.43MHz.

    Second carrier by giving 90 phase shift to carrier.

    Called Quadrature modulation.

    Resultant subcarrier phaser ca