L13: 6.111 – Spring 20041Introductory Digital Systems LaboratoryL13 L13 --VideoVideoSlides 2-10 courtesy of Tayo AkinwandeTake the graduate course, 6.973 – consult Prof. AkinwandeSome modifications of these slides by D. E. TroxelL13: 6.111 – Spring 20042Introductory Digital Systems LaboratoryHow Do Displays Work?How Do Displays Work? Electronic display is a “Language Translator” that converts “Time Sequential Electrical Signals” into spatially and temporally configured light signal (images) useful to the viewer. Translation Function carried out by two intertwined sub-functions Display element address wherein electrical signals are appropriately routed to the various display elements (similar to memory addressing) Display element (pixel) converts the routed electrical signal at its input into light of certain wavelength and intensity (inverse of image capture)L13: 6.111 – Spring 20043Introductory Digital Systems LaboratoryEmissive DisplaysEmissive DisplaysEmissive Displays generate photons from electrical excitation of the picture element (pixels). Can generate energy byz UV absorbed by a phosphorz injection by a PN junctionz Electron Beam hitting a phosphor This energy causes excitation followed by excitation relaxation.z Hole & Electron recombinationz Exiton formation and annihilationz Relaxation of excited ions or radicals in a plasma Sometimes the energy first goes to a dopant and then to photons, especially when changing the wavelength of the emitted light. Examples of Emissive Flat Panel Displaysz Electroluminescence (Light Emitting Diode), z Cathodoluminescence (Cathode Ray Tube)z Photoluminescence (PLasma Displays)L13: 6.111 – Spring 20044Introductory Digital Systems LaboratoryLight Valve DisplaysLight Valve DisplaysLight Valve Displays spatially and temporally modulate the intensity pattern of the picture elements (pixels) Displays that “spatially and temporally” modulate ambient lighting or a broad source of lighting and redirect it to the eye. The display element changes the intensity of the light usingz Refractionz Reflectionz Polarization change Examples of Light Valve Displays Liquid Crystal Displays (active & passive matrix) Deformable Mirror Displays Membrane Mirror Displays Electrophoretic Displays (E-Ink)L13: 6.111 – Spring 20045Introductory Digital Systems LaboratoryCathode Ray TubeCathode Ray TubePhosphor ScreenAnodeCathodeCRT DisplayDisplays in the lab available for projects are CRT displays.An electron beam “boiled off a metal” by heat (thermionicemission) is sequentially scanned across a phosphor screen by magnetic deflection. The electrons are accelerated to the screen acquiring energy and generate light on reaching the screen (cathodoluminescence) Courtesy of PixTechL13: 6.111 – Spring 20046Introductory Digital Systems LaboratoryFlat Panel DisplaysFlat Panel Displays Time sequential electrical signals describing an image need to be routed to the appropriate picture element (pixel). Typical flat panel displays are two-dimensional arrays of picture elements (pixels) that are individually addressed from the perimeter or the back. Methods of scanning include Sequential addressing (CRT) Row scan addressing (Thin-CRT, Plasma, Mirror, LCD) Row scanning of a matrix of pixels requires picture elements with non-linear Luminance – Voltage (L-V) characteristics. If the L-V characteristics is linear (or is not non-linear enough), a non linear switch element is required in series with the pixel.L13: 6.111 – Spring 20047Introductory Digital Systems LaboratoryThinThin--CRTCRTAnodeCathodeField Emission Device (FED) DisplayIn principle similar to the CRT except that it uses a two-dimensional array of electron sources (field emission arrays) which are matrix addressed allowing the vacuum package to be thinCourtesy of PixTechPhosphorsL13: 6.111 – Spring 20048Introductory Digital Systems LaboratoryPlasma DisplaysPlasma Displays• Electrons are accelerated by voltage and collide with gasses resulting in ionization and energy transfer.• Excited ions or radicals relax to give UV photons.• UV photons cause hole-electron generation in phosphor and visible light emission.Weber, SID 00 Digest, p. 402.L13: 6.111 – Spring 20049Introductory Digital Systems LaboratoryDigital Mirror DeviceDigital Mirror DeviceApplied voltage deflects Mirror and hence direct lightCourtesy of Texas InstrumentsL13: 6.111 – Spring 200410Introductory Digital Systems LaboratoryLiquid Crystal DisplaysLiquid Crystal DisplaysLiquid Crystals rotate the plane of polarization of light when a voltage is applied across the cellCourtesy of Silicon GraphicsL13: 6.111 – Spring 200411Introductory Digital Systems LaboratoryRaster ScanRaster Scan Television and most computer displays use raster scan.This is like television: 60 halfNon−Interlaced: Frame rate mayframes/sec. Scan lineRetrace lineElectron beam "scans" tube. Beam location is shown here. Beam currentdetermines brightness of display.be 60, 72, etc. frames/sec.Interlaced: Frames alternate.L13: 6.111 – Spring 200412Introductory Digital Systems LaboratoryComposite FramesComposite Frames The ‘frame’ is a single picture (snapshot). It is made up of many lines. Each frame has a synchronizing pulse (vertical sync). Each line has a synchronizing pulse (horizontal sync). Brightness is represented by a positive voltage. Horizontal and Vertical intervals both have blanking so that retraces are not seen (invisible).Sync u 51.8 secuActive video:63.6 sec1/60 secVertical Sync andRetrace BlankingAnalog Video SignalHoriz. Sync PulsesComposite FrameHorizontal LineBlankingL13: 6.111 – Spring 200413Introductory Digital Systems LaboratoryHorizontal SynchronizationHorizontal Synchronization The picture consists of white dots on a black screen. White is the highest voltage. Black is a low voltage. Sync is below the black voltage. Sync pulses are surrounded by the blanking intervalso one doesn’t see the retrace.~~~~~~u~~uu~~White 3 VBlack 0.8 VSync 0 VDigital VideoSignal:BlankingSync Pulse 2 sec 4.9 sec11.8 secHorizontal Sync:"Front Porch"~"Back Porch"~L13: 6.111 – Spring 200414Introductory Digital Systems LaboratoryComposite SynchronizationComposite Synchronization Horizontal sync coordinates lines. Vertical sync coordinates frames. They are similar except for the
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