Digital Video ECE 6354
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This 8 page Class Notes was uploaded by Karolann Wiegand on Saturday September 19, 2015. The Class Notes belongs to ECE 6354 at University of Houston taught by Thomas Hebert in Fall. Since its upload, it has received 85 views. For similar materials see /class/208287/ece-6354-university-of-houston in Electrical Engineering at University of Houston.
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Date Created: 09/19/15
Analog Television Two lectures of material Probably the single greatest human invention is television Although we may have some concerns about the content of what we see on TV the technology of TV has completely and irrevocably changed the human race As with audio recording television is designed to be perceived by biological components which may differ from person to person However the biological peculiarities of the human eye are much more exploited in television than in audio I The physiology of vision A Trireceptor theory of vision or why we use RGB monitors If you ask someone why red green and blue are used in computer monitors the immediate answer is quotBecause these are the primary colorsquot If you then ask quotBut why are these the primary colorsquot the answer you get is that quotIf you miX light of these colors together you can make any colorquot However if you then ask quotWhy is this soquot usually the answer is dead silence No doubt the person being asks the question thinks you are too weird for words However there are some very fascinating reasons why RGB are the primary colors and they have nothing to do with either light or Fourier series They lie in the nature of the human eye The major light sensing element in the human eye is the retinam I lighl l lighr my livmling L Lari uplic nerve bcr layer r ganglion onquot V 39l ganglionic ganglion cell 3 at call Iuynr s adugs iEnlrifugul ipolar cell inle pl1 cll lay 7 v amucrme cell E I Od bipolun nhipuml imuM nucleiquot cells 07039 horizon loi cell LEOHC body i with nucleus rod body inner sngmcnls nf ml nut mw outer Pgnenls of rod and cone run hi hlnr o 1quot quot t3 of Muller unlw lwifum lawn l qum native 4 I J r 5 we mm limilinq 3 39 39 Inemhr na rod and cone 2 P T pigmnnlvxj luynr l bragging The retina consists of a number of different and important layers of cells Notice however that from an engineering viewpoint the retina is constructed upside down The major light receptors the rods and cones are at the bottom of the retina not at the top Thus light must pass through nerves blood and tissue before reaching the main photoreceptors This means that some serious signal processing is required by the retina and the brain to edit out these structures in your visual perception There are two types of photoreceptors rods and cones Rods are responsible for low level light detection and are most sensitive in the bluegreen They are very light sensitive and motion sensitive but at the cost of resolution Rods are virtually missing in the fovea the center of the visual eld but are scattered elsewhere throughout the eye thus governing peripheral vision as well as nighttime vision At night the fovea is very insensitive and most of the visual information is being carried by rods in the periphery of your eye Cones carry the color information and provide higher resolution but at the cost of sensitivity Cones are concentrated in the fovea providing high resolution central daytime vision Rods contain a bluegreen pigment called rhodopsin Cones contain three pigments a bluesensitive pigmentm 447 nm a green sensitive pigment called chlorolabe 540 nm and a red sensitive pigment called erythrolabe 577 nm These three pigments are the pigments responsible for quotprimary colorsquot Individuals missing one or two of these pigments will have various forms of color blindness Notice that three pigments is by no means standard There is a species of shrimp that has 20 different photoreceptor pigments The graph below indicates the wavelength dependence of the three cone pigments in the human eyem 75 391 PERCENT OF MnxtMUM ABSORPTION U1 C 39 n I 7 e 25 O x an 40 50 50 so 39 am WAV El ENGTH NANDMETE H S The idea of trireceptor vision was worked out far before the physical mechanism of retinal pigments was understood A common diagram for describing human color perception was developed by the International Commission on Illumination CIE The CIE diagram is an attempt to precisely quantify the trireceptor nature of human vision olor pereeptrons were rneasured by glvmg subjects yanous eornbrnauons of the three standard ClE pnmary eolors y dlagram c 435 8 nrn5461 nrn and 700 nrn and measunng therr pereepuons These pereeptrons are plotted on an he ealledthe Chromatrertydagram The pure eolors he along the outer loeus of the dlagram andthe eenter of the dlagram CIE lllurnrnant C ls the CIE e Therefore m s d Thus any pornt on tlus dlagram ean be unlquely rndeated by an xy fund tal way only two basls funeuons are really requlre to desenbe any partreular ery rrnportantto us m eolor TV valu orne eolor or hue Thls faet wlll 1n the follow graph the CIE Chrornauerty dlagram ls overlald agarnst surface eolors of eornrnon palms and dyes the gr y blob as well as the pnmary eolor mangle ofthe Arnenean NTSC auonal Televlslon Systems Cornrnrttee l r tele lo systern and the European PAL Phase Alternatron Llne rate and SECAM Sequenual Couleur ayee vls n Memolre systemsj LT v mco39 05m my mm B Vlsual persrstenee Motron pretures onglnally set the frame rate at 16 frames per seeond Thls was rapldly found to be unaeeeptable and the frame rate was rnereasedto 24 frame per seeond In Europe tlus was ehangedto 25 frames per seeond as the Euro Tu V w m1 A F A 25 Hz m Europe the 4 dlfference does not seem to bother anyone When NTSC teleyrsron standards were rntro d th frame rate was set at 30 Hz 12 the 60 Hz llne frequency Then the rate wa ed to 29 97 Hz o mam am 4 5 MHz between the vlsual and audlo earners As we wlll see re tlus declslon has lead to some problems m developlng an HDTV standard Movles lmed at 24 frames per seeond are slmply eonyertedto 29 97 frames per seeond on teleyrsron broadeastrn Now there ls aglrteh For some reason the bnghter the strll lmage presentedto the vlewer the shorter the persrstenee H n man the penod of persrstenee of vlslon re then the lmage lckers Large bnght theater projectors avold thrs problem by plaerng rotaung shutters m front of the lmage m order to lncrease the repetrtron rate by a faetor of 2 to 48 or three to 72 wrthout ehangrng the aetu s Unfortunately there ls no easy way to put a shutter m front of a teleyrsron broadeastl Therefore to arrange for two ashes per frame the fl shes are ereated by rnterlaerng The basl ldea here ls that a slngle frame ls seannedtwree The frrst sean lncludes only the odd llnes the next sean lncludes only the even llnes Wrth thrs method the number of ashes per frame ls two and the freldrate ls double the frame rate Thu Although rnterlaerng sounds llke a great ldea re a number of aberratrons appear due to the faet that you really do not have aframe rate of 5060 Hz For example yertreally adjacent preture elements do not appear at the same trme lfthe tlungs as mrsalrgnment where the honzontal edges of one sean do not mateh wth the next and rnterlrne lcker The other situation that must be considered is rapid motion If the still frame images are presented at too low a rate rapid motion becomes jerky and odd looking This is especially a problem in action movies where high speed chase scenes are common However as of yet there has been little interest in converting movie projectors to either 2997 or 30 Hz due to the large investments in such equipment 11 Basic black and white television In a basic black and white TV a single electron beam is used to scan a phosphor screen The scan is interlaced that is it scans twice per photographed frame The information is always displayed from left to right After each line is written when the beam returns back to the left the signal is blanked remember the standard was invented before digital video When the signal reached the bottom it is blanked until it returns to the top to write the next lineIQJ C C A f I 2 5 H a 4 5 a 7 Va 6 I a f x Jquot B Odd lines Inactive lines 539 Even lines D Inactive lines in 1st vertical trace ll39l ist vertical retrace in 2d verlical trace in 2d veriical relrace L L i 15 eld A 262 llnes 2d fied 252 lines Frame 525 lines Conventional NTSC has 525 vertical lines However lines number 248 to 263 and 511 to 525 are typically blanked to provide time for the beam to return to the upper left hand comer for the next scan Notice that the beam does not return directly to the top but zigzags a bitl Hnnzumal vanes e Hnnzcmal remces Vumca Hyhnck ume Ilt 7 61quot held 4 a m 2 u w m Deueelmg 7 D Hllaces uana 44 euaees nmpmude pivme 4 mm r v mlracu The vemcal scanning signal for eonvenuonal black and whxte NTSC 15 qune strmghtforward It 15 slmply aposmve scan lmes 24am 5112525 2247 2547mm meme menB s w h e m quotm h e m 52529 97 15734 Hz Therefore 63 6 Typically about L w There are 427 pixels per horizontal scan 1mm and so each pixel 15 scanned for approximately 125 us The 427 pixel number comes from takmg IAMTT umw u n r n u Vlw ln eleetlon beam and thus gray scale levels on the pletule seleen Blanking signals Sync PUISES Tne eolnblnatlon nonzontal blanklng slgnal and synenlonlzatlon pulse ls qulte well de ned Fol black and wnlte TV the front poleln ls 0 02 tunes the alstanee between pulses and the back poleln ls 0 no tunes the alstanee between pulses 9 F H 4 008H l Honzona Hm I 1 sync pulse Front porch 006 H U 75 0 02 H Back porch cuquot g 50 Picture 015 H fl Information Hor zomal E blanklng lt 25 pulse I Tlme For l r l n l l n l eslleoltlne l l backporch oftllellonzontal blanklng slgnal Not only oloes tnls set the phase ofthe eolol slgnal mole on tnls latel lt also provldes a really easy way to tell lfyou have a blackrandrwhlte slgnal no eololbulst on a eolol slgnal wltn a 8710 eyele eolol burst l
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