colour tv systems

8/12/2019 Colour TV Systems

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54 EC 702: Television Engineering Page 1

UNIT3: COLOURTELEVISIONSYSTEMS

L20 COLOURCHARACTERISTICS

Acolour

picture

is

amonochrome

picture

on

awhite

raster

with

colours

added

for

the

main

part

of

the

scene.

Advantagesofcolourinapicture:

Enhancesthecontrastbetweenthedifferentpictureelements; Addsanewdimension; Moredetailsappeartobeavailableincolourimages.

IncolourTVitisnecessarytopickupandreproduceextrainformationaboutcoloursinascenewhich

wasmadepossibleaftertheinversionandfabricationofthethreecolourshadowmasktubebyNTSC.

Compatibility:

Compatibilityimplies:

ThecolourTVsignalmustdevelopanormalblack&whitepictureonamonochromereceiverwithoutanychangeofthereceivercircuit.

Reversecompatibilityimplies:

AcolourreceivermustbeabletodevelopaB&Wscenefromanormalmonochromesignal.Requirementsforaccomplishingcompatibility

ColoursignalmusthavethesameBWasthemonochromesignal.Colour

signal

must

have

the

same

brightness

information

as

that

of

the

monochrome

signal.

Locationandspacingofpictureandsoundcarrierfrequenciesshouldremainthesame. Coloursystemmustusethesamedeflectionfrequenciesandsyncsignalasemployedformonochrometransmissionandreception.

Compositecoloursignalshouldconsistofcolourinformationtogetherwiththeauxiliarysignalsrequiredtopermitthistobedecoded.

Colourinformationshouldnotaffectthereproductionofmonochromepicture.Theabovementionedconditionwillbemetifthecolourinformationofthescenewillbeencodedand

sentwithinthesamechannelBWof7MHzandwithoutchangingthebrightnesssignal,andatthe

receivingendadecodermustbeemployedtoreconstructthecoloursignalbythreecolourtube.

Colourfundamentals:

Lightisaformofelectromagneticenergyconsistingofaspectrumofwavelengthsrangingfrom

7000to4000lightvisibletotheEYE.Eachwavelengthisperceivedbytheeyeasacertaintintorhuethroughtheinfinitelylargenumberof

wavelengthsinthespectrumproducesagradationofcontinuoushuesfromVIOLETtoRED.


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Eachdistincthueiscalledaspectralcolourthatisperceivedbytheeyeasthepuresthighlysaturated

colour.

Nonesaturatedcoloursormixedcoloursmayconsistof:

Afairlywidecontinuousgroupofwavelengthsgivingimpressionoftemperedcolours. Awholespectrumofwavelengthswithonlyagroupofwavelengthsaccentuated,givingtheimpressionofasoftdrabcolour;

Twoormoreindependentcontinuousgroupsofwavelengths,givingtheimpressionofasingletemperedcolourinthespectrumoranewcolournotpresentinthespectrum.

Thethirdvarietyofcolourmixingformsthebasisof:

i. Paint, ii. Colourprinting, iii. ColourtelevisionThecolourofanobjectisafunctionoftheunabsorbedwavelengthoflightreflectedfromit.

Opaqueobjects: Reflectedcoloursdecideitscolour. Transparentobjects: transmittedcoloursdecideitscolour.

Mixingof

Colours

and

Colour

Perception

Three

Colour

Theory

Mixingofcolourcantakeplaceintwoways;

Subtractivemixing;Reflectivepropertiesofpigmentsareused.

Thesepigmentsabsorballwavelengthsexceptfortheircharacteristiccolourwavelength.

o Takesplaceinpaintingorcolourprinting. Additivemixing;Lightfromtwoormorecolourfromindependentsourcescancreateacombinedsensationofa

differentcolour.

Thiscanhappenwhentinylightemittingdotsareveryclosetoeachotheranddifferentcoloursfallonthesameplaceonawhitescreenwhichcanreflectallofthemtogether.

TheadditivemixtureofthreeprimarycoloursRED,GREENandBLUEinadjustableintensitiescancreateanycolour.

Bypairwiseadditivemixing,complementarycolourscanbeproduced:o RED+GREEN=YELLOW,o RED+BLUE=MAGENTA,o BLUE+GREEN=CYAN


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GrassmansLaw:

Theeyeperceivesnewcoloursdependingon

thealgebraicsumofR,G,Blightfluxes.

100%(W)=30%(R)+59%(G)+11%(B)

Hence Y=30%(R)+59%(G)

C=59%(G)+11%(B)

100%(W)=89%(Y)+11%(B)=70%(C)+30%(R)

TristimulusValuesofSpectralColours

Tristimulusvaluesofthedifferentspectral

coloursarethestandardizedcomponentvalues

ofthethreeprimarycolours(R,G,B)thatcan

be

mixed

to

generate

various

other

colours.

ThereferenceWHITEforcolourTVhasbeenselectedtobeamixtureof:

30%(R)+59%(G)+11%

(B) ThepercentagesforLIGHTFLUXESarebasedonthesensitivityofthehuman

eye.

Characteristicof

Colour

Picture

Anycolourpicturehasthreeimportcharacteristicstospecifyitsvisualinformation.

i. Brightness/Luminance Theamountoflightintensityasperceivedbythehumaneyeregardlessofthecolour.

o InB&Wpictures,morelightedpartshavemoreluminancethandarkareas.o Differentcolourshavedifferentshadesofluminancethoughequallyilluminated.

ii. Hue/Tint Thepredominantspectralcolourofthereceivedlight.

o Thecolourofanobjectisdifferentiatedbyitshue/tint Saturation

Thespectralpurityofthecolour.o Anindicationofhowlittlethecolourisdilutedbywhite.

Hue+Saturation=Chrominance(Chroma)

Chromadoesnotcontainthebrightnessinformation.


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L21: COLOURTVCAMERAandCOLOURSIGNALGENERATION

ColourTVCamera

AColourTVCameraconsistsofthreecameratubes,eachreceivingselectivelyfilteredprimarycolours

andgenerating

asignal

voltage

proportional

to

the

respective

colour

intensity

received.

Lightfromthepictureformsanimagebytheobjectivelens.Theimageissplitintothreeimagesby

dichroicmirrors,namelyred,blueandgreen.

Adichroicmirrorallowsonewavelengthandrejectsotherwavelengths.Raysfromeachofthesplitted

lightpassesthroughcolourfilter(Trimmingfilters)whichgivepreciseprimarycolourimages,are

convertedtovideosignalbyimageorthiconorvidiconcameratubes.Hencethethreecoloursignalsare

generated.

Simultaneous

scanning

of

the

camera

tube

is

done

by

the

deflection

oscillator

and

sync

generator

which

drivesallthethreetubes.

Tofulfillcompatibilityrequirements,theluminancesignalwhichrepresentsbrightnessofelementsbeingscannedissendalongwiththecoloursignals.

ColourPicturetube(TheTRINITRONthreeinlinecathode)

TheTrinitroncolourpicturetubeconsistsofasinglegunhavingthreeinlinecathodes.


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Thethreebeamsarebentbyanelectrostaticlenssystemandemergefromthesamepointinthelens

assembly.Asthebeamshaveacommonfocusplane,asharperimageisformedwithgoodfocusover

theentirepictureareatosolveconvergenceproblems.

ColourSignalGeneration

ThethreeprimarysignalsproducedbythethreecameratubesrepresenttheproportionsofRED,

GREEN,andBLUElightcomponentsofthePIXELbeingscanned.

ColourVoltageAmplitudes

Thecameratubeoutputsarenormalized.Their

amplitudesaremadeequalbyadjustinggainsof

amplifiersassociatedwiththethreecamera

outputswithwhitelightincidentonthecamera

lens.

VR=VB=VG=1V(100%whitelight)

Hindicatesonehorizontallineperiodof

scanning.Atanyoneinstant,onlyonecamera

tubedeliversoutputdependingontheprimary

colourbeingscanned.

Cameraoutputsforcomplimentarycolour.

Theoutputofyellow,sinceitincludesredand

green,

VR

and

VG

are

produced

simultaneously.

Theoutputofwhite,allthethreeoutputsisproducedtogether.

Theoutputvoltagecanvaryfrommaximumi.e.1V

for100%brightnessto0Vfordarknessas

illuminationvariesfrommaximumbrightto

absenceofanylight.

DesaturatedColours

Acolourisdesaturatedwhenwhiteismixedwithit.Thepercentageofdesaturationdependsonhow

muchwhiteismixedwiththecolour.

SincewhiteisamixtureofR,GandB,allthecamerapickuptubeswilldevelopoutputsforany

desaturatedcolourbutthemagnitudesofVR,VBandVGwillnotbethesame.

N.B.100%desaturation=white


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LuminanceSignal(Y)

Inacolourcamera,theluminancesignal(Y)isformedbyadding ; inthefollowingproportions; 0.30 0.59 0.11; droppingV; 0.30 0.59 0.11

YSignalmatrix

IntheYsignalmatrixforobtainingY,thevalues , , , aresochosenthatthethreecolourvoltagesaddsupintheratiosof0.30: 0.59: 0.11togiveY=1.WhenR=G=G=1VBrightwhite: 0.301 0.591 0.111 1 WhenR=G=G=0VDarkness: 0.300 0.590 0.110 0 ThusthemagnitudeofYvariesfrom100%(1V)to0%(0V)aslightintensitychangesfrommaximum

brighttodarkness.

Atreducedlightlevels,theamplitudeofYdecreasesaccordingly.

e.g.

at

50%

brightness

i.e.

R

=

G

=

G

=

0.5V;

0.300.5 0.590.5 0.110.5 0.5 .

AmonochromecamerawillalsodevelopthesamemagnitudeofYundertheseilluminationlevels.


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L22: GENERATIONOFCOLOURDIFFERENCEVOLTAGES

ThecolourdifferencesignalsaregeneratedtoavoidtransmissionofR,G,andBseparately.

Thethreepossiblecolourdifferencesignalsare(RY),(GY)and(BY).SinceYcontainsR,GandB,itisnotnecessarytotransmitallthethreecolourdifferencesignalsbecause

anycolourcanberecoveredfromtheremainingtwo.

Productionofcolourdifferencesignals

Resistancematrixforobtaining

Yandcircuitblocksfor

deliveringcolourdifference

signals.

TheYsignalisamplifiedtheninvertedtoobtainYandpassedontotheADDERcircuits.

OneADDERcircuitcombinesREDcameraoutputwithYtoproduce(RY); 0.30 0.59 0.11 0.70 0.59 0.11

ThesecondADDERcombinesBLUEcameraoutputwithYtoproduce(BY) 0.30 0.59 0.89

Reasonswhy(GY)isnotchosenfortransmission

TheproportionofGinYislarge(59%),hencetheamplitudeof(GY)isverysmallincomparisonwith(RY)and(BY).

o Thiscauses tobesmallatthetransmittingend. (GY)beingsmall,itisnecessarytouseamplifiersinthematrixnetworktorecover(RY)or(BY)atthereceivingend.


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Polarityofcolourdifferencesignals

(RY)and(BY)caneitherbepositiveor

negativedependingonthehuethey

represent.

Compatibilitywithcolourdifferencesignals

Compatibilitybetweenmonochromeandcoloursystemsismaintainedbytheuseofcolourdifference

signals.

Proofs

i. Foranypictureelementhavingnocolour,themagnitudeofR=B=G. LetR=B=G=1V;then 0.30 0.59 0.11 1

0.301 0.591 0.111 1 Hence(RY)=11=0V;(BY)=11=0V.

ThecolourdifferencesignalsdisappearforwhiteinthepictureandtheonlysignalthatgetstransmittedisYsignal,hencethereisnointerferencefromcolourdifference

signalsinthereproductionofB&Wpartsofthepicture.

ii. Thecolourdifferencesignalsaresmallinmagnitudeascomparedtobothcolour(R,B,G)andluminance(Y)signals.

Thismakestheirtransmissionandlaterprocessingatthereceivermucheasier.iii. ThecolourdifferencesignalsdonotneedalargebandwidthasnecessaryfortheYsignal.

ThissimplifiesthedesignofCHROMAsectioninthereceiver.Valuesofluminance(Y)andcolourdifferencesignalsoncolour

WhentelevisingasceneevenwhenthepotentialsR,G,andBarenotthesame,theYsignalstilldenotes

monochromeequivalentofthecoloursincetheproportions0.30; 0.59; 0.11stillpresentthecontributionwhichRED,GREENandBLUElightsmaketothebrightness.

Considerdesaturatedpurple:DesaturatedpurpleisashadeofmagentawhichismixtureofREDandBLUE. Sincethecolouris

desaturated,itmeansthatsomewhitelightisalsopresent.


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ThewhitelightcontentwillgiveallthethreevoltagesR,GandB,themagnitudeofwhichwilldependon

theintensityofdesaturationofthecolour.

Therefore,RandBpotentialswilldominateandbothmustbegreaterthanthemagnitudeofG.o Example: letR=0.7Volts;G=0.2VoltsandB=0.6Volts

Whiteis

due

to

0.2R,

0.2G

and

0.2B

Themagentahueisdueto0.5Rand0.4B.

Luminance(Y)signal; 0.300.7 0.590.2 0.110.6 0.394 Thecolourdifferencesignal; 0.7 0.394 0.306

0.6 0.394 0.206 Themissingcolourdifferencesignalthatisnottransmitted(GY)canbereconstructedbyusinga

suitablematrix; .. .. 0.194


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L23: COLOURTELEVISIONSYSTEMS

Thethreedifferentcoloursystemsare:

NationalTelevisionSystemCommittee (NTSC) AmericanTelevisionSystem, PhaseAlterationbyLine (PAL) GermanTelevisionSystem, SequentialCouleuresAMemoire (SECAM) FrenchTelevisionSystem.

Considering: Quality of the reproduced pictureCost of the equipment It is difficult to establish the superiorityof one system over the other

Thechoiceofwhichsystemistobeadoptedinothercountrieshasbeenmostlyinfluencedbythe

monochromesystemalreadyinuseinthatparticularcountry.

NTSCColourTelevisionSystem

TheNTSCsystemiscompatiblewiththeAmerican525linemonochromesystem.Ithastwonewcolour

signals.

Q Signal I Signal

LocatedalongREDDISHBLUEandYELLOWISH

GREENaxis(at330w.r.t.colourburstcircuits)

LocatedalongYELLOWISHREDandGREENISH

BLUEaxis(330counterclockwiseto+(RY).

Itisobtainedfromcolourdifferencesignals(RY)

and(BY)byasuitablematrix

Q=0.48(RY)+0.41(BY)Q=0.21R0.52G+0.31B

Itisobtainedfromcolourdifferencesignals(RY)

and(BY)byasuitablematrix

I=0.74(RY)0.27(BY)I=0.60R0.28G0.32B

Bandwidthreduction

TheQsignalistransmittedusingdoublesidebandandoccupiesachannelbandwidthof1MHz

0.5

TheIsignalistransmittedbyvestigialsideband: 2 0.5 1.5

Hence1MHzisservedcomparedtothebandwidthrequirementsof3MHzif(RY)and(BY)weretobe

sentdirectly.


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Thisreductionincoloursignalsidebandshelpstohaveanarrowchannelbandwidthof6MHz.Thebandwidthreductionresultsin

lessinterferencewiththesoundand

picturesignalsidebands.

ExactColourSubcarrierFrequency

Thecoloursubcarrierfrequencyisselectedtohaveanexactvalueof 3.579545 inordertomaintainthecompatibilitywithcolourandmonochromesystems.

Anyinterferencebetweenchrominancesignalandhighervideofrequenciesisreducedbyemploying:

o Suppressedcarriertransmitters,o Anotchfilterinthepathoftheluminancesignal.

Thesoundcarrierandcoloursubcarrierbeatwitheachotherinthedetectorabeatnoteof0.92MHz(4.53.58=0.92MHz)isdeveloped.

o Thisinterferespictureproduction.Tocancelthiscumulativeeffect,thesubcarrierfrequencymustbeanexactmultipleofaneven

harmonicofthelinefrequency.

Hence4.5MHzischosentobethe286thharmonicofthehorizontaldeflectionfrequency.o . 15734.26 .Thisfrequencyisclosetothevalueof15750Hzusedforhorizontalscanninginmonochrometransmitter.

,.


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