Ntsc Colour Tv System

7/26/2019 Ntsc Colour Tv System

1/50

ESSENTIALS OF COLORTRANSMISSION AND NTSC SYSTEM


2/50

It should produce a normal black

and h!te p!cture on a monochromerece!"er !thout an# mod!$cat!on o%rece!"er c!rcu!tr#&

A color rece!"er must be able toproduce a black and h!te p!cture%rom monochrome s!'nal&

Compatibility requirements of Color system.


3/50

It should occup# same band!dth as themonochrome s!'nal&

Locat!on and spac!n' o% p!cture and soundcarr!ers must be the same&

Same lum!nance !n%ormat!on as a monochromes!'nal transm!tt!n' the same scene&

Color !n%ormat!on should be carr!ed such that !tdoes not !nter%ere !th p!cture on the screen o%

a monochrome rece!"er&Same de(ect!on %re)uenc!es and s#nc s!'nals as

!n monochrome s#stem&

Requirements of colorsignal


4/50

E*pla!ned b# +elm+olt, theor# To t#pes o% l!'ht sens!t!"e or'ans- rods and cones Rods pro"!de br!'htness sensat!on and cones are

sens!t!"e to color Three sets o% cones !th each sens!t!"e to red- blue

and 'reen l!'hts respect!"el#& Thus each set o% cones detects the amount o% R./

components !n l!'ht& The opt!c ner"e s#stem !nte'rates d!0erent color!mpress!ons as per the response cur"e o% e#e toperce!"e the actual color&

How Color is Perceived by Eyes?


5/50

Response curve of eyes


6/50

Add!t!"e m!*!n' %orms the bas!s o% color tele"!s!on +ere l!'ht %rom to or more colors comb!ne

to'ether to '!"e sensat!on o% a ne color&The add!t!"e m!*!n' o% pr!mar# colors1 red- blue

and 'reen can create most o% the colors&

Additive Color Mixing &Grassmans law


7/50

GRASSMANS LAW

Eye is not able to distinguish each of thecolors that mix to form the new color

Eye beha!es as if the out"ut of di#erentset of cones are additi!e

$rightness im"ression created bycombined light source is e%ual to thenumerical sum of & "rimary colors thatconstitute that color

'he "ro"erty of eye "roducing a res"onsebased on algebraic sum of red( blue andgreen in"uts is called Grassmans law


8/50

/ased on spectral response cur"e and .rassman2s

la - the tr!st!mulus "alues ha"e been de"eloped&

Tr!st!mulus "alues are the component "alues o%three pr!mar# colors to produce "ar!ous othercolors&

Thus the re%erence h!te %or T3 has been chosen to

be a m!*ture o% 456 red- 786 'reen and 996 blue&There%ore 9 lumen o% h!te:5&4 lm o% red;5&78lmo% 'reen; 5&99 lm o% red&

Alternat!"el# 9lm o% h!te :5&


9/50

Luminance or Brigtness It !s the amount o% br!'htness asperce!"ed b# e#e re'ardless o% color& /etter l!'hted parts ha"e more lum!nance than dark areas&

Hue It !s the predom!nant spectral color o% rece!"ed l!'ht& D!0erent hues result %rom d!0erent a"elen'ths o% spectral rad!at!on&

Saturation It !s the spectral pur!t# o% color&

Ind!cat!on o% amount o% other colors present& Or can be de$ned as ho much the color !s d!luted b# h!te& A %ull# saturated color has no h!te&

Caracteristics of Color


10/50

+ue and saturat!on o% a color put to'ether !scalled chrom!nance&

Also called chroma&

A color s#stem has to handle the chroma!n%ormat!on apart %rom lum!nance&

)hrominanance


11/50

)olor 'ele!ision )amera


12/50

)olor !oltage am"litudes


13/50


14/50

* signal am"litudes


15/50

In color s#stems the Y s!'nal !s transm!tted as !t!s&

/ut !nstead o% transm!tt!n' all three colors!'nals separatel# -onl# color d!0erence s!'nals

are transm!tted& Color d!0erence s!'nals are obta!ned b#

subtract!n'

lum!nance "olta'e %rom color "olta'e& Onl# R1Y and /1Y s!'nals are sent and the color

s!'nals are der!"ed %rom these at the rece!"er&

)olor +i#erence !oltages


16/50


17/50

>e ha"e Y:5&4R; 5&78.; &99/ !e- ?5&4; 5&78 ;5&99@Y:5&4R ;5&78. ;5&99/ Rearran'!n' -

!y "#$ signal not transmitted?


18/50

/ut !% .1Y !s transm!tted-

9& I% R1Y !s the m!ss!n' s!'nal- then

R1Y:?15&785&4@ .1Y ; ?1&995&4@ /1Y

Th!s !mpl!es a 'a!n %actor !n the res!stor matr!*&

B& I% /1Y !s not sent then

/1Y: ?15&785&[email protected] ?O&45&99@R1Y


19/50

Moreo"er- proport!on s!nce proport!on o% .

!s lar'e !n man# s!'nals then- "alue o% .1Y!ll be small&

It !s e!ther the smallest o% the s!'nals or

almost e)ual to smallest o% other to

The smaller ampl!tude o% .1Y !ll also!ntroduce s!'nal to no!se rat!o problems !%selected %or transm!ss!on&


20/50

COLOR !" S#S!$MS

The bas!c color t" s#stems are

9@ The Amer!can NTSC ? Nat!onal Tele"!s!onS#stems Comm!ttee@ s#stem

B@ The .erman AL ? hase Alterat!on b#L!ne@

4@ The French SECAM?Se)uent!al Couleuresa memo!re@ s#stem&


21/50

In color t" s#stems- the problem o% con$n!n'color s!'nals !th!n the same band!dth o%monochrome s!'nal !s ach!e"ed us!n' Fre)uenc#Interlea"!n'&

Fre)uenc# Interlea"!n'

It has been %ound that the ener'# content o%

compos!te "!deo s!'nal occur as bundles h!choccur at harmon!cs o% l!ne %re)uenc#? 97&B7-49&B75G&&Hh,@

Shape o% ener'# bundle shos a peak at e*actharmon!cs o% l!ne %re)uenc#


22/50

Compos!te 3!deo s!'nal

Ener'#


23/50

From the $'& e can see that ener'# contentdecreases s!'n!$cantl# a%ter !ncrease !nharmon!cs

and !s much smaller a%ter 4&7 Mh,&

Also a part o% monochrome s!'nal band!dth 'etunused due to spac!n' beteen the bundles&

It !s here the color !n%ormat!on !s packed b#modulat!n' color d!0erence "olta'es us!n' hat!s knon as a color sub carr!er&

Th!s %re)uenc# !s so chosen that !ts s!deband %alle*actl# m!da# beteen harmon!cs o% l!ne%re)uenc#&


24/50


25/50

Thus the %re)uenc# o% subcarr!er must be anodd mult!ple o% hal% the l!ne %re)uenc#&

In order to a"o!d crosstalk !th p!cture !n%o-!t !s selected at the h!'her s!de o% channelband!dth&

In NTSC s#stem the subcarr!er !s 77 t!mesone hal% the l!ne %re)uenc# !e-

?BJBB= ;9@J?97=75B@ : 4&7< Mh,


26/50

The problem o% transm!tt!n' to colord!0erence "olta'es !th same carr!er !ssol"ed us!n' %uadrature &odulation.

+ere to separate modulators are used-one %or the /1Y and other %or R1Y& /ut thecarr!er %re) %ed to one modulator !s '!"en arelat!"e phase sh!%t o% 85 !th respect to

another&Thus the to s!'nals are sa!d to be !n)uadrature and the to outputs arecomb!ned to #!eld C- the subcarr!er phasor&

&odulation of Color 'i(ererencevoltages


27/50

The !nstantaneous ampl!tude o% C !e-chrom!nance s!'nal- '!"es the color

saturat!on at that !nstant& Ma*!mum amp& Impl!es 'reatest saturat!onand ,ero to no saturat!on !e- h!te&

The !nstantaneous phase an'le o% C "ar!es

%rom 5 to 45 and '!"es !n%o on the hue o%the color&

Thus phasor C conta!ns %ull !n%o on hue andsaturat!on&

% d M d l i


28/50

%uadrature Modulation


29/50

Suppressed carr!er !th double s!debands !sthe normal method %or modulat!n' color d!0"olta'es !th color sub carr!er&

In order to ensure proper demodulat!on atthe rece!"er - a short sample o% subcarr!erosc!llator- ?ha"!n'


30/50

Location of Color Burstsignal


31/50

The chrom!nance s!'nal phasor- C !s addedto lum!nance s!'nal- Y be%ore modulat!n' !t!th channel carr!er&

The ampl!tude o% Y s!'nal becomes thele"el l!ne %or th!s purpose&

/ut d!rectl# us!n' th!s "alues !ll lead to'ross o"ermodulat!on& To a"o!d th!s

e!'ht!n' %actorsare used&

Weighting factors and why are theyre%uired,


32/50

For e'& The peak to peak ampl!tudes o%'reen s!'nal?;1 5&


33/50


34/50

There%ore to a"o!d o"ermodulat!on- the"alues o% color d!0erence "olta'e s!'nal arereduced be%ore the# are modulated us!n'color sub carr!er&

Th!s !s ach!e"ed b# scal!n' don the color

s!'nal "olta'es b# mult!pl#!n' !th "aluesknon as the K>e!'ht!n' Factors2 The e!'ht!n' %actors are 0.877%or R1Y

component and 5&84 %or /1Y component&

Then $nall# %or proper recept!on theuncompensated "alues are obta!ned at therece!"er s!de us!n' su!table netork&


35/50


36/50


37/50

T#p!cal "alues o% ampl!tudes o% color subcarr!ers are 5&4 %or red and c#an- 5&78 %or'reen and ma'enta and 5& %or blue and

#ello&Thou'h these ampl!tudes !ll st!ll cause

o"er modulat!on b# about 446 !t !sacceptable& Th!s !s because saturat!on o%

hues !n natural and sta'ed scenes seldome*ceed =76&

)ormation of Crominance signal


38/50

Due to e!'ht!n' %actors the phase an'leso% color c!rcle chan'es to '!"e ne "alues

Pasor diagram of Croma signal


39/50

Th!s s#stem !s compat!ble !th the 7B7 l!neAmer!can monochrome s#stem&

+ere band!dth %or color transm!ss!on !sreduced b# us!n' propert!es o% human e#e&

+ere ad"anta'e !s taken o% the l!m!tat!ons o%human e#e to restr!ct color s!'nal band!dth&

E#e2s resolut!on o% color2s o the redd!sh blue#ello!sh 'reen a*!s !s much less than colors

around #ello!sh red 1'reen!sh blue a*!s &There%ore to ne color s!'nals I and are

'enerated&

*+,C ,$,+E&


40/50

I signal lies in a region 33 degree counter

clockwise to R-Y where eye has max resolution.

I=0.7!R-Y" # 0.$7!%-Y"

&hich gi'es I as

I=0.(0 R- 0.$8)-0.3$%

I signal is located at *7 degree with res+ect to the,urst signal.


41/50

imilarly / signal is deri'ed as

/=0.8!R-Y"0.1!%-Y"

=0.$1R-0.*$)0.31% It is located 33 degree counter clockwise to !%-Y"

and is in 2uadrature with I signal.


42/50

ince eye is ca+a,le o resol'ing ine details ocolors in the I region it is gi'en a ,andwidth o u+

to 1.* 4h5.6s eye is less sensiti'e to colors in the / region it

is gi'en a ,andwidth o only -0.* 4h5 withres+ect to color su,carrier. ou,le side,and transmission is allowed or /

signal and it occu+ies a ,andwidth o 14h5!-0.*"

%ut or I signal the % is restricted to 0.*4h5,ut 9% is allowed to ha'e re2uencies u+ to

1.*4h5.


43/50

:hereore o'erall a ,andwidth o $ 4h5 isre2uired or color signal transmission.

:his im+lies a sa'ing o 1 4h5 i color di 'oltages

were directly sent. :his reduced ,andwidth results in reduced

intererence with +icture and sound carriers.

C i C l S '


44/50

:he exact 'alue o color su, carrier in ;:$($.*=*?.? @5 instead o (0@5.

:hus A=!$$$71"1*73.$(>$=3.*7?** 4h5. It isderi'ed rom a crystal controlled oscillator.

:he small changes n line re2 a,out 1*.7@5 and ield

re2 0.0( @5 has +ractically no eect on delectionoscillators as an oscillator which can ,e triggered ,y (0

@5 +ulses can also ,e synchronised or *?.? @5

6lso 6A< circuit can easily adBust the line re2


49/50


50/50

Ntsc Colour Tv System

Documents

Transcript of Ntsc Colour Tv System