PAL Encoder and Decoder

By Rajender KumarDy. Director(Engg.)

STI(T), AIR & Doordarshan

Color reproduction in Television System

• Colors are generated by additive mixing of primary colors, Red (R), Green (G), Blue (B).

• The mixing is done in the color picture tube where three R-G-B electron guns will hit the respective R-G-B phosphor dots on the screen, resulting in emission of red, green and blue light from the respective dots. When these colors of different intensities are emitted, human eyes would perceive a particular color

Additive Mixing of Primary Colors

Color separation using optical system

Color Generation in TV Picture Tube

Conversion of RGB signals into CCVS

• TV Transmission is carried out in color composite video signals (CCVS), instead of RGB component video.

• TV receiver responds only to CCVS signals. • PAL encoder convert RGB signal into CCVS

signal and PAL Decoder takes CCVS as input and gives RGB as output

• Therefore conversion from RGB to CCVS is required.

CCVS

PAL Encoder

R-Signal

B-Signal

G-Signal CCVS

Constrains for migration from monochrome to color Transmission

• Compatibility: Color transmission must respond to B/W receivers.

• Reverse Compatibility: Monochrome transmission must be received by color TV.

• Bandwidth:Color composite video signal must utilize the same bandwidth of 5 MHz as utilized by monochrome composite signal.

How to maintain compatibility

• Three RGB signals are converted into Luminance Y, and color difference signals using simple mathematical relation

• Y=0.3R+0.59G+0.11B, Luminance Signal• U=0.493(B-Y) Color Difference Signals • V=0.877(R-Y)• (B-Y) and (R-Y) are weighted to avoid

over modulation

Generation of Luminance and color difference signals

(R’ – Y’)adder

γ

Cameraoutputs

Simple matrix

R1

R2

R3

R4

G G’

γ R R’

γ B B’

Inv.amp.

-Y’(R’ – Y’)

(B’ – Y’)adder

-Y’(B’ – Y’)

Amp.

Gamma correction R1 = 0.59, R2 = 0.3, R3 = 0.11

Luminance signal Y’ = 0.3R’ + 0.59G’ + 0.11B’

How to retain 5MHz BW

• The color difference signals, U and V are accommodated in the luminance Y signal using frequency interleaving.

• The frequency interleaving is achieved by modulating U and V using QAM at color sub-carrier frequency which is given by

vh

hSC ffff −−=4

284

Energy Spectrum of color video signal

Quadrature Amplitude Modulation

• QAM is a type of modulation scheme where two information, U and V can be conveyed on a single carrier, means saving BW.

• QAM signal or Chrominance signal C :

)sin()cos( tVtUC cc ωω ∗+∗=

UV

VUC

tCC c

1

22

tan

)cos(

−=

+=

+=

θ

θωU-U

-V

V C

θ

QAM Modulator

UBalanced modulator

Sub carrier oscillator

VBalanced modulator

Adder

U

)sin()cos( tVtUC cc ωω ∗+∗=

)cos( tcU ω∗

)sin( tV cω∗

C

o90+

)cos( tcω

)sin( tcω

V

Phase Angle for yellow ColorR = 1.0 , G = 1.0, B = 0.0Y= 0.30+0.59 = 0.89U = 0.493(B-Y) = 0.493(-0.89) = -0.4387, V = 0.877( R-Y) = 0.877(0.11) = 0.0965,

22 VUC += ( ) ( )22 0965.04387.0 +−= = 0.44

oo 1674387.00965.0tan180 1 =−= −θ

Chrominance signal phasor positions for primary and complimentary colors

Advantage of PAL over NTSC

Phase Alteration by Line : Changing phase of the sub-carrier by 180 degree at each alternating line to minimize the phase error. The phase error causes error in color reproduction. Correction of colors is done in the Human Visual System.Color correction is not done in NTSC system

Phase error correction in PAL system

θ - ∆ θ- U

Received vector with NTSC line

U

V

- V

θ - ∆ θθ

θ + ∆ θTransmitted vector

At angle θ

Received vector with PAL line

PAL Encoder

Luminance Delay(400-500ns)

Matrix G’

R’

B’

Burst gate

Y’

U

V U Modulator

fsc ∠90°

V Modulator

fsc ∠0° fsc ∠180°

Phase shift

U Carrier

V CarrierColor Sub-carrier 4.43 MHz

Summing Amplifier

+

-

+++

Summing Amplifier

4.43 MHz BPF ∆f = 2.6 MHz fc = 4.43 MHz

Out

put

Com

posi

te

PA

L

0° / 180° Phase switch

Composite syncs

Ident PAL switch 1/2fh

PAL Decoder

THANK YOU