(2) Pal Encoder And Decoder 662p2n

PAL Encoder and Decoder By Rajender Kumar Dy. 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 RG-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 R-Signal

G -Signal

B-Signal

PAL Encoder

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 Gamma correction

R1 = 0.59, R2 = 0.3, R3 = 0.11 Luminance signal Y’ = 0.3R’ + 0.59G’ + 0.11B’

Simple matrix G

γ

G’

R1 R2

Amp.

R3 R4

Camera outputs

Inv. amp. -Y’

R

γ

R’

(R’ – Y’) adder

(R’ – Y’)

-Y’

B

γ

B’

(B’ – Y’) adder

(B’ – Y’)

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

f SC = 284 f h

fh − − fv 4

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 :

C = U ∗ cos(ωct ) + V ∗ sin(ωct ) C = C cos( ω c t + θ ) C =

U

θ = tan

−1

2

+V

V U

2

V C θ

-U

U -V

QAM Modulator U

U

Balanced modulator

U ∗ cos(ω t ) c

cos(ω t ) c Sub carrier oscillator

Adder

C

C = U ∗ cos( ω c t ) + V ∗ sin( ω c t )

+ 90 o sin(ω t ) c

V

V Balanced modulator

V ∗ sin(ωct )

Phase Angle for yellow Color R = 1.0 , G = 1.0, B = 0.0 Y= 0.30+0.59 = 0.89 U = 0.493(B-Y) = 0.493(-0.89) = -0.4387, V = 0.877( R-Y) = 0.877(0.11) = 0.0965, C = U +V 2

2

θ = 180 − tan o

= −1

(− 0.4387 )2 + (0 .0965 )2 0 . 0965 = 167 0 . 4387

o

= 0.44

Chrominance signal phasor positions for primary and complimentary colors

Advantage of PAL over NTSC Phase Alteration by Line : Changing phase of the subcarrier 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 Received vector with NTSC line

V

Transmitted vector At angle θ

θ+∆θ

Received vector with PAL line

θ

θ-∆θ

U

-U θ-∆θ

-V

PAL Encoder Y’

R’

Luminance Delay (400-500ns)

U V

B’ fsc ∠90°

Burst gate

V Modulator

U Carrier

Color Sub-carrier 4.43 MHz

fsc ∠0° fsc ∠180°

Phase shift V Carrier

Composite syncs 0° / 180° Phase switch Ident PAL switch 1/2fh

+ +

+ -

Summing Amplifier

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

Summing Amplifier

Composite PAL

U Modulator

Output

G’

Matrix

PAL Decoder

THANK YOU