Television - the art of image transmission and reception

CAMERA

TELEVISION

Early days, camera adopted line scanning

Vidicon -

line scanning

TELEVISION

Electronic signals in PAL, NTSC standards

No scanning required now, standard remains the same

CCD - solid state image recording

TELEVISION

Electronic signals in PAL, NTSC standards

Figure 1 J.S. Zarach and Noel M. Morris, Television principles & practice

Figure 2

Amplitude Modulator

Amplitude Modulator

Frequency Modulator

Frequency Modulator

Syn. PulsesGenerator

Syn. PulsesGenerator

RFModulator

RFModulator

AudioSignal

VideoSignal

MYY

(I.F.)

Figure 3

Resolution: • The smallest distance that can exist between

two points• Maximum number of points that can exist in

an area

x

YB/W

RB/W

GB/W

BB/W

+ Y

Weighted sum

Figure 4a

YCamera

Figure 4b

Negative AM MY

fY - I.F. Carrier

fY

D.C.

USBLSB

fY +5.5MHz

Y

5.5MHz

Figure 5a

fYD.C.

fY

FilterD.C.

Signal USBLSB

fYD.C.

SSBSignal

fY + 5.5MHz

fY + 5.5MHz

Passband

Figure 5b

fYD.C.

fY

FilterD.C.

Signal USBLSB

fYD.C.

VSBSignal

USBVSB

fY + 5.5MHz

Passband

fY + 5.5MHz

What about Color?Straightforward approach

fRD.C.

fR+5.5MHz

fG

fG+5.5MHz

fB

fB+5.5MHz

But this will require THREE TIMES the bandwidthFigure 6

Figure 7

A

freq1T1

A

freq1T2

T1

T2

S1(t)

S2(t)

Time Spectrum

a. Frequency spectrum is not continuous

b. Frequency components can only occur in regular spaced slots

c. The positions of the slots are determined by the smallest repetitive frequency of the signal

THREE important findings for repetitive signals

e. For a continuous sine wave (i.e. infinite duration), the frequency spectrum is a single impulse.

f. The position of the impulse of a continuous sine wave is dependent only on the frequency

TWO important findings for continuous sine wave

1st cycle2nd cycle

3rd cycle4th cycle

A

freq1T1

T1 f2

f2

Figure 8a

S1

S2

f1

(Enlarged Frequency Scale)

1st cycle2nd cycle

3rd cycle4th cycle

A

freq1T1

T1 f2

f2

Figure 8b

S1

f1

S3

(Carrier)

Answer: No

Can RGB components be interleaved?

freq

fR

Figure 8c

fG fB

The 3 color components are only roughly periodic

freq

R

Figure 8d

G B

Result: Partial Overlapping between componentsDistortion is very prominent in smooth region

B/W

B/W

B/W

LuminanceY

R

G

B

RGB

TO

YUV

ChrominanceU, V

.

Y = 0.3R + 0.59G + 0.11BU = B - YV = R - Y

RGB to YUV transform

R = V + YG = (Y - 0.3R - 0.11B)/0.59B = U + Y

YUV to RGB transform

• Y - Luminance (intensity information)• U and V - Chrominance (color information)

• Y - Wide band (5.5 MHz)• U and V - Narrow band (about 2MHz)

The Eye is not sensitive to • Lumninance at high frequency (e.g. texture)•Chrominance, as compare with Luminance

f

VSB

Chroma at 4.43 MHz

-1.75 MHz

Sound at 6 MHz

Note: Y and UV are separated by interleaving, what about U and V?

Quadrature Modulation (QM)

U

V

cos c t

cos ( c t + 90o)

AM

AM

Y +

CU

CV

S

Figure 9

Note: Y and UV are separated by interleaving

Phasor representation of Quadrature Modulation

CU

CV

CU +CV

Color (hue) defined by

freq

Line frequency = 1/T = 15.6kHz

Line duration = T = 64s

Color Subcarrier frequency fsc = 283.5/T = 4.43MHz

Y

U

V

fsc

284/T

1/T

1/2T

Figure 26

Demodulation

S-Y

cos c t (LO)

cos ( c t + 90o) (LO)

X

X

U

V

Figure 10

LPF

LPF

CU cos c t = U cos2 c t

= U (cos 2 c t + cos (0))

= U after LPF

CU cos( c t + 90o) = U cos c t cos( c t + 90o)

= U (cos (2 c t+90) + cos (90o))

= 0 after LPF

CV cos( c t+90) = V cos2( c t+90o)

= V (cos (2 c t+180o) + cos (0))

= V after LPF

CV cos c t = V cos c t cos( c t + 90o)

= V (cos (2 c t+90) + cos (90o) )

= 0 after LPF

1. The two quadrature carrier signals are not sent to the receiver

2. Phase error in demodulation

CU

CV

CU +CV

The two quadrature carrier signals are regenerated in the receiver with a short burst of sine wave

The regenerated carrier signals may contain error

Consider an error ‘’ in the regenerated carrier

The carrier (LO) changes from: cos( c t) to cos( c t+ ) , andcos( c t+90o) to cos( c t+ 90o+)

CU cos( c t+ )= U cos c t cos( c t+ )

= U (cos (2 c t+ ) + cos ( ))

= U cos ( ) after LPF

CU cos( c t + 90o + ) = U cos c t cos( c t + 90o + )

= U (cos (2 c t+90 + ) + cos (90o + ))

= Ucos (90o + ) after LPF

CV cos( c t+90+ )

= V (cos (2 c t+180o + ) + cos ( ))

= V cos ( ) after LPF

CV cos( c t+ )

= V (cos (2 c t+90o + ) + cos (90o + ))

= V cos (90o + ) after LPF

Error Free phasor diagramError Free phasor diagram

CU

CV

Correct

CU + CV

CU

CV

Correct

Error

Error phasor diagram

Note: distortion is similar between

adjacent lines

Every line is subject to distortion

Color Distortion

Figure 11

OriginalDistorted (anticlockwise

Distorted (clockwise)

1. Odd lines:U modulated by cos( c t) V modulated by cos( c t+90o)

2. Even lines:U modulated by cos( c t) V modulated by cos( c t-90o)

Under Error Free condition U and V are fully recovered with quadrature demodulation

Odd Lines

CU

CV

Even Lines

CV

Correct

CU + CV

Error Free Signal

Error Free Signal

CV

CU

CV

CU

CV

CU

CV

CU

Line 1

Line 2

Line 17

Line 18

Implications in video signal

Line 1 Line 2 Line 3 Line 4

Y

Line 1 Line 2 Line 3 Line 4

U

Line 1 Line 2 Line 3 Line 4

V

fH

fH

fH/2

fH=15.625kHz is the line frequency

Odd Lines

CU

CV Error

CU + CV

Even Lines

CU

CV

Correct

CU + CV

LO with Error ‘’

Odd Lines

CU

CV Error

CU + CV

LO with Error ‘’

CU

CV Error

CU + CV

Even Lines (inverted)

Odd Lines (delayed)

CU

CV Error

CU + CV

LO with Error ‘’

CU

CV Error

CU + CV

+

Even Lines (inverted)

Phasor addition

CU

CVError (odd)

CU + CV

Error Free Resultant

LO with Error ‘’

CU

CVCorrect

CU + CV

=Error (even)

PAL Color Compensation - Graphical illustration

Figure 12a

Original Distorted

PAL Color Compensation

Figure 12b

Original Distorted Compensated

Line n-1

Line n

Line n+1

Line n-1

Line n

2222

2

ttjVttU

ttjVttU

ttjVttU

tCtCtC

coscos

coscos

coscos

speriodline 64

PAL Compensation by averaging consecutive lines: 1st case

2

50

ttVjttUttjVttU

tCtCtC Dcomp

coscoscoscos

.

2

ttjVttjV coscos

Subscript “D” denotes delay by 64 us

PAL Compensation by averaging consecutive lines: 1st case

2

50

ttVjttUttjVttU

tCtCtC

DD

Dcomp

coscoscoscos

.

ttjV

ttjVttjVcos

coscos

2

PAL Compensation by averaging consecutive lines: 2nd case

2

coscoscoscos

5.0

ttVttUttjVttU

tCtCtC

DD

Dcomp

ttU

ttUttUcos

coscos

2

Subscript “D” denotes delay by 64 us