View
219
Download
0
Embed Size (px)
Citation preview
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