Black and white TV fundamentals
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Transcript of Black and white TV fundamentals
TELEVISION ENGINEERING
Black and White Circuit Fundamentals
CCIR625 lines
Picture Signal – amplitude modulation
Sound signal – frequency modulation
Why?
BW in FM = 2(Δf+fm)
SSB, DSB-SC, AM
VSB
TV in India
General Block Diagram for TV Transmitter
Crystal
Oscillator
Power
Amplifier
RF
Amplifier
Scanning and Synchronizing
Circuits
Combining
Network
AM
Modulating
amplifier
Video
Amplifier
TV
Camera
FM Sound
Transmission
FM
Modulating
Amplifier
Audio
Amplifier
Antenna
Picture signal – space, time, amplitude
Scanning
Synchronization
Deflection
Basic TV operation
Camera face plate
Photoconductive plate Glass plate
Conductive coating
Video Signal
Current electron
General Block Diagram for TV Receiver
RF
Tuner
Video
Detector
Common IF
Amplifier
Scanning and Synchronizing
Circuits
Video
Ampr
Audio
amplifier
FM Sound
Detector
Sound IF
Amplifier 2-3
stages
Antenna
Speaker
Tuner
RF
Amplifier
IF
Amplifier Mixer
Antenna
LO
IF
Most motion horizontal
=width/height
=4/3
Same in motion picture
Irrespective of size of picture
Achieved by feeding desired deflecting current to deflecting coils.
Aspect Ratio
Persistence of vision
1/16 seconds
Picture stimulus on retinas ≥ 16 pictures per second
Motion picture – 24 pictures per seconds
TV- 25 pictures per seconds
Synchronization with line easy.
Scanning and image continuity
Horizontal scanning
W
H
Start End
Trace
Retrace
iH
iHmax
first line second line third line
trace retrace
Horizontal scanning
Trace Period
Retrace Period
Left Right Left
One cycle
Raster
Trace
Fly back / Retrace
Vertical scanning
Retrace
Top
Bottom
Trace
Top
Bottom
iV
iVmax
first frame second frame third frame
trace retrace
Bottom
Top
Vertical scanning
Raster
Trace
Period
Retrace
Period
Bottom
Top W
H
It is the ability of scanning beam to differentiate between two closely located points.
Better if number of lines increase.
Limited by --resolving capability of eye and beam.
Vertical resolution
Beam Spot
B
W
Nv = 1/(αρ) = H/(Dα) Where ρ = D/H = Viewing Distance/Height α = Min resolving angle of an eye
Vertical resolution
D
α
H
Picture has random distribution of black, grey and white.
70% lines get scanned properly.
Reason-
Finite beam size
Alignment of beam not coinciding with elementary resolution lines.
Kell factor k.
k between 0.64 to 0.84.
With k = 0.7,
602
Vertical resolution - practical constraints
Not much improvement after 500 lines.
BW increases with lines.
Channels reduce and cost increases.
Compromise between cost and quality.
625 lines in 625-B monochrome system.
Better suites with 50Hz line.
525 line system in US with 60Hz system.
Choice for 625 lines
25 pictures per second or 625 lines /s.
Flicker 1
625
Interlaced scanning
1
2
3
313
313
314
315
625
1
2
50 frames per second.
312.5 lines per frame.
Alternate lines scanned.
Frame 1- 1 to 312.5, stops at bottom center.
Reaches up at top center.
Frame 2 – 312.5, 313, ….625.
Interlaced scanning
Vertical sweep oscillator -50 frames per second with 312.5 lines per frame.
Horizontal sweep oscillator =50X312.5
15625 lines /s
Earlier – 25 X 625
15625 lines/s
Flicker reduces with only doubling V-Osc
US – 60 X 525 = 15750 lines/s
Interlaced scanning
Sweep rate = 15625 Hz Sweep Time = 64 µs 64 µs = 52µs + 12µs 52µs – Beam travels from left to right 12µs – Beam travels from right to left. Blanked out. Line
blanking period.
Horizontal sweep iH
iHmax
52 µs second line
retrace
trace
f = 15625 Hz
12 µs
Sweep rate = 50 Hz Sweep Time = 20 ms 20 ms = 18.720ms + 1.28ms 18.720ms – Beam travels from top to bottom 1.28ms– Beam travels from bottom to top. Blanked out.
Vertical blanking period.
Vertical sweep
f = 50 Hz
iv
iVmax
18.72 ms 1.28 ms
V. Retrace time – 1.28ms
20 lines for retrace per frame.
Field 1-
1 to 292.5 + 20 = 312.5
Field 2- ?
Vertical sweep
Ability of the scanning system to resolve picture details in vertical direction.
Vr = Na x k
Na = Number of active lines
= (625-40) x 0.69
400 lines
Vertical resolution
Ability of the scanning system to resolve maximum number of picture elements along the scanning line.
Horizontal resolution
To make same density as horizontal:
585 active lines X 4/3 aspect ratio
780 hypothetical lines/ elements
Eye can’t resolve more than Kell factor.
No of effective B/W elements = 780* 0.69
533 elements.
Horizontal resolution
Vertical signal along one line.
No of complete cycle in one line = 533/2
In 52 µs.
Horizontal frequency fh =?
IMP- 5MHz square wave requires atleast 11 harmonics -55MHz.
Eye can not see the shape of fine elements.
Square element replaced by dots.
5MHz sine wave.
Horizontal resolution
t
Corresponds to slow variations in picture e.g. plane screen.
Horizontal excursion – dc
Vertical excursion – 50Hz.
All amplifiers must be capable of operating from dc to 5MHz.
Low frequency response
V
Field 1 – 1 to 292.5. Sync pulse available at a to take to c.
Field 2 – 313 to 605. Sync pulse available at b to take to d.
Interlace error 1
2
3
313
313
314
315
625
1
2
c
a
d
b
Second field – Line starts from point other than c.
If starts from middle of c and d – 50% error.
If starts from d – 100% error.
No interlace. Same line traced again.
Need – Perfect synchronization at 50Hz and 15625Hz using crystal oscillator.
Interlace error
Contents of composite video signal:
Camera output signal
Blanking pulses to make retrace invisible.
Horizontal sync pulses after each line
Vertical sync pulses after each field
Composite video signal
Horizontal Sync
Amplitudes of H and V sync same for efficiency.
width of H and V sync different for ease of separation.
Sync pulses present consecutively with video signal. TDM.
Peak white level – 10 to 12.5% of max signal value.
Black level of signal – 72% of max signal value.
Blanking level – 75% of max signal value. Base of sync pulse.
Pedestal – difference between black level and blanking level. Small so merge with each other.
Picture between 10% to 75% of peak.
Voltage below 10% not used.
To minimize noise effect and gives enough margin for excessive bright spot without causing amplitude distortion..
Features of COMPOSITE VIDEO SIGNAL
Brightness:-
DC value of the signal.
Average brightness of the scene is average of all DC’s of all lines.
Higher the DC, lesser the brightness.
Contrast :-
Difference between min and max signal level
Increased by increased gain of amplifiers.
Pedestal height:-
Distance between pedestal level and DC value.
Indicates average brightness.
i.e. how away the average value from complete darkness.
Features of COMPOSITE VIDEO SIGNAL
Blanking pulses
Make retrace invisible.
Signal amplitude raised to slightly above black level (75%) during retrace.
PRF of H. Blanking – 15625Hz
PRF of V. Blanking – 50Hz
Blanking pulses
Horizontal Sync
Blanking pulse at 75% never used as sync pulse.
Reason-
Occasional signal or noise peak may reach 75% and trigger sync ckt.
Sync pulses placed on blanking pulses.
Signal clipped at 75%.
Lower portion - signal.
Upper portion – Sync pulses.
Leading edges of differentiated pulses used for synchronization.
Sync pulses
Horizontal Sync
Front porch – 1.5 µs.
Allows receiver to settle down from current level to blanking level.
Pulling – on – whites
Sync period – 4.7µs.
Triggers sync at leading edge.
Beam cut-off.
Back porch – 5.8µs
Allows enough time to retrace.
Allows saw tooth generator to change direction
Sync pulses
Vertical Sync
Chosen - 2.5H
If too small – can not be separated from H sync.
If too large – Power increases.
Vertical Sync
Extraction of SYNC
Sync Separator
Composite Video Signal
V
H
R
R
C
C
No Sync during V sync
H sync not available during V sync.
H oscillator may go out of sync.
REMEDY:-
Serrations are provided in V sync.
Achieves H sync without disturbing V sync.
Drawbacks of Vertical Sync
Serrations in Vertical Sync Serration width 4.7µs
5 serrations are given at H/2.
Rising edge occurring at ±4% of oscillator frequency will only help in synchronization.
Desired serrations will help synchronize the H oscillator without disturbing the V sync.
V oscillator uses level triggering.
Serrations in Vertical Sync
625 624 623
1 2 3 4 5
624 625 1 2 3 4 5
311 312 313 314 315 316 317
317 316 315 314 313 312 311
Drawbacks of Vertical Sync serrations
Even field vertical retrace starts earlier than desired.
Will disturb the interlace.
May cause interlace error.
Remedy:-
5 equalization pulses imposed before and after V pulse.
Equalization pulses of pulse gap H/2 and width 2.3µs.
Called pre and post equalization pulses.
2.5H duration of even and odd field become identical.
Pulses are thinner.
Capacitor charges less and discharges to zero much faster.
Capacitor discharges to zero in both field V pulse begins.
Drawbacks of Vertical Sync serrations
Equalization pulses
Pre-equalizing pulses
Post-equalizing pulses
After equalization pulses
What is need for modulation.
What are DSB-SC and SSB transmission.
Compare DSB-SC and SSB.
Drawbacks of using SSB.
Channel BW using DSB-SC – 11.5MHz
Signal transmission Pre-requisits
If -- DSB-SC
11.25MHz
fc 1 2 3 4 5 6
5.5
5.75
1 2 3 4 5 6
5.5MHz 5.5MHz
LSB USB
Guard Band
Picture Carrier
If -- VSB
Pc
USB
1 4 3 2 5
5.5
6
f MHz
Ps
1
0.75 1.25
0.75
2
7MHz
5.75
BW = 7MHz
Very sharp cutoff filters not required.
Attenuation curve from 0.75MHz to 1.25MHz.
Advantages of VSB?
625 line TV system allocates 7MHz to each channel.
Full carrier sent.
Helps in envelope detection.
Synchronous detection may lead to distortions visible to eye if frequency and phase error occur.
VSB (A5C)
Total band
54 55 56 57 58 59 60 60 61 62 63
Channel III Channel IV
fs =60.75
fp =55.25
Envelope detection.
Reception of VSB
1
2
1 2 3 4 5 6 5.5
0.75 1.25 2
0 to 0.75MHz – contribution from both side band.
Double amplitude.
0.75 to 1.25MHz – Full contribution from USB.
Contribution from LSB gradually reduces to zero..
Signal gradually reduces from double to single.
1.25MHz onwards – Contribution from USB only.
Single amplitude.
Total signal will be distorted.
Reception of VSB
Desired response -
Reception of VSB
1
2
1 2 3 4 5 6 5.5
0.75 1.25
IF correction
1 2 3 4 5 6 5.5 0.75 1.25 0 1 1.25 0.75
P
S
7MHz
1 2 3 4 5 6 5.5 0.75 1.25 0 1 1.25 0.75
fMHz
1
Total contribution from both side bands equal 1.
Total amplitude - single at every point.
Disadvantage –
Carrier reduced to 50%.
Some transmitted signal lost.
Accurate filter tuning required.
IF Correction
BW = 2nfm
n – number of significant side bands.
Δf = ±75KHz,
fm = 15KHz.
mf = 75/15 = 5
Using Bessel function – for mf = 5, n= 7
BW = 2X7X15
210KHz
In FM, BW α tone amplitude
In AM – BW α tone frequency.
FM for Sound (Commercial)
BW = 2nfm
n – number of significant side bands.
Δf = ±50KHz,
fm = 15KHz.
mf = 50/15 ≈ 3
Using Bessel function – for mf = 3, n= 5
BW = 2X5X15
150KHz
Carson’s rule – BW = ?
2(fm + Δf)
= 130K
FM for Sound (TV)
Requirement:
Sensitivity to visible light.
Wide dynamic range w. r. t. light intensity.
Ability to resolve details i. e. resolution.
Camera
Small size.
Ease of operation.
Principle of photoconductivity.
Selenium, tellurium, lead with their oxides .
Resistance decreases with increase in light.
Variation of resistance across the surface used to develop varying signal.
Scanned uniformly with electron beam.
Vidicon
Camera
Thin photoconductive layer of selenium or antimony compound.
Deposited on transparent conductive film.
Called Signal electrode or plate.
Load resistance RL between DC supply and signal electrode.
Target
Face plate
+40V
RL=50K
Electron Gun
Scanning beam
Black = 20M White = 2M
Signal Plate I = 0.3µA
Photo Layer
Electron beam by gun focused on photoconductive layer by combined action of uniform magnetic field of an external coil.
Grid 3 and 4 provide uniform de-acceleration field.
Avoids secondary emission.
H and V deflection coil deflect beam L-R and T-B.
Photo-layer thickness 0.0001cm.
R=20M Ω when dark.
R=2M Ω when brightly lit as energy takes electrons to conduction band.
Electrons are used up to make gun side potential zero.
Remaining electrons return back and collected.
Vidicon camera
Sudden change in potential on each element during beam scan.
Current flow in signal electron circuit.
Voltage across RL proportional to light intensity.
vo = Vcc – vRL.
Larger illumination, larger potential, larger current, larger voltage drop across RL smaller vo.
Vidicon camera
Another explanation
Gun
40V
Light
Glass Plate
Element – capacitance parallel to element resistivity.
One side connected to signal electron.
Other side open- towards Gun
Capacitance charged to 40V when not lighted. R very large.
With light, R falls, C discharges according to intensity.
Electron beam charges back C to 40V.
Hence current flow through RL proportional to intensity of light.
Dark Current:
R=20M Ω when screen is dark. R ǂ infinity.
Hence current is not Zero even in dark condition..
Current called dark current.
Vidicon camera
Image Lag:
Time delay in establishing a new signal current in camera.
Delay in following rapid changes in target illumination.
1. Photoconductive lag:
Due to property of material to respond.
2. capacitive lag or beam lag:
Due to storage effect of pixel capacitance and beam resistance.
More prominent if pixel brightly illuminated for prolonged time.
Recharging not complete in one scan.
Causes smear or comet tail effect following moving object.
Slow decaying produces after image.
Vidicon camera
New concept in MOS circuitry.
Shift register formed by string of closely spaced MOS capacitors.
Can store and transfer analog charge signals.
p-type substrate.
One side oxidized to form silicon dioxide – an insulators.
An array of metal electrodes – gates deposited by photolithography.
Creates very large number of tiny MOS capacitors.
Covers entire surface of chip.
Small potential to gates give depletion region potential wells.
Depth of wells vary with applied voltage magnitude.
CCD Image scanner
CCD Image scanner
ϕ3
ϕ2
ϕ1
SiO2
Si substrate p-type
CCD Image scanner
ϕ3
ϕ2
ϕ1
SiO2
Holes
Gate electrodes operate in group of three.
Every third electrode connected to a common conductor.
Spots under the conductors serve as light sensitive elements.
As image focuses on chip, electrons proportional to light intensity, are generated inside chip, close to surface.
Generated electrons are collected in nearby potential wells.
Pattern of collected electrons represent optical image.
CCD Image scanner
CCD Image scanner
Electron Energy
Surface Potential
Charge of one element transferred along surface by applying more positive voltage to adjacent gate or electrode while reducing voltage on it.
Electrons in the wells reduce their depths to fill adjacent well.
Accumulation of charge carriers under first potential wells of two consecutive trios are shown.
CCD Image scanner Charge Transfer
CCD Image scanner
ϕ3
ϕ2
ϕ1
t1 t2 t3 t4 t5
At t1, potential Ø1 exists at corresponding electrode.
Charge transfer effected by multiphase clock voltage pulses applied to gate in suitable sequence.
Charges move from Ø1 to Ø2 to Ø3 then to Ø1 under influence of continuing clock pulses till finally reach end of array for collection.
This forms signal current.
CCD Image scanner Charge Transfer
CCD Image scanner
Out
Readout register Address register Driving phases
1 2 3 1 3 2
1
2
3
Large number of CCD arrays are packed together to form image plate.
Does NOT need electron gun, scanning beam, high voltage vacuum envelope of conventional camera.
5 to 10 volts are required.
Spots under each trio serves as resolution cell.
Electrons generated proportional to light falling on cell.
Linear imaging structures are arranged to represent scan lines
Lines are independently addressed and read into common output diode through vertical output register.
Done by application of driving pulses through a set of switches controlled by address register.
CCD Image scanner Scanning
Phosphor coating inside.
Electrostatic focusing.
Electromagnetic deflection.
Monochrome picture tube
F = -eE
e = charge on electron = ?
E = electric field.
Beam diverge due to force of repulsion among electrons.
Electron refracts when passes from one electric field to another.
Electrostatic focusing
Electrostatic focusing
E1
E1<E2
E2
Electrostatic focusing
E2 E1
E1<E2 Co-axial cylindrical electrodes
E1 < E2.
Beam diverges when under E1.
Refracts and bends towards axis when at boundary.
E2 again forces the beam to diverge more.
E2 being higher, beam stays under E2 for lesser time.
Diverging action is lesser than converging.
Beam focuses.
Mostly used in picture tube.
Electrostatic focusing
F = BeV
B and V are perpendicular to each other.
Motion perpendicular to magnetic and electric field both.
Electrons come out with axial motion and a small transverse motion.
Cork screw right hand rule.
Electrons follow circular/spiral motion.
Small transverse component of velocity of electrons reacts with two fields.
All electrons converge at a point.
Mostly used in camera tube.
Electromagnetic focusing
B/W TV Picture Tube
Control grid
G1
G3
G2
Centering magnet
Pin cushion error magnet (EHT = 18K)
Final anode inner aqua dag coating
Aluminum Coating
Glass plate
Cathode –
cylinder of nickel coated at its end with thoriated tungsten or barium and strontium oxide.
Have low work function.
Emit electron when heated. (0V)
Picture Tube
Accelerating Grid 2-
400V.
G1 and G2 form strongly convergent electrostatic lens.
Accelerated by G2.
While converging, cross over at a point between G1 and G2.
Called first Cross over point.
Picture Tube
Control Grid 1-
-40V w.r.t. cathode.
Controls flow of electrons.
Cylindrical structure with small hole to confine electron beam to small area.
Picture Tube
Focus Grid G3-
Further divergence focused.
Higher voltage as G2.
G1, G2, G3 available at base for connection.
Wires soldered to socket and socket slid to pins.
G2 and G3 give second cross over point at screen through convergence and divergence.
Picture Tube
Beam velocity
Electron impact strong enough to produce illumination.
Achieved by final anode.
Conductive graphite coating called Aqua Dag.
Available at an outlet on bell.
12KV to 18KV for 14’ B/W to 24” B/W.
How does screen illuminate proportional to signal?
Secondary emission results.
Aqua dag collects these zero velocity electrons.
Path of electron current – cathode-screen-conductive coating-secondary electron-EHT.
Use of Aluminum coating
Increases light output by a factor of 2.
Prevents undesired ions from damaging the screen.
Ions (+ve) being heavy, do not attain enough velocity nor get deflected.
Heavy ions with low velocity can not penetrate aluminum coating.
They can not produce ion spot at center.
Screen burn
Can be due to +ve ions and electrons.
Ion burn saved by Al coating.
Occurs while switching off, if deflection drives disappear before EHT discharge.
Straight beam attracted by EHT damages screen center.
EHT must discharge before deflection drives disappear.
Implosion Protection
Vacuum inside and air pressure outside may implode tube .
Flying splinters of glass may hurt.
Protective metal banding to protect it from jerks damages etc.
Deflection Yoke
Coils split in 2 parts for gradual and uniform movement of beam in more uniform field.
Cosine winding- Thickness of deflection winding varies as cosine of angle from central reference.
Gives uniform magnetic field even if deflection angle increases.
Deflection Yoke – Cosine winding
Deflection Yoke
Deflection angle
Angle through which the beam can be deflected without striking the sides.
70º, 90º, 110º, 114º.
Large deflection angle Smaller tube length, smaller cabinet.
Large deflection power from deflection circuits.
Narrow neck
Used in TV.
Smaller deflection angle Larger tube length, smaller cabinet.
Smaller deflection power from deflection circuits.
Used in CRO
Deflection angle
Centering and pincushion magnets
Eccentricity of picture can be corrected.
Two ring magnets around neck.
Pincushion error can be corrected using small ring magnets on yoke.
Control wirings
TV Transmitter
Positive and negative modulation
Positive modulation Negative modulation
Effect on Noise Pulse on Positive and negative modulation
TV Receiver