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