Black and white TV fundamentals

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TELEVISION ENGINEERING Black and White Circuit Fundamentals

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  • TELEVISION ENGINEERING

    Black and White Circuit Fundamentals

  • CCIR625 lines

    Picture Signal amplitude modulation

    Sound signal frequency modulation

    Why?

    BW in FM = 2(f+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/() = H/(D) Where = D/H = Viewing Distance/Height = Min resolving angle of an eye

    Vertical resolution

    D

    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 s 64 s = 52s + 12s 52s Beam travels from left to right 12s Beam travels from right to left. Blanked out. Line

    blanking period.

    Horizontal sweep iH

    iHmax

    52 s second line

    retrace

    trace

    f = 15625 Hz

    12 s

  • 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 cant 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 s.

    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 DCs 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 s.

    Allows receiver to settle down from current level to blanking level.

    Pulling on whites

    Sync period 4.7s.

    Triggers sync at leading edge.

    Beam cut-off.

    Back porch 5.8s

    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 4.7s

  • 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 2.3s.

    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.

    f = 75KHz,

    fm = 15KHz.

    mf = 75/15 = 5

    Using Bessel function for mf = 5, n= 7

    BW = 2X7X15

    210KHz

    In FM, BW tone amplitude

    In AM BW tone frequency.

    FM for Sound (Commercial)

  • BW = 2nfm n number of significant side bands.

    f = 50KHz,

    fm = 15KHz.

    mf = 50/15 3

    Using Bessel function for mf = 3, n= 5

    BW = 2X5X15

    150KHz

    Carsons rule BW = ?

    2(fm + f)

    = 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 = 0.3A

    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 when dark.

    R=2M 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 when screen is dark. R 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

    3

    2

    1

    SiO2

    Si substrate p-type

  • CCD Image scanner

    3

    2

    1

    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

    3

    2

    1

    t1 t2 t3 t4 t5

  • At t1, potential 1 exists at corresponding electrode.

    Charge transfer effected by multiphase clock voltage pulses applied to gate in suitable sequence.

    Charges move from 1 to 2 to 3 then to 1 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

  • Electrostatic focusing

    E2 E1

    E1

  • 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.

    70, 90, 110, 114.

    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