Black and white TV 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.