Analog Television

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    ANALOG TELEVISION

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    Persistence of vision:

    the eye (or the brain rather) can retain the sensation of an

    image for a short time even after the actual image is

    removed.

    1 Frame merging

    This allows the display of a video as successive frames as

    long as the frame interval is shorter than the persistence

    period, The eye will see a continuously varying image in

    time.

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    When the frame interval is too long, the eye observes frame

    flicker. The minimal frame rate (frames/ second or fps or

    Hz) required to prevent frame flicker depends on display

    brightness, viewing distance.

    Higher frame rate is required with closer viewing and

    brighter display.

    For TV viewing: 50- 60 fps

    For Movie viewing: 24 fps

    For computer monitor: > 70 fps

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    3 Merging pixels

    Similarly, the eye can fuse separate pixelsin a line into one

    continuously varying line, as long as the spacing between

    pixels is sufficiently small.

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    4 Interlacing

    For some reason, the brighter the still image presented to the

    viewer ... the shorter the persistence of vision.

    If the space between pictures is longer than the period of

    persistence of vision then the image flickers. Therefore, to

    arrange for two "flashes" per frame,

    interlacing creates the flashes. The basic idea here is that a

    single frame is scanned twice. The first scan includes only

    the odd lines, the next scan includes only the even lines.

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    Basic black and white television

    In a basic black and white TV, a single electron beam is

    used to scan a phosphor screen. The scan is interlaced, that

    is -- it scans twice per photographed frame.

    The information is always displayed from left to right. After

    each line is written, when the beam returns back to the left,

    the signal is blanked. When the signal reached the bottom itis blanked until it returns to the top to write the next line

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    Trace and Retrace

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    NTSC has 525 vertical lines. However lines number 248 to

    263 and 511 to 525 are typically blanked to provide time for

    the beam to return to the upper left hand corner for the next

    scan. Notice that the beam does not return directly to the

    top, but zig-zags a bit.

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    Vertical Scanning signal

    The vertical scanning signal for conventional black and

    white NTSC is quite straightforward. It is simply a positive

    ramp until it is time for the beam to return to the upper left-

    hand corner. Then it is a negative ramp during the blanked

    scan lines.

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    Horizontal Scan signal

    The horizontal scan signal is very much the same. The

    horizontal scan rate is 525*29.97 or 15,734 Hz. Therefore,

    63.6 uS are allocated per line. Typically about 10 uS of this

    is devoted to the blanking line on the horizontal scan. There

    are 427 pixels per horizontal scan line and so each pixel isscanned for approximately 125 ns.

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    The electron beam is analog modulated across the horizontal

    line. The modulation then translates into intensity changes

    in electron beam and thus gray scale levels on the picture

    screen

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    Horizontal blanking signal and synchronization pulse is

    quite well defined. For black and white TV, the "front

    porch" is 0.02 times the distance between pulses, and the

    "back porch" is 0.06 times the distance between pulses.

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    The vertical blanking signal also has a number of

    synchronization pulses included in it. These are

    illustrated below.

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    The television bandwidth is 6 MHz.

    The sub-carrier for the color is 3.58 MHz off the carrier for

    the monochrome information.

    The sound carrier is 4.5 MHz off the carrier for the

    monochrome information.

    There is a gap of 1.25 MHz on the low end and 0.25 MHz

    on the high end to avoid cross talk with other channels.

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    TV Transmitter (B&W)

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    TV Receiver (B&W)

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    COLOR TELEVISION

    One of the great electrical engineering triumphs was the

    development of color television in such a way that it

    remained compatible with black and white television.

    A major driving force behind the majority of current color

    TV standards was to allow black-and-white TVs to continue

    to be able to receive a valid TV signal after color service

    was in place.

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    Trireceptor theory of vision

    why we use RGB monitors

    If you ask someone why red, green and blue are used in

    computer monitors -- the immediate answer is "Because

    these are the primary colors".

    If you then ask, "But why are these the primary colors?" --

    the answer you get is that "If you mix light of these colors

    together you can make any color".

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    Color information transmission in TV

    In the most basic form, color television could simply be

    implemented by having cameras with three filters (red,

    green and blue) and then transmitting the three color signals

    over wires to a receiver with three electron guns and three

    drive circuits.

    Unfortunately, this idealized view is not compatible withthe previously allocated 6 MHz bandwidth of a TV channel.

    It is also not compatible with previously existing

    monochrome receivers.

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    Therefore, modern color TV is carefully structured to

    preserve all the original monochrome information -- and

    just add on the color information on top.

    To do this, one signal, called luminance (Y) has been

    chosen to occupy the major portion (0-4 MHz) of the

    channel. Y contains the brightness information and the

    detail. Y is the monochrome TV signal.

    Consider the model of a scene being filmed with three

    cameras. One camera has a red filter, one camera a green

    filter and one camera a blue filter.

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    Assume that the cameras all adjusted so that when pointed

    at "white" they each give equal voltages. To create the Y

    signal, the red, green and blue inputs to the Y signal must be

    balanced to compensate for the color perception misbalance

    of the eye. The governing equation is:

    For example, in order to produce "White" light to the

    human observer there needs to be 11 % blue, 30 % red and

    59% green (=100%).

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    This is the "monochrome" part of the TV signal. It officially

    takes up the first 4 MHz of the 6 MHz bandwidth of the TV

    signal. However, in practice, the signal is usually band-

    limited to 3.2 MHz.

    Two signals are then created to carry the chrominance (C)

    information. One of these signals is called "Q" and the

    other is called "I". They are related to the R, G and B

    signals by:

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    The positive polarity ofQ is purple, the negative is green.

    The positive polarity of I is orange, the negative is cyan.

    Thus, Q is often called the "green-purple" or "purple-green"

    axis information and I is often called the "orange-cyan" or

    "cyan-orange" axis information.

    It turns out that the human eye is more sensitive to spatial

    variations in the "orange-cyan" than it is for the "green

    purple". Thus, the "orange-cyan" or I signal has a maximum

    bandwidth of 1.5 MHz and the "green purple" only has a

    maximum bandwidth of0.5 MHz.

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    Now, the Q and I signals are both modulated by a 3.58 MHz

    carrier wave. However, they are modulated out of 90

    degrees out of phase.(QAM) These two signals are then

    summed together to make the C or chrominance signal.

    The nomenclature of the two signals aids in remembering

    what is going on. The I signal is In-phase with the 3.58

    MHz carrier wave. The Q signal is in Quadrature (i.e. 1/4

    of the way around the circle or 90 degrees out of phase, or

    orthogonal) with the 3.58 MHz carrier wave.

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    New chrominance signal (formed by Q and I) has the

    interesting property that the magnitude of the signal

    represents the color saturation, and the phase of the signal

    represents the hue.

    Phase= Arctan (Q/ I) =hue

    Magnitude = sqrt (I 2+ Q 2) =saturation

    Now, since the I and Q signals are clearly phase sensitive --

    some sort of phase reference must be supplied. This

    reference is supplied after each horizontal scan and is

    included on the "back porch" of the horizontal sync pulse.

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    Conversion between RGB and YIQ

    Y = 0.299 R + 0.587 G + 0.114 B

    I = 0.596 R -0.275 G -0.321 B

    Q = 0.212 R -0.523 G + 0.311 B

    R =1.0 Y + 0.956 I + 0.620 Q

    G = 1.0 Y - 0.272 I -0.647 Q

    B =1.0 Y -1.108 I + 1.700 Q

    B d id h f Ch i Si l

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    Bandwidth of Chrominance Signals

    With real video signals, the chrominance component

    typically changes much slower than luminance

    Furthermore, the human eye is less sensitive to changes in

    chrominance than to changes in luminance

    The eye is more sensitive to the orange- cyan range (I) (the

    color of face!) than to green- purple range (Q)

    The above factors lead to

    I: bandlimitted to 1.5 MHz and

    Q: bandlimitted to 0.5 MHz

    M lti l i f L i d Ch i

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    Multiplexing of Luminance and Chrominance

    Position the bandlimited chrominance at the high end of the

    luminance spectrum, where the luminance is weak, but still

    sufficiently lower than the audio (at 4.5 MHz).

    The two chrominance components (I and Q) are multiplexed

    onto the same sub- carrier using QAM.

    The resulting video signal including the baseband

    luminance signal plus the chrominance components

    modulated tof c is called composite video signal.

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    I NTSC L i i AM VSB th Ch i QAM

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    In NTSC Luminance is AM VSB, the Chroma is QAM

    I&Q, and the Aural FM.

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    Transmitter Block Diagram

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    Color Decoder

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    Block diagrams of TV receivers

    PAL SECAM and NTSC

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    PAL , SECAM and NTSC

    There are three major TV standards used in the world today.

    These are the

    1. American NTSC (National Television SystemsCommittee) color television system,

    2. European PAL (Phase Alternation Line rate)

    3. French-Former Soviet Union SECAM (SequentialCouleur avec Memoire)

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    The largest difference between the three systems is the

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    The largest difference between the three systems is the

    vertical lines. NTSC uses 525 lines (interlaced) while both

    PAL and SECAM use 625 lines.

    NTSC frame rates are slightly less than 1/2 the 60 Hz power

    line frequency, while PAL and SECAM frame rates areexactly 1/2 the 50 Hz power line frequency.

    Lines a. lines v. resolution aspect h.resolution frame rate

    NTSC 525 484 242 4/3 427 29.94

    PAL 625 575 290 4/3 425 25

    SECAM 625 575 290 4/3 465 25

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    Color Encoding Principles for the PAL

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    Color Encoding Principles for the PAL

    All three systems use the same definition for luminance:

    The color encoding principles for the PAL system are the

    same as those of the NTSC system -- with one minor

    difference.

    In the PAL system, the phase of the R-Y signal is reversed

    by 180 degrees from line to line. This is to reduce color

    errors that occur from amplitude and phase distortion of the

    color modulation sidebands during transmission.

    Saying this more mathematically the chrominance signal

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    Saying this more mathematically, the chrominance signal

    for NTSC transmission can be represented in terms of the

    R-Y and B-Y components as

    The PAL signal terms its B-Y component U and its R-Y

    component V and phase-flips the V component (line by

    line) as:

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    Color Encoding Principles for the SECAM

    SECAM system differs very strongly from PAL and NTSC

    In SECAM the R-Y and B-Y signals are transmitted

    alternately every line. (The Y signal remains on for each

    line). Since there is an odd number of lines on any given

    scan, any line will have R-Y information on the first frame

    and B-Y on the second.

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    Furthermore, the R-Y and B-Y information is transmitted on

    different subcarriers. The B-Y sub-carrier runs at 4.25 MHz

    and the R-Y subcarrier runs at 4.4 MHz.

    In order to synchronize the line switching, alternate R-Y

    and B-Y sync signals are provided for nine lines during he

    vertical blanking interval following the equalizing pulses

    after the vertical sync.

    Summary

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    Summary

    Television is the radio transmission ofsound and pictures in

    the VHF and UHF ranges. The voice signal from a

    microphone is frequency-modulated. A camera converts a

    picture or scene into an electrical signal called the video or

    luminance Y signal, which amplitude-modulated

    Vestigial sideband AM is used to conserve spectrum space.

    The picture and sound transmitter frequencies are spaced

    4.5 MHz apart, with the sound frequency being the higher.

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    TV cameras use either a vacuum tube imaging device such

    as a vidicon or a solid-state imaging device such as the

    charged-coupled device (CCD) to convert a scene into a

    video signal.

    A scene is scanned by the imaging device to break it up into

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    A scene is scanned by the imaging device to break it up into

    segments that can be transmitted serially. The National

    Television Standards Committee (NTSC) standards call for

    scanning the scene in two 262 line fields, which are

    interlaced to form a single 525-line picture called a frame.

    Interlaced scanning reduces flicker.

    The field rate is 59.94 Hz, and the frame or picture rate is

    29.97 Hz. The horizontal line scan rate is 15,734 Hz or 63.6

    s per line.

    The color in a scene is captured by three imaging devices,

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    The color in a scene is captured by three imaging devices,

    which break a picture down into its three basic colors of red,

    green, and blue using color light filters. Three-color signals

    are developed (R, G, B). These are combined in a resistive

    matrix to form the Ysignal and are combined in other ways

    to form theIand Q signals.

    The I and Q signals amplitude-modulate 3.58-MHz

    subcarriers shifted 90 from one another in balanced

    modulators producing quadrature DSB suppressed signals

    that are added to form a carrier composite color signal. This

    A TV receiver is a standard superheterodyne receiver with

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    A TV receiver is a standard superheterodyne receiver with

    separate sections for processing and recovering the sound

    and picture. The tuner section consists of RF amplifiers,

    mixers, and a frequency-synthesized local oscillator for

    channel selection. Digital infrared remote control is used to

    change channels in the synthesizer via a control

    microprocessor.

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    The tuner converts the TV signals to intermediate

    frequencies of 41.25 MHz for the sound and 45.75

    MHz for the picture. These signals are amplified

    in IF amplifiers. The sound and picture IF signals

    are placed in a sound detector to form a 4.5-MHz

    sound IF signal. This is demodulated by a

    quadrature detector or other FM demodulator to

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    .The color signals are demodulated by two

    balanced modulators fed with 3.58-MHz

    subcarriers in quadrature. The subcarrier is

    frequency- and phase-locked to the subcarrier in

    the transmitter by phase-locking to the color

    subcarrier burst transmitted on the horizontal

    blanking pulse.

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    .To keep the receiver in step with the scanning

    process at the transmitter, sync pulses are

    transmitted along with the scanned lines of video.

    These sync pulses are stripped off the video

    detector and used to synchronize horizontal and

    vertical oscillators in the receiver. These

    oscillators generate deflection currents that sweep

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    .The color picture tube contains three electron

    guns that generate narrow electron beams aimed at

    the phosphor coating on the inside of the face of

    the picture tube. The phosphor is arranged in

    millions of tiny red, green, and blue color dot

    triads or stripes in proportion to their intensity and

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    The horizontal output stage, which provides

    horizontal sweep, is also used to operate a flyback

    transformer that steps up the horizontal sync

    pulses to a very high voltage. These are rectified

    and filtered into a 30- to 35-kV voltage to operate

    the picture tube. The flyback also steps down the

    horizontal pulses and rectifies and filters them into

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