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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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