5. TV Broadcasting
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Communication Systems
Prof. Hesham Tolba
Alexandria University
Faculty of Engineering
Electrical Engineering Department
Alexandria
2020
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Prof. Hesham Tolba
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TV Broadcasting
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5. TV Broadcasting
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Introduction
q TV is one the best sources for news, entertainment &
communications.
q TV provides great entertainment programs that include action-
packed dramas, comedies, soap operas, sporting events, cartoon,
and movies.
q TV brings pictures and sounds from around the world into
millions of homes.
q On the average, a TV set is in use in each home for about 7
hours each day.
q Many scientists contributed to the development of TV.
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Introduction …
q Different Types of TV
Direct-View TV
Rear-Projection TV
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Front-Projection TV
Flat-Panel TV
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Introduction …
q Direct-View TVs
q Built around a single large Cathode Ray tube
(CRT)
q Curved screen reflects glare at all angles
Curved Screen
q Flat screen reflects glare in only 1
direction, allows watching at different angles
Flat Screen
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Introduction …
q Projection TVs
q Usually built with 3 CRTs (red, green &
blue) project the image onto a mirror, which
reflects it onto the screen
Front-Projection TV
q Front Projection TV: consists of a separate
projector and a screen, allows watching at
different angles in a dark room.
q Rear Projection TV: Picture looks best when
viewed straight on, dimmer if viewing from
the side, allows watching in different
lighting conditions.
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Introduction …
q Flat-Panel TVs
q Plasma: “Emissive" display panel is self-
lighting
q Each pixel is composed of three gas-filled
cells or sub-pixels (one each for red, green
and blue) Plasma (ionized gas) reacts with
phosphors in each sub-pixel to produce light
(red, green and blue)
Plasma TV
LCD TV
q LCD: “Transmissive" display light created by
light bulb, shines light through the display
q Use red, green and blue color filters in
place of phosphor dots
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Introduction …
q TV Development
q Digital Television
q System of transmitting and receiving TV signals in digital
codes, and displaying those signals on a digital TV set,
such as a HDTV
q HDTV (High Definition TV)
q Type of digital television that offers a greater number of
scanning lines (1080i or 720p)
q Picture and Sound quality comparable to movie theater
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Introduction …
q The transmission and reception of still or moving images are by
means of electrical signals.
q The basic procedures for creating television video and audio
signals involve with the use of a television camera.
q While the camera changes the light into electronic signals
(video signals), its microphone also changes the sound waves
into electronic signals (audio signals).
q The most common video signal broadcast by television stations
is called composite color video signal.
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Introduction …
q This signal produces a color picture when received on a color
set, and a black and white picture on a black and white TV set.
q A typical TV signal requires 4 MHz of bandwidth (BW).
q When sound is added, a TV signal requires a total 6 MHz of BW.
q The eye (or the brain rather) can retain the sensation of an
image for a short time even after the actual image is removed
(persistence of vision).
q 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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Persistence of Vision
q When
the frame interval is too long, the eye observes frame
flicker.
q The minimal frame rate (frames/second or fps or Hz) required to
prevent frame flicker depends on display brightness, viewing
distance.
q Higher frame rate is required with closer viewing and brighter
display.
q For TV viewing: 50-60 fps
q For Movie viewing: 24 fps
q For computer monitor: > 70 fps
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Persistence of Vision …
q As with frame merging, the eye can fuse separate lines into one
complete frame, as long as the spacing between lines is
sufficiently small.
q The
maximum vertical spacing between lines depends on the
viewing distance, the screen size, and the display brightness.
q For common viewing distance and TV screen size, 500-600 lines
per frame is acceptable.
q Similarly, the eye can fuse separate pixels in a line into one
continuously varying line, as
pixels is sufficiently small.
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long
as
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the
spacing
between
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Persistence of Vision …
q For some reason, the brighter the still image presented to the
viewer, the shorter the persistence of vision.
q 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.
q The basic idea here is that a single frame is scanned twice.
q The
first scan includes only
includes only the even lines.
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the
odd
lines,
the
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next
scan
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Progressive Scanning
Progressive scanning pattern
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Interlacing Scanning
Interlaced scanning pattern.
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Interlacing …
Interlaced scanning pattern with rate 1/60 sec.
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Interlaced & Progressive scan
q Interlaced Scan
q Used by NTSC, PAL & SECAM video
q Screen shows every odd line at one scan of the screen, with
the even lines in a second scan
q Screen shows 1/2 of the frame every 1/60 of a second (30fps)
q Flicker or artifacts
q Progressive Scan
q Used by most Digital TV and some DVD players
q Displays the entire frame in a single scan
q Screen shows whole frame every 1/60 of a second (60fps)
q Eliminates flickers, smoother picture
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Interlaced Vs. Progressive Scan …
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Basic Black & White TV
q In a basic black and white TV, a
single electron beam is used to scan
a phosphor screen.
q The scan is interlaced, that is, it
scans twice per photographed frame.
q The information is always displayed
Trace and Retrace
from left to right.
q After each line is written, when the
beam returns back to the left, the
signal is blanked.
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Basic Black & White TV …
qWhen the signal reached the bottom it is blanked until it
returns to the top to write the next line.
qNTSC has 525 vertical lines.
qHowever 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.
qNotice that the beam does not return directly to the top, but
zig-zags a bit.
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Fields/images
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Trace and Retrace
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Vertical Scanning Signal
qThe 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 Scanning Signal
qThe horizontal scan signal is very much the same.
qThe horizontal scan rate is 525*29.97 or 15,734 Hz ➜ 63.6 uS
are allocated per line.
qTypically about 10 uS of this is devoted to the blanking line
on the horizontal scan.
qThere are 427 pixels per horizontal
scan line and so each pixel is
scanned for approximately 125 ns.
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Horizontal Scanning Signal …
qThe
electron beam is analog modulated across the horizontal
line.
qThe
modulation then translates into intensity changes in
electron beam and thus gray scale levels on the picture screen
qHorizontal
blanking signal and synchronization pulse is quite
well defined.
qFor
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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Horizontal Scanning Signal …
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Scanning signals
qVertical blanking signal also has a number of synchronization
pulses included in it, as shown.
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The TV Composite Signal
q The television bandwidth is 6 MHz.
q The sub-carrier for the color is 3.58 MHz off the carrier for
the monochrome information.
q The sound carrier is 4.5 MHz of the carrier for the monochrome
information.
q There is a gap of 1.25 MHz on the low end and 0.25 MHz on the
high end to avoid cross talk
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The TV Composite Signal …
q The audio portion of the TV
signal that is transmitted is
limited to W=10 kHz.
q The peak frequency deviation in
the FM-modulated signal is
selected as 25 kHz, and the FMsignal bandwidth is 70 kHz.
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TV Frequency Allocations
2-4
VHF-Lo
54 MHz-72 MHz
5-6
VHF-Lo
76 MHz-88 MHz
7–13
VHF-Hi
174 MHz–216 MHz
UHF
470 MHz–746 MHz
14–59
NOTE: Natural breaks occur between channels 4 and 5; channels 6
and 7; and channels 13 and 14. Each channel is 6 MHz wide.
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Basic Components of TV Camera
q Lens
q Collects light from scene to form a sharp image
q Dichroic Mirrors
q Split the full-color image into 3 separate primary color
images
q Image Sensors (3)
q Each has an electron gun that shoots a beam of electrons to
scan the image.
q A separate electronic signal for each of the three primary
colors is created by the scanning process.
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Basic Components of TV Camera …
q Encoder
q Combines the three primary color signals to produce a full-
color picture on the TV screen.
q Microphone
q Picks up sounds and changes them into audio signals
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Transmission of TV Signals
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Basic Components of TV Receiver
q Tuner
q Selects only the signal from the station the viewer wants to
receive
q Decoder
q Changes video signal into primary color signals
q Picture Tube
q Transforms the primary color signals into patterns of light
that duplicate the scene in front of the camera
q Images are created when an electron beam scans back and
forth across the back side of a phosphor-coated screen
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Basic Components of TV Receiver …
q Phosphor-coated screen
q Three phosphors arranged as dots or stripes to emit red,
green and blue light
q Three colored phosphors blend together to produce the colors
of the original scene
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Reception of TV Signals
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Deflecting signals
Signal waveforms applied to (a) horizontal and (b)
vertical deflection.
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CRT
q
Components
1.
2.
There is a cathode and a pair (or more) of
anodes.
There is the phosphor-coated screen. There
is a conductive coating inside the tube to
soak up the electrons that pile up at the
screen-end of the tube.
q
The beam will always land in a tiny dot
right in the center of the screen.
q
That's why, if you
you will find that
coils of wires. On
get a good view of
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look inside any TV set,
the tube is wrapped in
the next page, you'll
steering coils.
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B&W TV CRT
q In a B&W TV, the screen is coated with white phosphor and the
electron beam "paints" an image onto the screen by moving the
electron beam across the phosphor a line at a time.
q To "paint" the entire screen, electronic circuits inside the TV
use the magnetic coils to move the electron beam in a "raster
scan" pattern across and down the screen.
q The beam paints one line across the screen from left to right.
It then quickly flies back to the left side, moves down
slightly and paints another horizontal line, and so on down the
screen
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B&W TV CRT …
q Raster Scan
q Standard TVs use an
interlacing technique when
painting the screen.
q The screen is painted 60
times per second but only
half of the lines are
painted per frame.
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Color TV CRT
q A color TV screen differs from a B&W screen in three ways:
q There are three electron beams that move simultaneously
across the screen. They are named the red, green and blue
beams.
q The screen is coated with red, green and blue phosphors
arranged in dots or stripes.
q On the inside of the tube, very close
to the phosphor coating, there is a
thin metal screen called a shadow
mask. This mask is perforated with
very small holes that are aligned with
the phosphor dots on the screen.
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B&W TV Transmitter
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B&W TV Receiver
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Color Television
q One of the great electrical engineering triumphs was the
development of color television in such a way that it remained
compatible with B&W television.
q A major driving force behind the majority of current color TV
standards was to allow B&W TVs to continue to be able to
receive a valid TV signal after color service was in place.
q Color television could simply be implemented by having cameras
with three filters (red, green & blue) and then transmitting
the 3 color signals over wires to a receiver with 3 electron
guns and 3 drive circuits.
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Color Television …
q Unfortunately, this view is not compatible with the previously
allocated 6 MHz bandwidth of a TV channel.
q It is also not compatible with previously existing monochrome
receivers.
q Therefore, modern color TV is carefully structured to preserve
all the original monochrome information and just add on the
color information on top.
q To do this, one signal, called luminance (Y) has been chosen to
occupy the major portion (0-4 MHz) of the channel.
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Color Television …
! signal (the monochrome TV signal) contains the brightness
information.
q The
! signal is created from the red, green and blue inputs of
the three cameras (one has a red filter, the 2nd has a green
filter and the 3rd has a blue filter) used to film a scene in a
movie using the governing equation
q The
! = #. %×' + #. )*×+ + #. ,,×q This is the "monochrome" part of the composite TV signal.
q It officially takes up the first 4 MHz of the 6 MHz bandwidth
of the TV signal (usually band-limited to 3.2 MHz).
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Color Television …
q Two signals are then created to carry the chrominance (C)
information.
q One of these signals is called "." and the other is called "/".
q They are related to the
', + & - signals by:
. = #. 0,×' − #. )0×+ + #. %,×/ = #. 2×' − #. 03×+ − #. %0×11/5/20
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Color Television …
q Thus,
' − ! = ' − #. %×' + #. )*×+ + #. ,,×= #. 4' − #. )*+ − #. ,,- − ! = - − #. %×' + #. )*×+ + #. ,,×= #. %' − #. )*+ + #. 3*q Consequently,
/ = #. 344 ' − ! cos %% − #. 8*%(- − !) sin %%
. = #. 344 ' − ! sin %% − #. 8*%(- − !) cos %%
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Color Television …
. is purple, the negative is green;
the positive polarity of / is orange, the negative is cyan.
q The positive polarity of
. is often called the "green-purple" axis information and
/ is often called the "orange-cyan" axis information.
q Thus,
q The human eye is more sensitive to spatial variations in the
"orange-cyan" than it is for the "green purple".
/ signal has a maximum bandwidth of
1.5 MHz and the "green purple" only has a maximum bandwidth of
0.5 MHz.
q Thus, the "orange-cyan" or
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Color Television …
q The
/ & . signals are both modulated by a 3.58 MHz carrier.
q They are modulated out of 90 degrees out of phase (QAM).
q These two signals are then summed together to make the
=
or
chrominance signal.
q The
/ signal is In-phase with the 3.58 MHz carrier wave;
. 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.
q The
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Color Television …
/ & . ) has the interesting
property that the magnitude of the signal represents the color
saturation, and the phase of the signal represents the hue.
q New chrominance signal (formed by
>?@AB = @CDE@F .// = ?HB
I@JFKEHLB = /! + .! = A@EHC@EKMF
/ & . signals are clearly phase sensitive, some sort
of phase reference must be supplied.
q Since the
q This reference is supplied after each horizontal scan and is
included on the "back porch" of the horizontal sync pulse.
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Color Television …
q The phase reference consists of 8-
10 cycles of the 3.58 MHz signal.
q It is called the "color burst" and
looks something like this
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Color Television …
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Color Television …
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Color Television …
q Conversion between
'+- and !/.
! = #. 0** ' + #. )34 + + #. ,,8 / = #. )*2 ' − #. 04) + − #. %0, . = #. 0,0 ' − #. )0% + + #. %,, -
' = ,. # ! − #. *)2 / + #. 20# .
+ = ,. # ! − #. 040 / − #. 284 .
- = ,. # ! − ,. ,#3 / + ,. 4## .
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Color Television …
q Bandwidth of Chrominance Signals
q With real video signals, the chrominance component typically
changes much slower than luminance.
q The
human eye is less sensitive to changes in chrominance
than to changes in luminance
q The eye is more sensitive to the orange-cyan range ( / ) (the
color of face!) than to green- purple range (.)
q The above factors lead to
/: bandlimitted to 1.5 MHz
.: bandlimitted to 0.5 MHz
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Color Television …
q Multiplexing of Luminance and Chrominance
q 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).
q The two chrominance components ( / &
. ) are multiplexed onto
the same sub-carrier using QAM.
q The resulting video signal including the baseband luminance
signal plus the chrominance components
called composite video signal.
modulated
to
N! is
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Color Television …
q
/ & . are multiplexed using QAM as shown.
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Color Television …
q Color TV Signal
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Color Television …
q Color TV Signal
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Color Television …
q In NTSC Luminance is AM VSB, the Chroma is QAM
/&., and the
Aural FM.
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Color Television …
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Color Television …
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Color Television …
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Color Television …
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Color Television …
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Color Camera Block Diagram
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Color TV Transmitter
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Color TV Transmitter
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Color Decoder Block Diagram
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Color TV Receiver
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FM Stereo Trasmitter
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FM Stereo Receiver
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TV Standards
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Principal TV Systems
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TV Standards
q NTSC (National Television System Committee)
q Used in U.S., Japan, and several Asian countries
q Made up of 525 lines (262.5 odd-numbered and 262.5 even-
numbered lines)
q Produces 525 lines per frame at 30 frames per second
q PAL (Phase alternating lines)
q PAL is originated from Germany
q Made up of 625 lines per frame at 25 frames per second
q SECAM (Sequence electronique couleur avec memoir)
q SECAM is originated from France
q Made up of 625 lines per frame at 25 frames per second
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PAL, SECAM & NTSC …
q The largest difference between the three systems is the
vertical lines.
q NTSC uses 525 lines (interlaced) while both PAL and SECAM use
625 lines.
q NTSC frame rates are slightly less than 1/2 the 60 Hz power
line frequency, while PAL and SECAM frame rates are exactly 1/2
the 50 Hz power line frequency.
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PAL, SECAM & NTSC …
NTSC
PAL
SECAM
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Prof. Hesham Tolba
Lines/Frame
a. lines
525
625
625
484
575
575
V. Resolution
aspect
242
290
290
Prof. Hesham Tolba
Communication Systems
H. Resoulation
Frame Rate
4/3
4/3
4/3
29.94
25
25
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5. TV Broadcasting
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PAL, SECAM & NTSC …
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Communication Systems
79
PAL, SECAM & NTSC …
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40
5. TV Broadcasting
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PAL, SECAM & NTSC …
q World TV Standards
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81
PAL Coding Principle
q All three systems use the same definition for luminance.
q The color encoding principles for the PAL system are the same
as those of the NTSC system with one minor difference.
q In the PAL system, the phase of the
' − ! signal is reversed by
180 degrees from line to line.
q This is to reduce color errors that occur from amplitude and
phase distortion
transmission.
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Communication Systems
Prof. Hesham Tolba
of
the
color
Prof. Hesham Tolba
modulation
Communication Systems
sidebands
during
82
41
5. TV Broadcasting
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PAL Coding Principle …
q The chrominance signal for NTSC transmission can be represented
in terms of the ' − ! and - − ! components as
="#$% =
-−!
'−!
sin R& S +
cos R& S
0. #%
,. ,8
PAL signal terms its - − ! component P
and its ' − !
component Q and phase-flips the Q component (line by line) as:
q The
='() =
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P
Q
sin R& S ±
cos R& S
0. #%
,. ,8
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Communication Systems
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PAL Coding Principle …
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Communication Systems
84
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5. TV Broadcasting
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PAL Coding Principle …
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85
SECAM Coding Principle
q SECAM system differs very strongly from PAL and NTSC
' − ! and - − ! signals are transmitted alternately
every line. (The ! signal remains on for each line).
q In SECAM the
q Since there is an odd number of lines on any given scan, any
line will have ' − ! information on the first frame and B-Y on
the second.
' − ! and
subcarriers.
q The
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Communication Systems
Prof. Hesham Tolba
-−!
information
Prof. Hesham Tolba
is
transmitted
Communication Systems
on
different
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43
5. TV Broadcasting
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SECAM Coding Principle
- − ! sub-carrier runs at 4.25 MHz and the ' − ! subcarrier
runs at 4.4 MHz.
q The
' − ! and
lines during the
equalizing pulses
q In order to synchronize the line switching, alternate
- − ! sync signals are provided for nine
vertical blanking interval following the
after the vertical sync.
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87
NTSC Bandpass Characteristics (Color)
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Communication Systems
88
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5. TV Broadcasting
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Summary
q Television is the radio transmission of sound and pictures in
the VHF and UHF ranges.
q The voice signal from a microphone is frequency-modulated.
q 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.
q The
picture and sound transmitter frequencies are spaced 4.5
MHz apart, with the sound frequency being the higher.
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Summary …
q TV cameras use either a vacuum tube imaging device such as a
vidicon or a solid-state imaging device such as the chargedcoupled device (CCD) to convert a scene into a video signal.
q A scene is scanned by the imaging device to break it up into
segments that can be transmitted serially.
q The
National Television Standards Committee (NTSC) standards
call for scanning the scene in two 262½ line fields, which are
interlaced to form a single 525-line picture called a frame.
Interlaced scanning reduces flicker.
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Communication Systems
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45
5. TV Broadcasting
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Summary …
q 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
µs per line.
q 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.
q Three-color signals are developed (R,
G, B).
q These are combined in a resistive matrix to form the
Y signal
and are combined in other ways to form the I and Q signals.
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Summary …
/ & . 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.
q The
q This color signal is then used to modulate the AM picture
transmitter along with the ! signal.
q A TV receiver is a standard superheterodyne receiver with
separate sections for processing and recovering the sound and
picture.
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Communication Systems
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5. TV Broadcasting
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Summary …
q The
tuner section consists of RF amplifiers, mixers, and a
frequency-synthesized local oscillator for channel selection.
q Digital infrared remote control is used to change channels in
the synthesizer via a control microprocessor.
q The tuner converts the TV signals to intermediate frequencies
of 41.25 MHz for the sound and 45.75 MHz for the picture.
q These signals are amplified in IF amplifiers.
q The sound and picture IF signals are placed in a sound detector
to form a 4.5-MHz sound IF signal.
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Summary …
q This
is demodulated by a quadrature
demodulator to recover the sound.
detector
or
other
FM
q Frequency-multiplexing techniques similar to those used in FM
radio are used for stereo TV sound.
q The picture IF is demodulated by a diode detector or other AM
demodulator to recover the Y signal.
q The
color signals are demodulated by two balanced modulators
fed with 3.58-MHz subcarriers in quadrature.
q 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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Communication Systems
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5. TV Broadcasting
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Summary …
q To keep the receiver in step with the scanning process at the
transmitter, sync pulses are transmitted along with the scanned
lines of video.
q These sync pulses are stripped off the video detector and used
to synchronize
receiver.
horizontal
and
vertical
oscillators
in
the
q These oscillators generate deflection currents that sweep the
electron beam in the picture tube to reproduce the picture.
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Summary …
q 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.
q The phosphor is arranged in millions of tiny red, green, and
blue color dot triads or stripes in proportion to their
intensity and generate light of any color depending upon the
amplitude of the red, green, and blue signals.
q The
electron beam is scanned or deflected horizontally
vertically in step with the transmitted video signals.
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Prof. Hesham Tolba
Communication Systems
and
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5. TV Broadcasting
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Summary …
q Deflection signals from the internal sweep circuits drive coils
in a deflection yoke around the neck of the picture creating
magnetic fields that sweep the three electron beams.
q 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.
q These are rectified and filtered into a 30- to 35-kV voltage to
operate the picture tube.
q The flyback also steps down the horizontal pulses and rectifies
and filters them into low-voltage dc supplies that are used to
operate most of the circuits
in the
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49