Presented by: Yara Naguib May Mohamed Dr.Mohamed Abd El...
Transcript of Presented by: Yara Naguib May Mohamed Dr.Mohamed Abd El...
Presented by: Amany Mohamed Yara Naguib May Mohamed Sara Mahmoud Maha Ali Supervised by: Dr.Mohamed Abd El Ghany
Analogue Terrestrial TV.
No satellite Transmission
Digital Satellite TV.
Uses satellite and provides wide channel range.
Cable TV or (CAT).
Distributes audiovisual content using co-axial cables.
New Technologies:
Digital Terrestrial TV.
HD TV.
Uses NTSC, PAL, SECAM.
Web-TV.
IP TV.
The cathode ray tube (CRT) is a vacuum tube containing one or more
electron guns, and a phosphorescent screen used to view images. It has a
means to accelerate and deflect the electron beam onto the screen to create
the images.
Color television is a television transmission technology that includes
information on the color of the picture, so the image can be displayed in
color on the television screen. It is an improvement on the earliest
television technology, monochrome or black and white television, in which
the image is displayed in grey scale.
A cathode-ray tube (CRT) television displays an image by scanning a beam
of electrons across the screen in a pattern of horizontal lines known as a
raster. At the end of each line the beam returns to the start of the next line;
the end of the last line is a link that returns to the top of the screen. As it
passes each point, the intensity of the beam is varied.
A color television system is identical except that an additional signal
known as chrominance controls the color of the spot.
The chrominance is the signal used in video systems to convey the color
information of the picture
When analog television was developed, the luminance signal has to be
generated and transmitted at the same time (displayed on CRT).
It is therefore essential to keep the raster scanning in the device generating
the signal in exact synchronization with the scanning in the television.
The human eye has a characteristic called Phi phenomenon.
Quickly displaying successive scan images will allow the apparent illusion
of smooth motion.
The quick movement of the image can be partially solved using a long
persistence phosphor coating on the CRT, so that successive images fade
slowly.
The maximum frame rate depends on the bandwidth of the transmission
system and the number of horizontal scan lines in the image.
A frame rate of 25 or 30 Hz is a satisfactory compromise, while the process
of interlacing two video fields of the picture per frame is used to build the
image.
Plasma screens and LCD screens have been used in analog television
sets. These types of display screens use lower voltages than older CRT displays.
Many dual system television receivers, equipped to receive both analog transmissions and digital transmissions have analog tuner receiving capability and must use a television antenna.
A TV channel usually consists of 2 signals.
The picture information is transmitted using amplitude modulation.
The sound information is transmitted using frequency modulation.
Different methods of modulation are used to minimize interference between the picture and sound signals.
The picture information is transmitted on a separate carrier located 4.5 MHz lower in frequency than the sound carrier.
Vestigial sideband modulation technique is used in Television system.
The video signal is most often generated by a TV camera.
TV camera is used to convert the scene to an electric
signal this is done by transmitting a voltage of 1 V for
black and 0 V for white.
The scene is divided into smaller segments that can be
transmitted serially over a period of time.
Scanning is a technique that divides a rectangular scene
into individual lines.
the scene is subdivided into many fine horizontal lines
called scan lines.
Each line represents a very narrow portion of light
variations in the scene.
TV standards call for the scene to be divided into a
maximum of 525 horizontal lines.
The greater the number of scan lines, the
higher the resolution and the greater the detail that can be
observed.
The RF Tuner consists of an RF amplifier, mixer, and local
oscillator.
The tuner is used to select the TV channel to be viewed by
choosing an appropriate intermediate frequency (IF).
The receiver employs two or three stages of intermediate
frequency (IF) amplifiers.
The output from the last IF stage is demodulated to recover the
video signal.
The signal that carries picture information is amplified and
coupled to the picture tube
The picture tube converts the electrical signal back into picture
elements of the same degree of black and white.
The picture tube is very similar to the cathode-ray tube
used in an oscilloscope.
The glass envelope contains an electron-gun structure that
produces a beam of electrons.
When electron beam strikes the screen, This beam is
deflected by a pair of deflecting coils.
The video signal is fed to the grid or cathode of the
picture tube.
Relation between signal voltage and the control grid.
IF signal consists of a video carrier wave and the sound carrier
A demodulator recovers the video signal
New carrier frequency is sent to an FM demodulator
Advantage of intercarrier sound
So the FM sound carrier is then demodulated, amplified, and used to drive a loudspeaker.
Color signal conveys picture information for each of the
RGB components of an image
Not simply transmitted as three separate signals, because
• Monochrome receivers
• Three times the bandwidth of existing television
• Problems with signal transmission
The RGB signals are converted into YUV form
Y signal represents the lightness and darkness
(luminance) of the colors in the image.
The Y signal is ideal for transmission as the luminance
signal.
The U and V signals are
"color difference" signals
The U signal is the difference
between the B signal and the
Y signal
The V signal is the difference
between the R signal and the
Y signal
Advantage of this scheme
Two demodulators
• X demodulator
• Z demodulator
Color difference signals are further matrixed into
three color difference signals.
R, G, B signals in the receiver are electronically derived
by matrixing
Low resolution portion of the Y signal cancel out
The higher resolution portions of the Y signals do not
cancel out
U and V are transmitted by using quadrature amplitude
modulation
Before transmission, the subcarrier, a 3.58 MHz sine
wave, is removed from the active portion of the video
Subcarrier sidebands that carry all the information of U
and V signals are produced
The subcarrier sidebands are known as "Chroma" or "chrominance".
It is a 3.58 MHz sine wave which changes phase as compared to the subcarrier according to the changing values of U and V representing hue of the color
It also changes amplitude which represents the approximate saturation of a color
The chrominance signal represents only the U signal, after 70 ns V signal, 70 ns later –U and 70 later –V
To extract U, a synchronous demodulator samples the Chroma every 280 nanoseconds
The output is a train of discrete pulses having amplitude same as original U(discrete time analog samples)
The pulses are low-pass filtered (analog continuous time signal)
To extract V, a 90 degree shifted subcarrier samples the Chroma signal every 280 nanoseconds
The mentioned process uses the subcarrier HOW?!! The receiver must reconstitute the subcarrier.
Color burst, is transmitted during the back porch of each scan line
Phase-locked loop (oscillator) locks onto the signal to achieve a phase reference
Such a process causes poor color reproduction due to phase errors in the received signal
Synchronizing pulses added to the video signal at the end of every scan line
video frame remains locked to reconstruct the image
The Sync signal separator circuit separates the composite signal into
2 signals.
a sync separator circuit detects the sync voltage levels
Loss of horizontal synchronization results in an unwatchable picture
Loss of vertical synchronization produces an image rolling up or
down the screen.
Signals the beginning of each new video line.
A back porch, which is used as a reference level to remove any DC
components from the floating (AC-coupled) video signal.
For composite color signals, the clamping occurs during the
horizontal sync pulse
It is a series of pulses that occur between fields to signal the monitor
and prepare to scan the next field.
Some lines contain only HSYNC pulses, while others contain
VSYNC pulses
Changes to the image are kept in step with the vertical
synchronization pulse to avoid visible discontinuity of the image.
If the image is updated with a new one while being transmitted, the
display shows a mishmash of both frames
Vertical synchronization times frame buffer to ensure that only
whole frames are seen on screen.
the program has to wait until the video controller finishes
transmitting the image
Triple buffering reduces this latency significantly.
Channel selector switch
The fine tuning control
The hold control
The brightness control
The contrast control
The volume and tone controls
http://www.tecnicontrol.pt/en/wiki/item.html?id=81-the-various-methods-of-tv-transmission
http://www.daenotes.com/electronics/communication-system/tv-television-transmitter
https://en.m.wikipedia.org/wiki/Analog_television