Television

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Television
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Simplified TV System
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Simplified TV System

• The aural or sound transmitter is an FM system
  similar to broadcast FM radio.
  
• The video, or picture, signal is
  amplitude-modulated onto a carrier.
  
• Thus, the composite transmitted signal is a
  combination of both AM and FM principles.
  

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Transmitter Principles

• The TV camera converts a visual picture or scene
  into an electrical signal. The camera is thus a
  transducer between light energy and electrical
  energy.
• At the receiver, the CRT picture tube is the
  analogous transducer that converts the electrical
  energy back into light energy.

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Transmitter Principles

• The microphone and speaker are the similarly
  related transducers for the sound transmission.
  
  
• There are actually two more transducers, the
  sending and receiving antennas. They convert
  between electrical energy and the electromagnetic
  energy required for transmission through the
  atmosphere.

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Transmitter Principles

• The diplexer feeding the transmitter antenna
  feeds both the visual and aural signals to the
  antenna while not allowing either to be fed back
  into the other transmitter.
• Without the diplexer, the low-output impedance of
  either transmitter's power amplifier would
  dissipate much of the output power of the other
  transmitter.

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Transmitter Principles TV Cameras

• The most widely used image pickup device is the
  charge couple device (CCD). CCD cameras are used
  in many applications such as broadcasting and
  imaging.
• The CCD is a solid-state chip consisting of
  thousands or millions of photosensitive cells
  arranged in a two-dimensional array. When light
  (photons) strike the CCD surface, the light
  information is converted to an electronic analog
  of the light. The electronic information is then
  shifted out of the device serially in what is
  call a bucket brigade.

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Transmitter Principles TV Cameras
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Transmitter Principles - Scanning
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Transmitter Principles - Scanning

• In this simplified system, the camera focuses the
  letter "T' onto the photosensitive cells in the
  CCD imaging device.
  
• Instead of a million cells, this system has just
  30, arranged in 6 rows with 5 cells per row.
  
• Each separate area is called a pixel, which is
  short for "picture element." The greater the
  number of pixels, the better the quality (or
  resolution) of the transmitted picture.

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Transmitter Principles - Scanning

• The letter "T" is focused on the light-sensitive
  area so that all of rows 1 and 6 are
  illuminated.
  
• All of row 2 is dark and the centers of rows 3,4,
  and 5 are dark.
  
• If we scan each row sequentially and if the
  retrace time is essentially zero, then we have a
  sequential breakup of information.
  
• The retrace interval is the time it takes to move
  from the end of one line back to the start of the
  next lower line.

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Transmitter Principles - Scanning

• The variable light on the photosensitive cells
  results in a similar variable voltage being
  developed at the CCD's output.
  
• The visual scene has been converted to a video
  (electrical) signal and can now be suitably
  amplified and used to amplitude- modulate a
  carrier for broadcast.
• The picture for broadcast National Television
  Systems Committee (NTSC) TV has been standardized
  at a 43 ratio of the width to height. This is
  termed the aspect ratio and was selected as the
  most pleasing picture orientation to the human
  eye.

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Transmitter/Receiver Synchronization

• When the video signal is detected at the
  receiver, some means of synchronizing the
  transmitter and receiver is necessary
  
• 1. When the TV camera starts scanning line 1, the
  receiver must also start scanning line 1 on the
  CRT output display. You do not want the top of a
  scene appearing at the center of the TV screen.
• 2. The speed that the transmitter scans each line
  must be exactly duplicated by the receiver
  scanning process to avoid distortion in the
  receiver output.
• 3. The horizontal retrace, or time when the
  electron beam is returned back to the left-hand
  side to start tracing a new line, must occur
  coincidentally at both transmitter and receiver.
  You do not want the horizontal lines starting at
  the center of the TV screen.
• 4. When a complete set of horizontal lines has
  been scanned, moving the electron beam from the
  end of the bottom line to the start of the top
  line (vertical flyback or retrace) must occur
  simultaneously at both transmitter and receiver.

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Transmitter/Receiver Synchronization

• In the scanning process for a television, the
  electron beam starts at the upper left-hand comer
  and sweeps horizontally to the right side.
  
• It then is rapidly returned to the left side, and
  this interval is termed horizontal retrace.
  
• An appropriate analogy to this process is the
  movement of your eye as you read this line and
  rapidly retrace to the left and drop slightly for
  the next line.

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Transmitter/Receiver Synchronization

• When all the horizontal lines have been traced,
  the electron beam must move from the lower
  right-hand corner up to the upper left-hand
  corner for the next "picture."
• This vertical retrace interval is analogous to
  the time it takes the eye to move from the bottom
  of one page to the top of the next.
  

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Horizontal Synchronization
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Horizontal Synchronization

• The Transmitter send a synchronization (sync)
  pulse between every line of video signal so that
  perfect transmitter-receiver synchronization is
  maintained.
• Three horizontal sync pulses are shown along with
  the video signal for two lines.
  
• The actual horizontal sync pulse rides on top of
  a so-called blanking pulse, as shown in the
  figure. The blanking pulse is a strong enough
  signal so that the electron beam retrace at the
  receiver is blacked out and thus invisible to the
  viewer.

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Horizontal Synchronization

• The interval before the horizontal sync pulse
  appears on the blanking pulse is termed the front
  porch, while the interval after the end of the
  sync pulse, but before the end of the blanking
  pulse, is called the back porch.
• Notice that the back porch includes an
  eight-cycle sine-wave burst at 3,579,545 Hz. It
  is appropriately called the color burst, because
  it is used to calibrate the receiver color
  subcarrier generator.
• Naturally enough, a black-and-white broadcast
  does not include the color burst.

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Horizontal Synchronization

• The two lines of video picture signal shown in
  the figure can be described as follows
  
• Line 2 It starts out nearly full black at the
  left-hand side and gradually lightens to full
  white at the right-hand side.
  
• Line 4 It starts out medium gray and stays there
  until one-third of the way over, when it
  gradually becomes black at the picture center. It
  suddenly shifts to white and gradually turns
  darker gray at the right-hand side.

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Vertical Synchronization
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The Television Signal

• The maximum modulating rate for the video signal
  is 4 MHz.
  
• Because it is amplitude-modulated onto a carrier,
  a bandwidth of 8 MHz is implied.
  
• However, the FCC allows only a 6-MHz bandwidth
  per TV station, and that must also include the FM
  audio signal (only is a relative term here
  because 6 MHz is enough to contain 600 AM radio
  broadcast stations of 10 kHz each).

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The Television Signal
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The Television Signal
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The Television Signal

• The lower visual sideband extends only 0.25 MHz
  below its carrier with the remainder filtered
  out.
  
• The upper sideband is transmitted in full.
• The audio carrier is 4.5 MHz above the picture
  carrier with FM sidebands as created by its
  25-kHz deviation.

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Principle of Colour Television
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Principle of Colour Television

• The colour camera scan scene in unison, with red,
  green and blue colour content separated into
  three different signals.
  
• This process is accomplished within the camera.
• The lens focuses the scene onto a beam splitter
  that feeds three separate light filters.

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Principle of Colour Television
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Principle of Colour Television

• Color receiver CRTs are a marvel of engineering
  precision.
  
• They are made up of triads of red, blue, and
  green phosphor dots.
  
• The trick is to get the proper electron beam to
  strike its respective colored phosphor dot. This
  is accomplished by passing the three beams
  through a single hole in the shadow mask, as
  shown in the figure.
• The shadow mask prevents the "red" beam from
  spilling over onto an adjacent blue or green
  phosphor dot, which would certainly destroy
  colour rendition.

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Digital Television

• The DTV standard is based on the standard
  recommendations by the Advanced Television System
  Committee (ATSC).
  
• This standard provides for the transmission of
  television programs in the HDTV screen format, 16
  X 9, as shown the figure.
  
• It also provides for the transmission of a
  standard definition television (SDTV) format that
  provides a digital picture with comparable
  resolution to analog NTSC formats.

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Digital Television
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Digital Television

• The format typically used to convert the analog
  video to a digital format is the ITU- R. 601
  422 format. This is an international standard
  for digitizing component video.
• The base sampling frequency for the ITU-R 601
  standard is 3.375 MHz. The 422 represents the
  sample rate for the following elements of a
  component video signal.
• luminance Y red-luminance R-Y and
  blue-luminance B-Y

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Digital Television

• A video signal is composed of green, red, and
  blue components.
  
• In addition to providing green color information,
  the green channel provides the luminance
  information. Luminance is the black-and-white
  detail.
• The R- Y and B- Y values provide the
  color-difference values.
  
• These components, the Y, R- Y, and B- Y, are then
  converted to a digital signal using a PCM
  technique. The base sample rate for the ITU-R 601
  standard is 3.375 MHz, and 10-bit sampling is
  used.
• This means that the luminance channel is sampled
  at four times the base rate, and the R- Y and B-
  Y channels are sampled at two times the base
  rate.

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Digital Television

• The three digital samples (Y, R- Y, and B- Y) are
  time-division-multiplexed together with a
  resulting serial data bit rate of 270 Mbps.
  
• This data rate must undergo some form of data
  compression so that the data will fit into the
  6-MHz bandwidth available for broadcast
  television. The video-compression technique
  selected for DTV transmissions is MPEG2. (MPEG is
  an abbreviation for the Motion Pictures Expert
  Group.)
• The compression techniques rely on the
  redundancies in the video signal.

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Digital Television
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Digital Television

• The digital compression technique specified for
  digital television, as defined by ATSC document
  N/52, details the digital audio compression
  (AC-3) standard developed by Dolby Laboratories.
  
• This system provides five full-bandwidth audio
  channels (3 Hz to 20 kHz). The five channels are
  for the left, center, right, and left-right
  surround-sound channels.
• The standard also provides one low-frequency
  enhancement channel, which has a reduced
  bandwidth (3 Hz to 120 Hz).

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Digital Television

• The new audio system is commercially called the
  5.1 Channel Input.
  
• The standard provides for various sample rates
  and input word lengths (up to 24 bits) for
  compatibility to the many available digital audio
  encoding formats.
• The six audio outputs are multiplexed together,
  which results in a 5.184-Mbps data stream. This
  data stream is then compressed to a 3S4-kbps data
  stream.