Ed's AV Handbook.com
Home Theater & High Fidelity Stereo Audio


Chapter Four
Video Reproduction

Page 3

Batting practice for the audio/video pro and a primer for the novice 



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Digital Broadcast Television
Off-air, Cable, & Satellite



Digital Television
          A digital TV needs five video voltages from a video source to create each picture frame:

                      1.  Red     2.  Green
    3.  Blue     4.  Horizontal Sync      5.  Vertical Sync
                            (
H & V are the synchronizing start and end points of each line and frame.)

          A digital video source 
feeds component video [(Y) (R-Y) (B-Y)]* to a television video processor.  The television decoding processor converts component video to RGB and builds each picture frame progressively line by line.  The horizontal and vertical syncronizing data is decoded from encoded digital video data.  OLED, LCD, and plasma TV processors receive the progressive data but hold it until the processor has a complete frame of lines.  They then flash the entire frame to the screen.  DLP projectors build each frame progressively line by line on the screen.

*Handbook Note: Refer to NTSC Television for an explanation of component video.

Digital TV Broadcast
          Digital television broadcast bandwidth is not large enough to fit digital video as is.
Therefore Off-air, cable, and satellite TV broadcasters convert RGB to component video,
encode the syncronizing signals to digital data, and
employ MPEG video compression to reduce the data even further for broadcast.

               1. Component Video reduces RGB to (Y) (R-Y) (B-Y)       

               2. MPEG compression reduces video frame data.

In addition, local broadcast off-air 
ATSC TV tuners employ the ATSC 8VSB codec*.  Cable TV tuners employ QAM codec.

*Handbook Note: Codec is an acronym for Compress/Decompress


Handbook Note: Audio codec is discussed in Chapter 3 Page 3

MPEG Video Compression
          MPEG video compression is a cycle of incomplete compressed video frames inserted between complete uncompressed frames.  
         Compression eliminates redundant video frame data.
 This is somewhat similar to animation.  Unchanging elements of a moving scene, such as a static background of blue sky, are held and repeated frame after frame.  Changing data of motion and color passes without interuption.
          The amount of data reduction plus the number of incomplete frames placed between complete frames is determined by the version of MPEG used:
MPEG 1, MPEG 2, MPEG 4,
or MPEG HEVC.  

          Each succeeding version increases the amount data reduction needed to handle the bandwidth needs of each broadcast format.  
HDTV off-air broadcast employs MPEG2, Satellite TV broadcast uses MPEG4, UltraHDTV ATSC 3.0 broadcast will use MPEG HEVC.  

ATSC 8VSB
          The final stage of an off-air digital ATSC TV broadcast employs ATSC 8VSB RF modulation. 8VSB divides MPEG video and audio into interleaving packets of data called PIDS.  It is a continuing cycle of a video PID followed by an audio PID.  An ATSC 8VSB TV tuner receives, decodes, and restores the order of the data.

QAM
         Digital cable TV tuners use QAM RF modulation.  QAM is a variation of the previous interweaving of the audio and video data PID method.

Satellite TV Distribution
          TV programming is distributed via an umbrella of communication satellites to local broadcasters, cable companies, and “small dish” satellite TV providers.  Several dozen communication satellites hover in geo-synchronous orbit more than 22,000 miles above the equator.  They receive and redirect programming to your TV provider’s “tree farm” of large dish satellite antennas.
        Local broadcasters retransmit the programming via a broadcast tower to roof top antennas.  Cable TV providers redistribute programming via cable wired directly to residences. Small dish satellite providers up-link their programming to proprietary satellites which bounce the programming back to small roof top satellite dish antennas.  A small number of viewers receive their programming directly from the same satellites as the TV providers.  This requires a large 8 to 10 foot diameter dish antenna that must be aimed at the intended satellite.

Dish antenna, why ?
          A local TV broadcaster may engage thousands of watts of broadcast power.  As a result, the local TV receiving antenna can be as little as a “rabbit ear” antenna sitting on the TV.  
In contrast, satellites are limited to only five to fifteen watts of down-link broadcast power.
In addition, they broadcast at radio frequencies that are impractical for an electronic tuner to manage.  Consequently the satellite receiving system, small or large dish, needs a major assist.

         Assistance comes in the form of the satellite dish and a low noise block feedhorn (LNBF) mounted in front of the dish.  The dish magnifies the incoming signal hundreds of fold by reflecting and focusing the energy at a mounting arm that holds the actual antenna element located in the LNBF.
          The LNBF amplifies the signal still further and shifts the incoming signal from the 4GHz or 12GHz region to the more manageable neighborhood of 950 MHz to 1450 MHz.
 
In addition, small dish operators (DishNet / DirectTV) employ MPEG4 digital compression to squeeze hundreds of channels within their bandwidth.
          The combination of the dish antenna, the LNBF, and MPEG 4 produce a signal that a satellite receiver can easily handle and display on a TV.

Digital TV Result

         RGBHV (derived from a TV camera prismatic lens and CCD sensors) is converted to component video.  Broacasters digitally encode the video and compress its data.  A digital TV tuner and video processor restore the broadcasted data to RGBHV and recreate the original sequence of pixel framed images on the TV screen.

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Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288   Revised 2018