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HDTV Picture Formats

The type of screen your HDTV has (wide screen or standard screen) determines how programs are displayed on the screen. The picture format for an HDTV is a combination of aspect ratio and screen resolution and is different for standard-screen and wide-screen HDTVs. 

Aspect Ratio

An aspect ratio is the ratio of the width to the height of the TV screen. The aspect ratios differ because the television industry manufactures both standard-screen and wide-screen HDTVs to appeal to consumer viewing preferences.

A standard-screen HDTV has a 4:3 aspect ratio. The screen is 4 units wide for every 3 units tall.

HD Picture Format1

A screen resolution of 480p or 480i fills the screen.

A wide-screen HDTV has a 16:9 aspect ratio. The screen is 16 units wide for every 9 units tall.

HDPictureFormat2

A screen resolution of 720p or higher fills the screen.

Screen Refreshes

Note: The screen resolution (1080i, 480p, and so forth) is sometimes referred to as the scan rate. The terms are interchangeable.

  • The image displayed on a television screen is actually an illusion, a trick of the eye.  In reality, the complete image you see is actually made up of horizontal rows of glowing pixels that have been stimulated by an electron beam and only stay glowing for a fraction of a second.  In order to preserve the illusion of the complete image and to depict motion on the screen, these rows of pixels must be re-stimulated (or refreshed) often enough to trick the eye into believing the image is complete and continuous.
  • It turns out that a refresh rate of 60 times per second is just about perfect to preserve the illusion of a complete image on the screen to the human eye; refreshing less often produces flicker and refreshing more often wastes bandwidth.

There are two methods used for refreshing screens:

  1. Interlacing (i)
    • Interlacing means that every other line is updated with each refresh.  The complete screen is updated 30 times per second because it takes 2 refreshes to completely update the screen.  Since there are 60 half-screen refreshes per second, the screen is completely refreshed only 30 times per second.
    • Interlacing reduces the amount of bandwidth required to refresh the TV image.
    • On small TV sets, this type of refresh is hardly noticeable.
    • As analog TV sets become larger, interlacing may produce a noticeable flicker.
  2. Progressive (p)
    • Progressive means that every line is updated, in sequential order, with each refresh.  The complete screen is updated 60 times per second.
Progressive refresh requires more bandwidth than interlacing, but produces a much smoother picture with almost no flicker.  

Screen Resolution

The screen resolution indicates the amount of detail that the picture displays. Resolution is identified by the number of display lines on the screen. The techniques that an HDTV uses to "paint" the picture on the screen are referred to as progressive and interlaced.

For example, a resolution of 1080i indicates that the screen shows 1080 horizontal lines in an interlaced display, and 480p indicates that the screens shows 480 horizontal lines in a progressive display.

Note: The screen resolution (1080i, 480p, and so forth) is sometimes referred to as the scan rate. The terms are interchangeable.

With the progressive method, every pixel on the screen is refreshed simultaneously. The interlace method involves refreshing pixels in alternation; first the odd lines and then the even lines.

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Picture Effects

There are two methods stations broadcast HD signal that can result in three picture effects on your TV.

  1. Centercut
    • One of two methods a station broadcasts high definition content, and switches program sources and commercials between high definition, and standard definition, content. This fills the 16:9 screen of an HDTV, the cable processing equipment acquires the content displayed with a 4:3 ratio section for analog output.
    • The Result: Analog televisions have little change in their picture, some lost content on the sides, but a full display on the television screens

Centercut

    • Stations decide how to process their transmission
  1. Letterbox
    • One of two methods a station broadcasts high definition content, and switches program sources and commercials between high definition, and standard definition, content.
    • When a station is broadcasting high definition content, again filling the 16:9 screen of a HDTV, the cable processing equipment acquires the entire 16:9 content to be used for analog output.
    • The Result: Analog television displays the rectangular picture and black bars appear on the top and bottom to fill the screen.

Letterbox

    • The "letterbox" election causes most concerns and complaints.
    • Stations decide how to process their transmission
  1. Postage Stamp
    • The "Postage Stamp" or black bars on all four sides, result when the down-conversion of a high definition broadcast is enabled for "letterbox" processing and the program content changes from a 16:9 HD to 4:3 SD program or commercial.
    • The Result: Analog television displays a smaller picture flanked with black bars.

Postage Stamp

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