"Aspect Ratio" refers to the 'shape' of the video image, and is commonly referred to by the ratio between the horizontal and vertical dimensions, much in the same way as the gradient of roads is labeled on road signs.
The first defined aspect ratio was 4:3 or 1.33:1, which was also known as "Academy Standard". This was based upon the standard shape of a theatre stage, and was used for almost all films up until the early 1950's.
When televisions were introduced, they were also built to the same shaped screen, because it was the accepted ratio of the film industry. The problem was that TV became too popular and as a result people were staying at home and watching television, instead of visiting their local cinema.
Movie studios started to look for ways to entice people back by offering them something they could not experience in the home, such as 3D and wider images.
Many different options were tried, and the two most popular ratios survive today - 1.85:1, also known as "Academy Flat" and 2.35:1 which is known as "Panavision / Cinemascope".
Widescreen Television has an aspect ratio of 1.78:1 or 16:9, which most modern programming is recorded in. The correct aspect ratio of an image in usually maintained by a process known as letterboxing, whereby the image is 'squeezed' vertically, and black bars are added at the top and bottom of the image until the correct shape is achieved within the 4:3 frame.
The process unfortunately results in a loss of image quality as vertical information is lost in the 'squeezing' process. The most efficient use of 16:9 is known as "Full Height Anamorphic" where the video image is horizontally 'squeezed' to make use of the full height of the image frame.
|
|
DVD players can accept the two main aspect ratios - 4:3 and 16:9. To help the DVD player display these aspect ratios correctly, you must tell the player what aspect ratio your television or display device is.
Within the DVD player's set-up menu, you can configure the machine to display Anamorphic 16:9 footage 'natively' to a 16:9 screen, or if you want to display 16:9 footage on a 4:3 screen, whether to 'letterbox' or 'pan and scan' the image.
When a DVD player 'letterboxes' an image, it takes every 4 lines and re-samples them down to 3 until the correct aspect ratio is achieved (the black bars are added electronically by the player).
For 'pan and scan' the DVD player 'stretches' the image to the correct shape as it would for a 16:9 display, and then crops the sides of the image, to display the central 4:3 section.
When encoding video for DVD, the encoding system is told the aspect ratio of the source footage so that this information can be stored in the 'header' of the MPEG-2 video stream.
When the footage is brought into the authoring application, the aspect ratio is set within each video track, with a 'preference' as to whether 16:9 footage should be 'letterboxed' or 'pan and scan' - this gives the DVD Author the opportunity to 'force' the display format, ignoring any preference set within the DVD player (Most filmmakers do not like for their image to be cropped via 'pan and scan' as it alters the original intended framing, although television programmes are often filmed 'pan and scan safe' for broadcast).
'Academy Flat' and 'Cinemascope' are both wider images than a 16:9 display and so these would also appear letterboxed when displayed correctly. |
