TV WIDE-SCREEN MODES
INTRODUCTION
There is much confusion about what constitutes "wide-screen" television, and how it relates to the standard shape of TV screen that TV started with. Why is wide-screen being introduced anyway? This is an attempt to explain clearly in a simplified and as non-technical a way as possible the relationship between wide and classic TV pictures. Standard-shaped pictures, just like they've been since the start of conventional analogue broadcast TV, are known as "4:3 format". This means that the ratio of picture width and height, also known as the "aspect ratio", is 4:3. Thus a true 4:3 TV screen could be, for instance, 40cm wide by 30cm high, or 12 inches wide by 9 inches high, or any other dimensions which have the ratio 4:3. These dimensions were based on the "Academy" film format shape for motion pictures, which was in normal use in cinemas before the second-world-war when the TV system was being defined. Naturally TV was made to be similar, especially as film was then the only way of recording pictures.
WHY WIDE SCREEN?
After the second-world-war, partly as a response to the growing threat from television, wide-screen films started to become popular at the cinema; "Cinemascope" format films being probably the best known example, although there are others. These became successful because wide screen format is a more natural representation of the world, especially on a large screen. The reason for this is that human beings have two eyes, and if you take in to account the total field of vision from both eyes this is much wider than it is high, as the eyes are side-by-side. If you stand facing a blank wall, and mark on the wall the extent that you can see, you will end up with a patch approximately 16 units wide by 9 units high, i.e. an "aspect ratio" of approximately 16:9. It was thus just a matter of time before wide-screen television was being suggested as a possibility, especially as the larger screen sizes which make wide-screen so attractive were becoming technically feasible. Television has not been historically known for the universal adoption of world technical standards, but one of the significant world-wide standards to be agreed was that wide-screen TV would be universally based on an aspect ratio of 16:9. Interestingly, Cinemascope movies are still significantly wider than this, and we haven't mentioned "Cinerama" either, which was so wide it took three projectors to achieve! However, 16:9 represents a good compromise.
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A classic 4:3 aspect ratio picture on a 4:3 screen. |
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A 16:9 wide-screen picture on a 16:9 screen. |
INTRODUCTION OF WIDE SCREEN
Wide-screen TV can not be universal overnight. Some programmes, especially older archive programmes will still be in 4:3, whilst others will be new 16:9 material. This means that for the forseeable future, both picture shapes need to co-exist together. Unfortunately, anyone familiar with basic DIY will tell you that if you want to fit a piece of wood 16 inches wide by 9 inches high in a space 12 inches wide by nine inches high, it won't fit! Something has got to give. And that's the basic problem with having the two formats running together. In the end it's all down to simple geometry. There are two sides to this problem; how do you display a true wide-screen 16:9 picture satisfactorily on an old 4:3 TV, and secondly, how do you display a non-wide-screen 4:3 picture on a 16:9 wide-screen set? The following describes the options available.
THE 4:3 VIEWER
Most legacy 4:3 TV sets have no way for the viewer to change the basic picture shape. In the case of the analogue broadcasts these TVs were designed for, the decision as to how to display the picture had to be made by the broadcaster before transmission. If a 4:3 TV is being used with a digital set-top-box then this box can usually be configured to automatically give acceptable results with wide-screen material. There are several options for viewing a 16:9 programme on a 4:3 screen. These are:
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Do nothing. The true 16:9 picture is "anamorphic", which simply means it fills all of the active picture area, so if we do nothing, the 16:9 anamorphic picture will exactly fill the 4:3 screen. However, it will be badly distorted, as the image needs to be stretched vertically to fit. All of the screen area is used, but the distortion of the image means tall thin people, and is clearly unacceptable! | |
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Make the 16:9 picture exactly fit the width, and have black bars top and bottom to make up the 4:3 shape. This means that the image is not distorted, but not all of the screen area is used. This is often called "letter box" mode, and has been a popular method of transmitting Cinemascope movies on TV for many years, especially in mainland Europe, where it is well accepted. This is the only method that shows all of the original image whilst giving the largest possible picture area, and with no distortion. | |
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Make the picture exactly fit the height, but "crop" the image to 4:3, such that the left and right edges of the original picture fall off the sides of the screen. This can work with some material, but can also lead to the "two noses talking to each other" syndrome! Interestingly, UK broadcasters have used this technique for years for transmitting Cinemascope movies in preference to "letterbox", calling it "pan and scan". This is so-called as it has the added complication that the visible area is panned left or right across the original image during the film to follow the action, which means a panning control track has to be recorded by someone watching the film before broadcast, and then this is used to perform the pan automatically during transmission. This is not really an option for a live programme! | |
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A compromise, 14:9 Letter Box. You can also represent a 4:3 picture as 12:9, and wide-screen is 16:9, so between these it is possible to devise a compromise format of 14:9 Letter Box. This is currently often broadcast on analogue TV in the UK. This loses a small amount of the left and right picture edge, and has smaller black bars above and below than with full "letter box". This is considered less disturbing than full letter-box to UK viewers brought up on a diet of "pan and scan" movies! This can be an acceptable mode, as most wide-screen programmes are shot to "protect" the central area, keeping critical action, captions etc. away from the extreme edges. |
THE 16:9 VIEWER
The wide-screen viewer, as well as wide-screen programmes, will also encounter a considerable amount of non wide-screen 4:3 material, especially archive programmes. Diplaying these to best effect is the reverse problem to that already discussed, and the options this time are:
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Do nothing, just allow the 4:3 picture to fully fill the 16:9 screen. Again unnaceptable as there is severe picture distortion. This time the image is compressed vertically to fit, and results in short, fat people! | |
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Make the 4:3 picture exactly fit the 16:9 picture height, keeping the image aspect ratio the same. This means there will now be vertical black bars either side of the picture. The picture is not distorted, but not all of the screen area is used. This method is often called "pillar box", and is the equivalent of "letter box". This is the only method that shows all of the original image whilst giving the largest possible picture area, and with no distortion. | |
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Make the picture exactly fit the width and crop the top and bottom. Usually unacceptable as important parts of the picture are lost, especially captions or sub-titles. | |
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The 14:9 compromise the other way round. A smaller amount of the top and bottom are lost, and smaller black bars are present either side. As before, this can be an acceptable mode, as most wide-screen programmes are shot to "protect" the central area, keeping critical action, captions etc. away from the extreme edges. | |
 © Amcro Technology Ltd. 2002-9 |
So-called "Panoramic" modes. These are a feature of some wide-screen sets, and work by leaving the important central area of the picture un-distorted, but progressively distorting the edges to make the picture fit the screen. This can be done by fitting the picture vertically, as in Pillar Box mode, and stretching the left and right edges progressively to fit the screen. Alternatively, the picture can be fitted horizontally and the top and bottom edges progressively compressed to make the picture fit, as in the example shown here. A combination of the above methods may be used in some TVs. Although the central area is un-distorted, objects in the picture change size as they move away from the central area. This used to be known as scan non-linearity, and usually required a visit from a TV service engineer! Whether Panoramic mode is acceptable depends upon the kind of programme material, and the degree of discernment of the viewer. |
WIDE-SCREEN SWITCHING
What is really needed is a TV that can adjust its own aspect ratio, and a means of signalling the required aspect ratio with the TV broadcast. This is possible with transmissions that carry the "Wide-Screen Signalling" (WSS) signal on line 23, which can be added to an analogue broadcast. However this has been largely superseded by the advent of Digital TV which has wide-screen signalling built-in. If a digital "Set-Top-Box" (STB) is used, it is possible this will be used with a 4:3-only TV. In this case, the decision as to how to view a wide-screen picture can now be made by the viewer when the box is installed, as most STBs have a set-up menu which will re-size the picture and select either "letter box" or full-height (see above) as the default. With some STBs the options can be also be changed directly "on the fly" by the viewer by means of a button on the remote control handset. If a 16:9 TV is used, the STB is set to output the full "anamorphic" image, filling the active picture area. The STB then signals to the wide-screen TV either via the WSS, or via pin 8 of the SCART connector whether the image is 4:3 or 16:9, and the TV then switches its scan size accordingly. Note that some recent 4:3 TV sets will reduce their vertical scan to "Letter Box" mode (see above) when a wide-screen signal is flagged.
