Understanding TV

About 4:3 aspect images

An aspect ratio is the measured comparison between the width and height of a 2 dimensional image. Conventional analog Cathode Ray Tube (CRT) TV and computer screens have an aspect ratio of 4 to 3 (written 4:3) also referred to as 4 by 3 (written 4x3). That is, the screen width is four units of length, and the screen height is 3 units of length when compared to each other, as illustrated in Example 1 below:

Example 1. 4:3 Aspect ratios

     

See that there are 4 width items to 3 height items,
in the ratio of 4:3 in each example image.

An aspect ratio does not have fixed lengths, just a fixed ratio or proportion between the width and height lengths. This means that screens of different sizes can still have the same aspect ratio as illustrated in Example 2 below:

Example 2. 4:3 Screen image sizes

     

See that even though the screen image sizes are very different,
they all still display an aspect ratio of 4:3.

The ratio of 4:3 can also be expressed as 1.33:1 or as 12:9. These ratios are the same as each other, and mean the same thingthey're just expressed differently, as illustrated in Example 3 below:

Example 3. 4:3 ratios (1.33:1, 4:3, 12:9)

     

See that they are all the same shape and ratio,
even though they're called 1.33:1, 4:3, and 12:9 respectively.

Sources of 4:3 aspect images include analog terrestrial (antenna) TV, Cable TV, Satellite TV, digital Standard Definition TV (SDTV), Video Cassette Recorders (VCR), DVD Players, Personal Computers (PC), Video cameras, digital still cameras, and video gaming equipment (like XBox, PlayStation, etc).

About 16:9 aspect images

Modern digital display devices such as monitors, TVs, and projectors more commonly have a (widescreen) aspect ratio of 16 to 9 (written 16:9) and can be referred to as 16 by 9 (written 16x9) or expressed as 1.78:1.

When compared with a 12:9 (4:3) screen, the 16:9 screen is clearly a third wider, hence the name "widescreen", as illustrated in Example 4: below:

Example 4. 16:9 compared to 4:3

  &    = 

When a 4:3 (12x9 grid) unshaded image (left) is displayed over a 16:9 (16x9 grid)
shaded image (centre), the combined resultant image is displayed (right).

In the resultant image (right) in Example 4 above, you can clearly see that the 16x9 shaded grid is 4 squares wider than the 12x9 unshaded grid. When these images are centred, this extra 16:9 grid space is split on either side of the 4:3 grid as illustrated in Example 5 below:

Example 5. 4:3 grid centred on a 16:9 grid.

See that this leaves the extra 16:9 grid space
split to either side of the 4:3 grid image.

Displaying 4:3 aspect images in a 16:9 display

When a 4:3 aspect ratio image is displayed on a 16:9 aspect ratio screen, it clearly doesn't fit as its width is too narrow for the wider screen. If the 4:3 aspect image's ratio is retained (not stretched), the result is visible as a vertical band of unused screen (usually unlit and black) displayed on the left and right of the image on the 16:9 screen, similar to the result exhibited in Example 5 above. This is sometimes referred to as "pillarbox" format (as opposed to letterbox format).

To overcome this pillarbox (and other) display behaviour, modern digital graphic display device manufacturers have added image manipulation circuitry to their devices (monitors, TV's, and projectors) so that the displayed images can be resized, stretched, shrunk, cropped, and altered to suit your personal viewing preferences.

For example, a 4:3 aspect image need only be stretched horizontally by a third of its original width to match the width of a 16:9 aspect image, as illustrated in Example 6 below:

Example 6. 4:3 aspect image stretched horizontally to 16:9 aspect.

  

See that the 4:3 aspect image of unshaded squares and circles (in the left 16:9 shaded frame) has been stretched horizontally to fit the 16:9 aspect frame (right), but has linearly distorted all the circles and squares by evenly stretching them horizontally into ovals and rectangles to fit the wider frame size.

This linear or even stretching of the width of the image distorts the whole picture, and makes people and objects in the image look wider than normal. To enable this effect, select the linear resizing aspect control of the display device. On some devices, this is called the "Anamorphic" aspect ratio.

An alternative effect, is one that stretches the width of an image non-linearly, where the centre of the image remains unaltered, and the remainder of the image is horizontally stretched, increasing progressively towards the sides of the image, as illustrated in Example 7 below:

Example 7:  4:3 aspect non-linear stretching to 16:9 aspect.

  

See that the 4:3 aspect image of unshaded squares and circles (in the left 16:9 shaded frame) has been stretched horizontally to fit the 16:9 aspect frame (right), and that the circles and squares in the horizontal centre of the image remain unaltered, but the circles and squares near the horizontal edges have been proportionately and non-linearly distorted by stretching them horizontally into ovals and rectangles to fit the wider frame size.

This non-linear or uneven stretching of the width of the image leaves people and objects in the middle of the image appearing normal, however it distorts gradually and increasing towards the edges of the picture. To enable this effect, select the non-linear resizing aspect control of the display device. On some devices, this is called the "Widescreen" (or "Panorama" or "Theatre" as appropriate) on the display device aspect control.

Unfortunately, this creates a slight myopic effect which can be somewhat similar to looking down a tunnel. It is more suited for use with still images or slow moving scenes where the focus is in the centre of the image, and unsuited to scenes where objects are fast moving or the focus is other than in the centre of the image. Scrolling text across the screen speeds in and out nearer to the edges, and appears to slow down across the centre. Once noticed, this behaviour is quite distracting.

Displaying original image aspect ratio

Most digital display devices have the ability to dynamically resize an image to fit the display size, and the difference between the original image size and the display image size will determine the amount of resizing necessary. To view an image in its original unaltered size so that you can determine its original aspect ratio, sizing, and proportions, select "Real" (or "Thru" or "1:1" or "True" as appropriate, or disable the auto-resizing function) on the display device aspect control.

Real (or equivalent) is the best setting to select when you are viewing a 16:9 image on a 16:9 display. Sources of 16:9 aspect images include digital High Definition TV tuners (built into TVs or separate in a so-called Set Top BoxSTB).  DVDs, cable TV and satellite TV may be converted to widescreen aspect in due course, but do not offer such services at the time of writing this document (June 2005).

Displaying 16:9 aspect images in a 4:3 display

Now for the opposite end of the scale. You may experience the situation where you need to display a 16:9 aspect image on a 4:3 aspect display device. Necessarily, this will involve the reduction in size of the 16:9 image so that it fits into a 4:3 (12:9) frame. As described above, modern digital display devices come fully equipped with dynamic rescaling ability (known as scalers) which can resize, stretch, shrink, and scale images from one resolution (and aspect) to another, up or down.

Accordingly, when viewing a 16:9 image within a 4:3 frame, to maintain the aspect ratio of 16:9 without distortion, the 16:9 image must be scaled down to fit within the width of the 4:3 frame. This is performed by scaling the image proportionally (what is done to the height, must be equally done to the width so as to keep the image in proportion).

The 16 sections of the 16:9 width can be reduced to fit within 12 sections of a 4:3 (12:9) width by reducing the image width by a quarter (1612=4). To scale this image proportionally, reducing its width by a quarter requires that you reduce its height by a quarter too. This calculates to (3/4=). Therefore, a 16:9 image displayed proportionally within a 4:3 frame must display within only of the full height of a 4:3 frame to retain its proper aspect ratio proportions.

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