Defining Aspect Ratios

What is an aspect ratio? Aspect ratio is typically described as the ratio of screen width to screen height. For instance, a 2:1 (referred to as "2 to 1" or "2 by 1") aspect ratio means the width is twice that of the height. Figure 1 shows two common aspect ratios, the first is that of a standard television which has a 4:3 (referred to as "4 by 3") aspect ratio. Also note that the television aspect ratio is listed as 1.33:1. This is another way of listing aspect ratios—dividing the width by the height (e.g., 4/3 = 1.33). This is referred to as "1.33:1" or "1.33 to 1".

A widescreen display, such as the plasma panel shown on the right of Figure 1, will usually have a 16 by 9 aspect ratio (16:9). Since 16/9= 1.78, the aspect ratio is also known as "1.78:1" or "1.78 to 1".

Figure 1

Most American TV screens have an aspect ratio of 4:3/1.33:1, i.e., the TV screen width is roughly 1/3 wider than the height. That dimension was selected because TV sets were made to closely match the aspect ratio of movie screens (referred to as the Academy aspect ratio), which, in turn, enabled movies to be shown on TV.

The film industry later threw a curve ball at television, which was cutting into their business, by offering movies in widescreen—a format television couldn't match. Over time, a number of widescreen movie formats, with varying aspect ratios, appeared under various names such as CinemaScope, Cinerama, Panavision, and VistaVision. Each of these new movie aspect ratios, at 1.85:1 or more (Cinerama had a minimum aspect ratio of 2.65:1 and a maximum of 3.00:1), exceeded that of the television screen.

Because movies no longer fit on a standard television screen, the TV industry suddenly had a problem showing movies on television. Previously, movies had to be shown on television using either pan-and-scan or letterbox techniques. The pan-and-scan technique only shows part of the widescreen image at any time. The camera (for the TV image) will move across (pan-and- scan) the actual movie image, centering on the most significant parts. Unfortunately there is always a significant portion of the original image not seen by the television viewer. The letterbox technique solves this problem by displaying the full movie image in its original aspect ratio on the TV screen. To do this, the image has to be reduced in size vertically. The net effect is that the image is as wide as the television screen, but there is empty space (a.k.a. black bars) above and below the image.

The major disadvantage of the letterbox format is that vertical resolution is lost. With a letterbox format, only a portion of the vertical screen is used. With less lines of vertical resolution, the potential of a better picture is lost.

Resizing For Aspect Ratio in Home Theater

The same aspect ratio issues have surfaced in home theater environments. Viewers may watch anything from a standard television show to various widescreen movie formats. Of course, a home theater may consist of anything from a display with the standard TV aspect ratio to a plasma panel with a 16:9 aspect ratio. So how do we match aspect ratios?

In many cases, the display device has a built-in scaler that will automatically resize the image to fit the display. Plasma panels typically have such a scaler that will match the incoming signal to the size of the display. In other cases, external scalers can be used to match just about any movie format to any display. Scalers usually match the image to the size of the display device by resizing the image as needed.

Although the scalers can resize the image to fit the display, they may not maintain the same aspect ratio. If a scaler resizes a TV show made in a 1.33:1 aspect ratio to fit a 16:9 display, the original aspect ratio will probably be altered. Now the image may fill the 16:9 screen but the original width-to-height ratio is lost. This will result in the image looking vertically compressed (i.e., objects will look short and wide).

One feature to look for in a scaler is aspect ratio conversion—the ability to horizontally and vertically size the video image to match a specific aspect ratio. This feature is offered on Extron's DVS 406 Digital Video Scaler and USP 405 Universal Signal Processor. Figure 2 shows how aspect ratio conversion can improve the geometry of an image over simply scaling the image.

Figure 2

Figure 2 shows various movie formats that have been scaled to fit a 16:9 display. Note that although the image may be resized, the correct aspect ratio is lost and the image looks distorted. Using the aspect ratio control on the DVS 406 and USP 405, the correct aspect ratio can be restored. The images on the right side show the effect of restoring the proper aspect ratio—the proper proportions have been restored and the overall image looks normal again.

Anamorphic DVDs

Home theater owners have another option in terms of anamorphic or widescreen DVDs. Unfortunately the term "anamorphic" has lead to confusion as to how these DVDs work. There are at least two definitions of the term. In the movie industry, anamorphic is used to refer to the process where a movie is compressed horizontally with a special lens to fit it on a standard frame of film. When the movie is shown, another special lens is used to resize the image to its proper proportions. Many people misinterpret this usage of "anamorphic" with the way the term is applied to "anamorphic DVD". The belief is that the DVD player either resizes or stretches out a movie that was previously compressed.

An anamorphic or widescreen DVD is encoded with the contents of the widescreen movie allowing full 16:9 reproduction. Figure 3 illustrates the process. A widescreen image is stored on the DVD, but the image is stored in a horizontally squeezed format as shown on the right. This is how the image would appear if you could view it as it is stored, if you were viewing it on a screen with a 4:3 aspect ratio. Features on the image would appear tall and narrow.

Figure 3

So how is this squeezed image played back? Figure 4 demonstrates the process. When the DVD player is set for 4:3 output, horizontal lines are removed from the image to reduce the vertical height to produce a normal aspect ratio. The image still fits the 4:3 display horizontally but with the horizontal lines removed, it now appears in letterbox form. The proper aspect ratio now exists, but there is a disadvantage in that considerable vertical resolution is lost.

When the DVD player is set for 16:9 output, the full image is output as it is stored; however, the widescreen display will automatically stretch the image out horizontally and make it look normal again. The full vertical resolution is shown, resulting in a sharper image.

Figure 4

While HDTV is closer to movie aspect ratios, varying aspect ratios will continue to be an issue in the foreseeable future. Made for TV movies and programs will still be around for a while, and movies will still be issued to theaters with varying aspect ratios. Fortunately, products like Extron's DVS 406 allow the user to convert between aspect ratios as needed, while maintaining the correct aspect ratio on their display.