Aspect Ratio: It Used to Be Easy

Although aspect ratio has always been a factor in television, for many decades only one aspect ratio was available: 4:3. With apologies to accomplished punsters, that picture has changed.
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Way back before advanced television became a part of our world, most television engineers did not much concern themselves with aspect ratio. Although aspect ratio has always been a factor in television, for many decades only one aspect ratio was available: 4:3. With apologies to accomplished punsters, that picture has changed.

Aspect ratio is an expression of the geometrical relationship between the width and height of an image. In television, aspect ratio is typically expressed as a fraction containing two numbers, the picture's width being represented by the numerator and its height by the denominator. A familiar example is 4:3. In cinema, aspect ratio is typically expressed as the quotient obtained when the fraction is divided; 4:3 is thereby expressed as 1.33, it being understood that the complete ratio is 1.33/1.

When electronic television became commercialized, the aspect ratio of 4:3 (1.33) had been standardized by the first National Television Systems Committee (NTSC), in 1941. This was not a magic number; it was adopted simply because it was the most common cinematic aspect ratio of that time. The emergence of commercial television motivated the moviemakers to offer something more than television could offer, and this led to the development of the widescreen cinematic aspect ratios.

For many decades, 4:3 was the only aspect ratio used in television throughout the world. By the time development work on HDTV began, the most common aspect ratio for theatrical features was 1.85, ("Flat"); 2.4 ("Scope") was also common. To make the new HDTV formats more "cinematic," as well as to better accommodate widescreen cinematic features to television, it was understood that a widescreen aspect ratio was desirable in HDTV.

CONSUMER WIDESCREEN INTRODUCED

The earliest commercial HDTV format, the Japanese Hi-Vision system, originally had an aspect ratio of 15:9 (1.67). It was subsequently agreed, based on work that was probably originally done at RCA Laboratories in the early 1980s, that the ratio of 16:9 (1.78) was a better choice, as it is directly scaled from 4:3 by squaring that ratio. To date, the 16:9 aspect ratio is still the only HDTV standard that is agreed upon throughout the world, being standardized by the International Telecommunications Union. The ITU also amended Recommendation 601, the component digital standard for standard-definition video, to include the 16:9 aspect ratio.

There is really no such thing as compatibility between aspect ratios; there is only accommodation of one aspect ratio to another. Thus, most cinematic aspect ratios must be accommodated to the 4:3 aspect ratio of NTSC, PAL and SECAM rasters. We have seen that the most common cinematic aspect ratios, 1.85 and 2.4, are considerably wider than the 1.33 or 4:3 ratio. One method of accommodating a widescreen aspect ratio to a narrower-screen raster is to letterbox it. When this is done, the edge-to-edge width of the image is fit to the 4:3 raster. This results, of course, in the height of the image being insufficient to fill the raster, leaving a full-width image with black bars above and below it. The wider the cinematic aspect ratio, the taller the black bars.

While letterboxing has traditionally been the accommodation method of choice in Europe, it has enjoyed less popularity in the United States, and in fact, until some time in the 1980s, the FCC blanking rules prohibited it in NTSC broadcast. While the blanking rules have been eliminated, and the viewing public has become more accustomed to letterboxing in recent years, the broadcast networks are still typically reluctant to use this method, as it generates telephone calls from viewers.

Widescreen cinematic aspect ratios have traditionally been dealt with in NTSC broadcast by the use of pan-and-scan. In pan-and-scan, the widescreen aspect ratio is accommodated to 1.33 by filling the 4:3 raster vertically with the widescreen image but including only that horizontal portion of the image that will fit into the width of the 4:3 raster. The particular area of the image that is transferred to the 4:3 screen is selected to include the portion of highest interest at any given time: the telecine's mechanism is panned so that it scans the horizontal portion of the image that is of interest. This fills the screen, but we have all experienced those situations where someone on screen is having a meaningful dialogue with someone who is located off-screen.

FITTING FEATURE FILMS TO HDTV

In HDTV, these aspect ratio anomalies are dealt with in various ways. Images at 1.85 are just slightly wider than the 1.78 aspect ratio of HDTV, not enough of a difference to justify panning and scanning. This gives us two ways to deal with them. The 1.85 image may be fitted to the 16:9 raster horizontally (letterboxed), in which case, there will be very small black bars below and above the image. The 1.85 image may also be fitted to the 16:9 raster vertically, which will result in a small amount of the image overlapping the edges of the raster on either side; the overlapping portion is cut off.

Scope pictures, with their aspect ratios of 2.4, are considerably wider than those in the flat-aspect ratio, and must be letterboxed or panned and scanned. If they are letterboxed, the black bars above and below the image are of course appreciably shorter than those resulting when they are letterboxed into the 4:3 aspect ratio.

When 16:9 images must be fitted into the 4:3 aspect ratio, the situation is reversed from that discussed above. If a 4:3 image is sized to fill the 16:9 raster from top to bottom, black bars will result on the right and left sides of the raster. This is commonly called pillarboxing. If the 4:3 image is sized to completely fill the width of the 16:9 raster, the full height of the image cannot be included within the 16:9 raster, and the portion of interest must be selected, a process called tilt-and-scan. In practice, this very often results in the bottom of the image being cut off, but the deleted portion depends on the particular image and the artistic intent behind it.

The prevalence of the two aspect ratios 4:3 and 16:9, particularly in Europe, led the British Broadcasting Corporation to develop a compromise, the 14:9 aspect ratio. This compromise was developed to distribute "equal pain" to the 4:3 and the 16:9 aspect ratios: The percentage of black bars above and below when it is fitted into the 16:9 raster equals the percentage of black bars on the left and right sides when it is fitted into the 4:3 raster.

Although the overwhelming majority of cinematic films made in recent decades are intended to be projected in the flat aspect ratio of 1.85, and a number are intended to be projected in the 2.4 or scope aspect ratio, there are many other cinematic aspect ratios in existence. The MPAA Academy Camera Aperture for 35mm capture has an aspect ratio of 1.37, as does the currently little-used standard 16mm camera aperture. Super 35 can, in addition to Academy Aperture, be shot with aspect ratios of 2.4, 2.2, and 1.85. Super 16 has a camera aperture of 1.69. Other camera aperture aspect ratios that may be encountered include 65mm at 2.28, IMAX (65mm film run sideways), 1.35, and Vistavision (35mm film run sideways), at 1.50. Any of these may be accommodated by the 1.78 HDTV aspect ratio by one degree or another of letterboxing or pillarboxing, or panning and scanning.

Other methods of 4:3 to 16:9 aspect ratio accommodation exist. One is simply to enlarge the image until it fills the 16:9 raster, and cut off the portions of the image that fall outside the raster. Yet another is to fill the raster vertically, and "stretch" the image horizontally until it fits the wider-screen horizontally. This of course distorts the image, with results that are often undesirable. A refinement of the stretching approach, found on some widescreen monitors and receivers, is to horizontally stretch the image adaptively. In this approach, the amount of horizontal stretching varies from little or none in the center of the image, to progressively more as the left and right edges of the raster are approached. This can result in a stretched image that does not appear to the casual observer to be distorted.

When 16:9 images are letterboxed onto 4:3 screens in the home, the results can also be objectionable to the viewer. Consider what happens when a television broadcaster upconverts a 4:3 NTSC commercial to 16:9 HDTV, preserving its original aspect ratio by pillarboxing it. This, of course, results in the transmitted image having black stripes on its left and right sides. When this 16:9 image is received and displayed on a 4:3 screen, it is letterboxed to fit the 4:3 aspect ratio, which generates black bars above and below the image. The image already had black bars on its right and left sides, so it is now completely surrounded in black, and it becomes a small picture on a big screen. If the receiver knew that the original image was 4:3, it could simply display it in that aspect ratio, eliminating the black bars all around. It is technically possible to do this in both the ATSC and DVB broadcast systems. Aspect ratio metadata may be transmitted with the program, and this data can be used to cause the receiver to accommodate the transmitted aspect ratio automatically.

The blessing and the curse of advanced television is that both the broadcaster and the viewer are given many choices!