Production Clips: Aspect ratio conversion

As the transition to digital television continues in the United States, aspect ratio conversion has become one of the most confusing and critically important production issues facing television program producers and broadcasters.
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As the transition to digital television continues in the United States, aspect ratio conversion has become one of the most confusing and critically important production issues facing television program producers and broadcasters.

It is important to understand the active picture aspect ratio with a given display aspect ratio; for example, a 4:3 image on a 16:9 display.

In the early years of cinema, the aspect ratio, or the ratio of picture width to height, was fixed to about 4:3 by technologies of the era. It was logical to adopt this ratio for television, since it allowed films to become a ready source of program material.

Over the years, film technologies were developed to give directors the freedom to shoot in ever-wider aspect ratios, which were not immediately compatible with television. With the increasing adoption of 16:9 format widescreen television, program material shot for 4:3 will not suit widescreen, and vice versa.

With the presence of high-definition television increasing in the United States, it is crucial that broadcasters own an aspect ratio converter and that their staff is educated in the conversion process. There is much to know in order to achieve the best possible results.

Converters vary in quality, with noticeable differences in effectiveness. A quality product provides better images and should somewhat simplify the process.

While technical tools to change aspect ratio are advanced and simple to use, the creative choices facing producers are just the opposite. Difficult decisions on picture shape involve variables ranging from program genre to cultural tastes of viewers.

Ultimately, when creating a non-live program, it is not known whether what aspect ratio will be implemented. Therefore, it is necessary to understand the role of the safe area, the portion of the image that will be viewable regardless of aspect ratio. A producer filming in 16:9 often cuts the sides of an image to keep the action within the narrow 4:3 middle section. This ensures that the important parts of the image will be seen on both 16:9 and 4:3 screens. This could be referred to as the active picture aspect ratio within the display aspect ratio. (See Figure 1.)

Aspect ratio conversion has made this safe area smaller and more significant. Everyone in the chain has to be clear as to when a 16:9 program features a 4:3 safe area, or vice versa, so it is established which portion of the image will be preserved regardless of the aspect ratio at the output.

To create an original television program that will play well on both 4:3 and 16:9 sets, the master tape should contain as much information as possible. The simplest approach is to shoot in 16:9 and protect the sides of the image to allow the center of that image to be removed without a significant degree of loss. Decisions of final aspect ratio can be made following the post-production process.

A growing trend in aspect ratio conversion in areas of Europe is 14:9, which could also be considered in the United States. In the UK particularly, it has become common to shoot in 16:9 but confine the action to a 14:9 shoot and protect graticule, which provides the room to convert to either 4:3 or 16:9.

With 14:9, the aspect ratio on the tape is no different than 16:9. The 14:9 area is actually a masking as opposed to a new aspect ratio. This compromise provides the viewer with small black bars at the top and bottom of the screen, and a small bit of image loss on the sides. This loss is minimal and generally acceptable to viewers.

Another option is to shoot in 4:3 using a 14:9 display and protect graticule. Although portions at the top and bottom of the image are lost, a good aspect ratio converter can successfully tighten the image.

What sets a quality unit apart from one of lesser value is generally found in the design of the filters used in the conversion process. Additional functions within an aspect ratio converter can also raise the quality level of conversions. On some units the ratio format is fixed, while on others it is variable, allowing the user to select special formats such as 14:9. Some units on the market can process audio and provide an audio delay to match the video delay. Other extra capabilities allow the user to pan, tilt and zoom the image, or execute smooth transitions from one aspect ratio to another, rather than an instant switch.

Of course, the traditional, purely technical method of testing the signal and viewing the results is always recommended. An easy way to do this is to perform a test of rolling credits. Credits are hard-edged and filled with motion, easily the two most difficult areas of aspect ratio conversion.

For live coverage, particularly of sporting events, addressing motion during the conversion process becomes even more important. This is the big reason why converters should never be tested using only still images.

Aspect ratio issues essentially require a new approach to shooting sports. A tight shot of a motion-filled image in 16:9 is going to pack a lot more information into an image than what would appear in 4:3. The producer needs to determine what the viewer''s mind can register. In widescreen sports, the camera operator may follow the central action, but the viewer may be looking at other information within the frame, usually details and action that were not noticed in 4:3. It needs to be determined how much additional information will be delivered to the viewer.

Adding high-definition television into the equation adds a new layer of complexity. A single fixed camera can provide quality sports coverage on a large home HD display, though it would be inappropriate for the smaller 4:3 sets. Widescreen may also require the use of new, creative camera angles that may not be appropriate for 4:3 service. This means that certain events will require both 4:3 and 16:9 shots.

Live high-definition programming is also more complex for the producer on the mobile truck. As the very wide shots that work in high definition are incompatible on small 4:3 screens, it comes down to a shot-by-shot decision. High-definition 16:9 shots interspersed with tighter shots showing details of the action for 4:3 viewers is a plausible solution, though it won''t be perfect.

Furthermore, when shooting a live presentation that includes mixed aspect ratios, the timing of the aspect ratio change is paramount. A live presentation may bring in material as 4:3 with portions as 16:9, which requires frame-accurate switching between the ratios. If a non-frame-accurate converter is used, the change in the feed happens a few frames later than the change in the aspect ratio converter took place. As a result, the viewer sees the image cut and suddenly change shape — obviously not what was intended.

This also requires a frame-accurate control system. This could be a switcher that will accurately drive the converter while cutting between 4:3 and 16:9 sources, or a frame-accurate aspect ratio converter with Video Index signaling.

Control of the aspect ratio converter''s functions, especially to vary the aspect ratio on the fly, can be accomplished directly from the front panel. Control can also be external, as in remote control from a network. In Europe, converters are typically driven from signaling standards inserted within the video signal. Widescreen signaling line 23 (WSSL23) or Video Index can be used to change the aspect ratio converter''s mode as the incoming signal is altered.

The aspect ratio converter can also be combined with another unit, such as an upconverter or standards converter. Aspect ratio conversion from 4:3 to 16:9 is part of the upconversion process, so one advantage is that the filtering needed for upconversion and aspect ratio conversion can be combined. If designed correctly, this can provide a quality advantage. This combination can also minimize the delay because the unit has one less filter stage.

Broadcasters in the United States will find the aspect ratio conversion process easiest if they combine knowledge of what''s available on the market and of the process itself. This includes simultaneously supporting 4:3 and 16:9 for widescreen television, while also applying the process to high-definition television.

Richard Schiller is product manager, Gordon Scott is principal design engineer and Dan Burgess is design engineer for Snell & Wilcox.

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