Jerry Whitaker /
07.12.2006 12:00 AM
Using AFD to Resolve Aspect Ratio Discrepancy
Active Format Description solves a troublesome problem in the transition from conventional 4:3 display devices to widescreen 16:9 displays, and also addresses the variety of aspect ratios that have been used over the years by the motion picture industry to produce feature films.

AFD is described in ATSC document A/53, the Digital Television Standard, Annex A. While the AFD specification has been on the books for some time, the need for certain updates has been identified.

The ATSC is currently working through revisions designed to bring the AFD spec into alignment with work undertaken concurrently by the Society of Motion Picture and Television Engineers and the Consumer Electronics Association.

The changes, while relatively minor, are important to ensure that AFD data moves properly through the DTV system and effects the desired result on consumer displays. AFD and the related "bar data" are two functions that-by necessity-span the entire DTV spectrum from production through transmission and ultimately to the consumer's television set. As such, coordination with other industries is critical.

ABOUT THE NEED

There are, of course, many different types of video displays in common use-ranging from 4:3 CRTs to widescreen projection devices and flat-panel displays of various design.

Each of these devices may have varying abilities to process incoming video. In terms of input interfaces, these displays may likewise support a range of input signal formats-from composite analog video to IEEE 1394 to HDMI.

Possible video source devices include cable, satellite, or terrestrial broadcast set-top (or integrated receiver-decoder) boxes, media players (such as DVDs), analog or digital tape players, and personal video recorders.

Although choice is good, this wide range of consumer options presents two problems to be solved:

First, how does a program supplier communicate to the display device the "active area" of the video signal? It would be useful, for example, to signal that the 4:3 program being transmitted contains within it a letterboxed 16:9 image.

Second, how does a program supplier communicate to the display device, for all interface types, that a given image is intended for a 16:9 display? AFD answers these questions and, in the process, provides the following additional benefits.

Active area signaling allows the display device to process the incoming signal to make the highest resolution and most accurate picture possible. Furthermore, the display can take advantage of the knowledge that certain areas of video are currently unused and can implement algorithms that reduce the potential effects of uneven screen aging.

Aspect ratio signaling allows the display device to produce the best image possible. In some scenarios, lack of a signaling method translates to restrictions in the ability of the source device to deliver certain otherwise desirable output formats.

ACTIVE AREA SIGNALING

A consumer device such as a cable or satellite set-top box cannot reliably determine the active area of video on its own.

Even though certain lines at the top and bottom of the screen may be black for periods of time, the situation could change without warning. The only sure way to know active area is for the service provider to include this data at the time of video compression and to embed it into the video stream.

Fig.1 shows 4:3- and 16:9-coded images with various possible active areas. The group on the left is either coded explicitly in the MPEG-2 video syntax as 4:3, or the uncompressed signal provided in NTSC timing and aspect ratio information (if present) indicates 4:3. The group on the right is coded explicitly in the MPEG-2 video syntax as 16:9, provided with NTSC timing and an aspect ratio signal indicating 16:9, or provided uncompressed with 16:9 timing across the interface.

As can be seen in Fig. 1, a pillar-boxed display results when a 4:3 active area is displayed within a 16:9 area, and a letterboxed display results when a 16:9 active area is displayed within a 4:3 area.

It is also apparent that double-boxing can also occur, for example when 4:3 material is delivered within a 16:9 letterbox to a 4:3 display; or, when 16:9 material is delivered within a 4:3 pillar-box to a 16:9 display.

For the straight letterbox or pillarbox cases, if the display is aware of the active area it may take steps to mitigate the effects of uneven screen aging.

Such steps could involve using gray instead of black bars. Some amount of linear or nonlinear stretching and/or zooming may be done as well, using the knowledge that video outside the active area can be safely discarded.

The two double-boxed cases can occur as a result of poor or uninformed production choices made by the service provider, in some cases in concert with the content provider.

Whenever 4:3 material is coded as 16:9, double-boxing occurs when the 4:3 display places the 16:9-coded frame on screen. Whenever 16:9 material is coded as 4:3, double-boxing occurs when the 16:9 display pillar-boxes the 4:3 coded frame.

Regardless of the cause, two aspects of this problem are of prime importance to broadcasters. The display device should not be expected to process the double-boxed image to fill the screen to make up for incorrectly coded content; content and service providers should be expected to deliver properly coded content.

Native 4:3 content must be delivered coded as 4:3. Native 16:9 content must be delivered coded as 16:9. Letterboxed widescreen video in NTSC should not be coded as 4:3, but should be coded into a 16:9 coded frame.

Recognizing these issues, the ATSC undertook a study of available options and decided to endorse the basic signaling structure developed by the DVB consortium. The benefits of common AFD signaling across many different markets are easily understood.


(click thumbnail)
Fig. 1 Coding and active area.
COMMON FORMATS

Some common active video formats represented by the 4-bit AFD field include:

The aspect ratio of the active video area is 16:9; when associated with a 4:3-coded frame, the active video is top-justified.

• Active video area is 16:9; when associated with a 4:3 coded frame, the active video is centered vertically.

• Active video area is 4:3; when associated with a 16:9 coded frame, the active video is centered horizontally.

• Active video area exceeds 16:9 aspect ratio; active video is centered vertically (in whatever coded frame is used).

• Active video area is 14:9; when associated with a 4:3-coded frame, the active video is centered vertically; when associated with a 16:9 coded frame, it is centered horizontally.

In addition to AFD, ATSC defined another data structure-bar data-also for use in the video elementary stream. While the AFD gives a general view of the relationship between the coded frame and the geometry of the active video within it, bar data is able to indicate precisely the number of lines of black video at the top and bottom of a letterboxed image, or the number of black pixels at the left and right side of a pillar-boxed image.

For the ATSC system, AFD and/or bar data are included in video user data whenever the rectangular picture area containing useful information does not extend to the full height or width of the coded frame. Such data may optionally also be included in user data when the rectangular picture area containing useful information extends to the full height and width of the coded frame.

For more information on AFD and bar data, consult the following references:

ATSC Standard A/53E with Amendment No. 1, "ATSC Digital Television Standard," Dec. 27 2005, www.atsc.org/standards/a_53e-with-Amend-1.pdf ATSC Recommended Practice A/54A, "Guide to the Use of the Digital Television Standard," Dec. 4, 2003, www.atsc.org/standards/a_54a.pdf All ATSC Standards and Recommended Practices can be downloaded at no charge from the ATSC Web site ( www.atsc.org ).

Jerry Whitaker can be reached via TV Technology.


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