Active Format Descriptor
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In television technology, Active Format Descriptor or Active Format Description (AFD) is a signal that broadcasters can transmit with the picture to enable both 4:3 and 16:9 television sets to optimally present pictures transmitted in either format. Essentially, AFD is a standard set of codes sent in the MPEG video stream that provides aspect ratio and safe area information to television or set-top-box decoders. They can then use this information, together with knowledge of the display shape and user preferences, to choose a presentation mode. AFD is used in the generation of Widescreen signaling. AFDs are not part of the core MPEG standard; they are a DVB extension, which has subsequently also been adopted by ATSC.
By using AFDs broadcasters can also control the timing of Aspect Ratio switches more accurately than using MPEG signalling alone. This is because the MPEG signalling can only change with a new Group of Pictures in the sequence, which is typically around every 12 frames or half a second - this was not considered accurate enough for some broadcasters who were initially switching frequently between 4:3 and 16:9. The number of Aspect Ratio Converters required in a broadcast facility is also reduced, since the content is described correctly it does not need to be resized for broadcast on a platform that supports AFDs.
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[edit] Usage
A 16:9 film may be broadcast with AFD 2, indicating that the whole frame is important. On a 4:3 TV, this will then be shown as a 16:9 letterbox to ensure no image is lost. 16:9 sports coverage on the other hand may be broadcast with AFD 7, indicating that it is safe to display only the central 4:3 region. On a 4:3 TV, the image will be cropped and it will be shown full-screen.
As of 2006, AFDs are only broadcast in a minority of the countries using MPEG digital television. As a result, the quality of implementation in receivers is variable. Some receivers only respect the basic "active area" information. More fully-featured receivers also support the "safe area" information, and will use this to optimise the display for the shape of the viewer's screen. Display in the compromise 14:9 letterbox format was not supported by initial UK receivers, which limited the value of the AFD flags - this ratio is especially useful when watching widescreen material on smaller 4:3 sets.
Many receivers currently have a bug in their handling of AFD 7, arising from an error in a table in the 2001 UK Digital Television Group receiver implementation guidelines. This causes 4:3-safe content like the sport mentioned above to be shown letterboxed on a 4:3 TV, instead of full-screen.
[edit] Complete list of AFD codes
Numbers in brackets are new (correct) notation.
0 (8) as coded frame
1 (9) 4:3 active picture
2 (10) 16:9 active picture
3 (11) 14:9 active picture
4 (12) unused
5 (13) 4:3 with shoot and protect 14:9 centre
6 (14) 16:9 with shoot and protect 14:9 centre
7 (15) 16:9 with shoot and protect 4:3 centre
