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.
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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.
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
