Active Format Description
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In television technology, 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 information to the decoder about where in the coded picture the active video is and also the "protected area" which is the area that needs to be shown. Outside of the protected area, edges at the sides or the top can be removed without the viewer missing anything significant. Decoders can then use this information, together with knowledge of the display shape and user preferences, to choose a presentation mode. AFD can be used in the generation of Widescreen signaling, although MPEG alone contains enough information to generate this. AFDs are not part of the core MPEG standard; they are a DVB extension, which has subsequently also been adopted by ATSC (with some changes), and by SMPTE as standard SMPTE 2016-1-2007, "Format for Active Format Description and Bar Data".
Active Format Description is occasionally incorrectly referred to as "Active Format Descriptor", which is an error. There is no "descriptor" (descriptor has a specific meaning in ISO/IEC 13818-1, MPEG syntax). The AFD data is carried in the Video Layer of MPEG, ISO/IEC 13818-2. This mistake is very common in the UK, where the term Active Format Descriptor is used by the DTG amongst others. When carried in digital video, AFDs can be stored in the Video Index Information, in line 11 of the video.
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 2008, a concerted effort on the part of US broadcasters to begin broadcasting AFD is underway in preparation for the US digital television transition which will take place on February 17, 2009.
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
Values from ETSI TS 101 154 V1.7.1 Annex B and ATSC A/53 section 6.2.4 (values show as (N) are obsolete)
0: ESTI: reserved; ATSC: undefined
1: reserved
2: ETSI: 16:9 active picture (top aligned); ATSC: "not recommended"
3: ETSI: 14:9 active picture (top aligned); ATSC: "not recommended"
4: ETSI: box > 16:9 (center): wider than 16:9 active picture. The aspect ratio of the source area is not given, and the size of the top/bottom bars is not indicated. ATSC: bar data (indicating the extent of top, bottom, left, and right bars) should be transmitted when using this code.
5–7: reserved
8 (0): as coded frame
9 (1): 4:3 active picture
10 (2): 16:9 active picture
11 (3): 14:9 active picture
12 (4): unused
13 (5): 4:3 with shoot and protect 14:9 centre. The term "shoot and protect" is not explained in the standard, but means that the areas above and below the central 14:9 region of the 4:3 active picture can be trimmed without losing important detail.
14 (6): 16:9 with shoot and protect 14:9 centre. Here, the areas to the right and left of the central 14:9 region of the 16:9 active picture can be trimmed without losing important detail.
15 (7): 16:9 with shoot and protect 4:3 centre. Here, the areas to the right and left of the central 4:3 region of the 16:9 active picture can be trimmed without losing important detail.
[edit] See also
[edit] External links
- ETSI TS 101 154 v1.7.1 - the latest version of the DVB standard that defines AFDs
- ATSC A/53 - the ATSC standard
- Digital TV Group AFD tutorial
- DTG digital receiver implementation guidelines
- Explanation of the error in the DTG guidelines
- Active Format Descriptors explanation
- AFD codes explained
- Shoot and Protect 14:9 safe areas
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