Active Format Description
Encyclopedia
In television
technology, Active Format Description (AFD) is a standard set of codes that can be sent in the MPEG
video stream or in the baseband SDI
video signal that carries information about their aspect ratio and active picture characteristics. It has been used by television broadcasters to enable both 4:3 and 16:9
television sets to optimally present pictures transmitted in either format. It has also been used by broadcasters to dynamically control how down-conversion equipment formats widescreen 16:9
pictures for 4:3 displays.
Standard AFD codes provide information to video devices 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. Video decoders and display devices 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). SMPTE
has also adopted AFD for baseband SDI
carriage 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 common in Britain, where the term is used by the Digital TV Group
amongst others. 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. 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.
signal may be broadcast with AFD 8 or AFD 10, indicating that the entire frame includes important picture information and should not be cropped. On a 4:3 TV, this will then be shown as a 16:9 letterbox
to ensure no image is lost. Other widescreen 16:9
content (like sports coverage) may be broadcast with AFD 15, 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 British receivers, which limited the value of the AFD flags - this ratio is especially useful when watching widescreen material on smaller 4:3 sets.
which occurred on June 12, 2009.
After the DTV transition, 4:3 versions of programming are not available directly from a large percentage of US broadcasters. Cable and satellite providers down-convert 16:9
HD feeds from these broadcasters to generate the 4:3 SD versions for their SD viewers. The most common forms of down-conversion are letterbox
or center-cut (cropping off the left and right sides of the 16:9
image to fit into the 4:3 raster).
Some US broadcasters transmit AFD with their HD DTV signals in order to maintain control over how SD viewers will receive their programming. With AFD included in these signals, cable and satellite providers are able to dynamically control whether HD content is to be either letterbox
or center-cut for their SD viewers.
Without AFD, either a fixed letterbox
or center-cut will be required on a station-by-station basis. A fixed letterbox
will result in an undesirable windowbox
effect on SD originated programming. A fixed center-cut will result in loss of important picture content on certain HD content (e.g., an HD sports broadcast containing score graphics formatted for 16:9 display).
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
technology, Active Format Description (AFD) is a standard set of codes that can be sent in the MPEG
Moving Picture Experts Group
The Moving Picture Experts Group is a working group of experts that was formed by ISO and IEC to set standards for audio and video compression and transmission. It was established in 1988 by the initiative of Hiroshi Yasuda and Leonardo Chiariglione, who has been from the beginning the Chairman...
video stream or in the baseband SDI
Serial Digital Interface
Serial digital interface is a family of video interfaces standardized by SMPTE. For example, ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video...
video signal that carries information about their aspect ratio and active picture characteristics. It has been used by television broadcasters to enable both 4:3 and 16:9
16:9
16:9 is an aspect ratio with a width of 16 units and height of 9. Since 2009, it has become the most common aspect ratio for sold televisions and computer monitors and is also the international standard format of HDTV, Full HD, non-HD digital television and analog widescreen television ...
television sets to optimally present pictures transmitted in either format. It has also been used by broadcasters to dynamically control how down-conversion equipment formats widescreen 16:9
16:9
16:9 is an aspect ratio with a width of 16 units and height of 9. Since 2009, it has become the most common aspect ratio for sold televisions and computer monitors and is also the international standard format of HDTV, Full HD, non-HD digital television and analog widescreen television ...
pictures for 4:3 displays.
Standard AFD codes provide information to video devices 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. Video decoders and display devices 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
Widescreen signaling
In television technology, widescreen signaling is a digital stream embedded in the TV signal describing qualities of the broadcast, in particular the intended aspect ratio of the image...
, 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
Advanced Television Systems Committee
The Advanced Television Systems Committee is the group, established in 1982, that developed the eponymous ATSC Standards for digital television in the United States, also adopted by Canada, Mexico, South Korea, and recently Honduras and is being considered by other countries.-See also:*ATSC...
(with some changes). SMPTE
Society of Motion Picture and Television Engineers
The Society of Motion Picture and Television Engineers The Society of Motion Picture and Television Engineers The Society of Motion Picture and Television Engineers (SMPTE , founded in 1916 as the Society of Motion Picture Engineers or SMPE, is an international professional association, based in...
has also adopted AFD for baseband SDI
Serial Digital Interface
Serial digital interface is a family of video interfaces standardized by SMPTE. For example, ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video...
carriage 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 common in Britain, where the term is used by the Digital TV Group
Digital TV Group
The Digital TV Group is the industry association for Digital Television in the United Kingdom : The Group was formed in 1995 to set technical standards for the implementation of digital terrestrial television in the UK and now encompasses all digital TV platforms and convergence issues...
amongst others. There is no "descriptor" (descriptor has a specific meaning in ISO/IEC 13818-1, MPEG
Moving Picture Experts Group
The Moving Picture Experts Group is a working group of experts that was formed by ISO and IEC to set standards for audio and video compression and transmission. It was established in 1988 by the initiative of Hiroshi Yasuda and Leonardo Chiariglione, who has been from the beginning the Chairman...
syntax). The AFD data is carried in the Video Layer of MPEG, ISO/IEC 13818-2. 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
Aspect ratio (image)
The aspect ratio of an image is the ratio of the width of the image to its height, expressed as two numbers separated by a colon. That is, for an x:y aspect ratio, no matter how big or small the image is, if the width is divided into x units of equal length and the height is measured using this...
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
16:9
16:9 is an aspect ratio with a width of 16 units and height of 9. Since 2009, it has become the most common aspect ratio for sold televisions and computer monitors and is also the international standard format of HDTV, Full HD, non-HD digital television and analog widescreen television ...
. 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.
Usage
A widescreen 16:916:9
16:9 is an aspect ratio with a width of 16 units and height of 9. Since 2009, it has become the most common aspect ratio for sold televisions and computer monitors and is also the international standard format of HDTV, Full HD, non-HD digital television and analog widescreen television ...
signal may be broadcast with AFD 8 or AFD 10, indicating that the entire frame includes important picture information and should not be cropped. On a 4:3 TV, this will then be shown as a 16:9 letterbox
Letterbox
Letterboxing is the practice of transferring film shot in a widescreen aspect ratio to standard-width video formats while preserving the film's original aspect ratio. The resulting videographic image has mattes above and below it; these mattes are part of the image...
to ensure no image is lost. Other widescreen 16:9
16:9
16:9 is an aspect ratio with a width of 16 units and height of 9. Since 2009, it has become the most common aspect ratio for sold televisions and computer monitors and is also the international standard format of HDTV, Full HD, non-HD digital television and analog widescreen television ...
content (like sports coverage) may be broadcast with AFD 15, 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
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
. 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
14:9
14:9 is a compromise aspect ratio of 1.56:1. It is used to create an acceptable picture on both 4:3 and 16:9 televisions, conceived following audience tests conducted by the BBC...
letterbox format was not supported by initial British receivers, which limited the value of the AFD flags - this ratio is especially useful when watching widescreen material on smaller 4:3 sets.
AFD for the DTV transition
Since 2008, a concerted effort on the part of US broadcasters to begin broadcasting AFD is underway in preparation for the US DTV transitionDTV transition in the United States
The DTV transition in the United States was the switchover from analog to exclusively digital broadcasting of free over-the-air television programming...
which occurred on June 12, 2009.
After the DTV transition, 4:3 versions of programming are not available directly from a large percentage of US broadcasters. Cable and satellite providers down-convert 16:9
16:9
16:9 is an aspect ratio with a width of 16 units and height of 9. Since 2009, it has become the most common aspect ratio for sold televisions and computer monitors and is also the international standard format of HDTV, Full HD, non-HD digital television and analog widescreen television ...
HD feeds from these broadcasters to generate the 4:3 SD versions for their SD viewers. The most common forms of down-conversion are letterbox
Letterbox
Letterboxing is the practice of transferring film shot in a widescreen aspect ratio to standard-width video formats while preserving the film's original aspect ratio. The resulting videographic image has mattes above and below it; these mattes are part of the image...
or center-cut (cropping off the left and right sides of the 16:9
16:9
16:9 is an aspect ratio with a width of 16 units and height of 9. Since 2009, it has become the most common aspect ratio for sold televisions and computer monitors and is also the international standard format of HDTV, Full HD, non-HD digital television and analog widescreen television ...
image to fit into the 4:3 raster).
Some US broadcasters transmit AFD with their HD DTV signals in order to maintain control over how SD viewers will receive their programming. With AFD included in these signals, cable and satellite providers are able to dynamically control whether HD content is to be either letterbox
Letterbox
Letterboxing is the practice of transferring film shot in a widescreen aspect ratio to standard-width video formats while preserving the film's original aspect ratio. The resulting videographic image has mattes above and below it; these mattes are part of the image...
or center-cut for their SD viewers.
Without AFD, either a fixed letterbox
Letterbox
Letterboxing is the practice of transferring film shot in a widescreen aspect ratio to standard-width video formats while preserving the film's original aspect ratio. The resulting videographic image has mattes above and below it; these mattes are part of the image...
or center-cut will be required on a station-by-station basis. A fixed letterbox
Letterbox
Letterboxing is the practice of transferring film shot in a widescreen aspect ratio to standard-width video formats while preserving the film's original aspect ratio. The resulting videographic image has mattes above and below it; these mattes are part of the image...
will result in an undesirable windowbox
Windowbox (film)
Windowboxing in the display of film or video occurs when the aspect ratio of the media is such that the letterbox effect and pillarbox effect occur simultaneously...
effect on SD originated programming. A fixed center-cut will result in loss of important picture content on certain HD content (e.g., an HD sports broadcast containing score graphics formatted for 16:9 display).
Complete list of AFD codes
Values from ETSI TS 101 154 V1.7.1 Annex B, ATSC A/53 Part 4 and SMPTE 2016-1-2007| 0 | ETSI: 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 | Full Frame image, same as the frame (4:3 or 16:9). |
| 9 | 4:3 Image: Full Frame in 4:3 frame, Pillarbox in 16:9 frame. |
| 10 | 16:9 Image: Letterbox in 4:3 frame, Full Frame in 16:9 frame. |
| 11 | 14:9 Pillarbox/Letterbox image. |
| 12 | unused |
| 13 | 4:3 with shoot and protect Shoot and protect Shoot and protect is a technique used in video and film production, in which the material is shot in such a way that the areas of interest within a frame lie within a rectangular "protected area" within the frame, with margins at top and bottom and both sides. The action safe and caption safe areas... 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 | 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 | 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. |
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
- National Association of Broadcasters - AFD Ready Initiative
- Digital TV Group AFD tutorial
- Explanation of the error in the DTG guidelines
- AFD codes explained
- Shoot and Protect 14:9 safe areas