Active Format Description
This article includes a list of general references, but it lacks sufficient corresponding inline citations. (November 2011) |
In
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 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.[4]
AFD can be used in the generation of
Active Format Description is occasionally incorrectly referred to as "Active Format Descriptor". 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.
In 2012, a
Usage
A widescreen 16:9 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
As of 2006, AFDs are only broadcast in a minority of the countries using MPEG
AFD for the DVB DTV transition
The line 23 data format (compatible with the analog Widescreen signaling) allows signaling of the source (coded image) aspect ratio and the Active Format Descriptor.[7]
Bits | Format |
---|---|
000 | Active region same as coded frame (source material) |
001 | 4:3 |
010 | 16:9 |
011 | 14:9 |
100 | not used - reserved for future use |
101 | 4:3 with shoot and protect 14:9 center |
110 | 16 : 9 with shoot and protect 14:9 center |
111 | not used - reserved for future use |
AFD for the ATSC DTV transition
A concerted effort on the part of US broadcasters to broadcast AFD began in 2008 in preparation for the
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. However, there are cases where pay-TV providers completely disregard AFD instructions and for instance, present a 4:3 picture with widescreen elements cut off to assuage user complaints about letterboxing, on standard 4:3 sets (for instance for a secondary-market station available only in standard definition on a provider on the claim that an HD signal exists for the provider's 'primary' station for a network), to the displeasure of broadcasters.
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
Complete list of AFD codes
Decimal | Binary | ETSI[8][9] / DVB | ATSC[10] / SMPTE[11] |
---|---|---|---|
0 | 0000 | reserved | undefined |
1 | 0001 | reserved | |
2 | 0010 | 16:9 active picture (top aligned) | not recommended |
3 | 0011 | 14:9 active picture (top aligned) | not recommended |
4 | 0100 | 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. | bar data (indicating the extent of top, bottom, left, and right bars) should be transmitted when using this code. |
5 | 0101 | reserved | |
6 | 0110 | ||
7 | 0111 | ||
8 | 1000 | Full Frame image, same as the frame (4:3 or 16:9). | |
9 | 1001 | 4:3 Image: Full Frame in 4:3 frame, Pillarbox in 16:9 frame. | |
10 | 1010 | 16:9 Image: Letterbox in 4:3 frame, Full Frame in 16:9 frame. | |
11 | 1011 | 14:9 Pillarbox/Letterbox image. | |
12 | 1100 | reserved | |
13 | 1101 | 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 | 1110 | 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 | 1111 | 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. |
The following image illustrates the above codes and the resulting images as seen on 4:3, 16:9 and 21:9 displays. Green circles represent essential content, orange circles indicate optional image areas. Black areas are unused parts of the frame, i.e. bars. The red edge indicates the full frame.
See also
- Letterbox
- Shoot and protect
- Widescreen
- Widescreen signaling (WSS)
References
- ^ Active Format Description (AFD): An Overview (PDF). Tandberg Television. 2008.
- ^ "EBU QC - Details of 0001W: Active Format Description (v5.1)". qc.ebu.io. Retrieved 2023-03-20.
- .
- ^ Daniel, Peter (2020). "Digital Television - AFD codes explained". Peter Daniel. Retrieved 2023-03-20.
- ^ Group, Andy Finney ATSF for the Digital TV. "DTG :: News :: DTG collects Emmy Award". www.dtg.org.uk. Archived from the original on 2012-03-26. Retrieved 2012-01-17.
- ^ Group, Andy Finney ATSF for the Digital TV. "DTG Publications: D-Book". www.dtg.org.uk. Archived from the original on 2011-10-17. Retrieved 2012-01-17.
- ^ "Serial Digital Line 23 - Wide Screen Decoder" (PDF). MICROVIDEO. Archived from the original (PDF) on 2015-01-03. Retrieved 2014-09-25.
- ^ ETSI TS 101 154 V1.7.1 Annex B (PDF). p. 55.
- ^ ETSI TS 101 154 V2.3.1 (2017-02) (PDF). ETSI. January 2017. p. 179.
- ^ ATSC A/53 Part 4 (PDF). Advanced Television Systems Committee. August 2009.
- ^ SMPTE 2016-1-2007 (PDF).