02-28-2006, 02:57 PM
Join Date: Apr 2005
Originally Posted by jcsickz
I was talking to a guy who said that all HDTV was broadcast in 4:3. I don't think he knows what he is talking about because the only time I see the bars on the sides is when an SD signal is coming through an HD channel. Is this guy talking out of his ass?
one of the parameters that the MPEG2 HDTV encoder (broadcaster) specifies on the fly is Aspect Ratio
5.5.1 Active Area Signaling
4 December 2003
Guide to the Use of the ATSC Digital Television
A consumer device such as a cable or satellite set-top box cannot reliably determine the active area of video on its own. Even though certain lines at the top and bottom of the screen may be black for periods of time, the situation could change without warning. The only sure way to know active area is for the service provider to include this data at the time of video compression and to embed it into the video stream.
working in this area, MPEG has adopted an amendment to the MPEG-2 Video standard to include active area data.
Letterboxed movies can be seen on cable, satellite, and terrestrial channels today. If one observes closely, considerable variability in the size of the black bar areas can be seen. In fact, variations can be seen even over the course of one movie.
As mentioned previously, a display device may wish to mitigate the effects of uneven screen aging by substituting gray video for the black areas. It is problematic for the display to be required to actively track a varying letterbox area, and real-time tracking of variations from frame to frame would be ifficult (if not impossible).
Clearly, two approaches are possible. First, include—on a frame-by-frame basis—a video parameter identifying the number of black lines (for letterbox) or number of black pixels (for pillar-box). Second, standardize on just two standard aspect ratios: 16:9 and 4:3.
5.5.3 Treatment of Active Areas Greater than 16:9
Any wide-screen source material can be coded into a 16:9-coded frame. No aspect ratio for coded frames exceeding 16:9 is standardized for cable, terrestrial broadcast, or satellite transmission in the U.S. If the aspect ratio of given content exceeds 16:9, the coded image will be letterboxed inside the 16:9 frame, as shown in Figure 5.9, where 2.35:1 material is letterboxed inside the 16:9 frame.
Figure 5.9 Example of active video area greater than 16:9 aspect ratio.
It can be helpful for a display to know the actual aspect ratio of the active portion of the 16:9 coded frame for a variety of reasons, including:
• Reduction in the effects of uneven screen aging. The display device controller may wish to use gray instead of black for the bars.
• The display may offer the user a “zoom” option to make better use of available display area, and knowledge of the aspect ratio can automate the selection of this display option.
The zoom feature can be vertical scaling only, or a combination of horizontal and vertical where the leftmost and rightmost portions of the image are sacrificed to fill the screen area vertically.
Several standards include aspect ratio data. The MPEG-2 video syntax includes horizontal and vertical size data and aspect ratio indication of the coded image. An NTSC signal is normally thought to be intended for 4:3 display, but this is not always the case. EIA-608-B includes a “squeezed” bit, and IEC 61880 defines a method for NTSC VBI line 20. The line-20 method is currently used for playback of anamorphically coded DVDs, when the DVD player supports it and is properly set up by the user.
ATSC Guide to Use of the ATSC DTV Standard 4 December 2003
5.5.4 Active Format Description (AFD) and Bar Data
In recognition of these issues, the ATSC undertook a study of available options and—in Amendment 1 to A/53B—decided to endorse the basic signaling structure developed by the DVB consortium. The benefits of common active format description signaling across many
different markets is easily understood.
Some common active video formats represented by the 4-bit AFD field include:
• The aspect ratio of the active video area is 16:9; when associated with a 4:3 coded frame, the active video is top-justified
• Active video area is 16:9; when associated with a 4:3 coded frame, the active video is centered vertically
• Active video area is 4:3; when associated with a 16:9 coded frame, the active video is centered horizontally
• Active video area exceeds 16:9 aspect ratio; active video is centered vertically (in whatever coded frame is used)
• Active video area is 14:9; when associated with a 4:3 coded frame the active video is centered vertically; when associated with a 16:9 coded frame, it is centered horizontally
It should be noted that certain active formats signal to the receiver that active video may be safely cropped in the receiver display (see , “Digital Receiver Implementation Guidelines and Recommended Receiver Reaction to Aspect Ratio Signaling in Digital Video Broadcasting.”
In addition to AFD, Amendment 1 defined another data structure, bar_data(), also for use in the video Elementary Stream. The bar_data() structure, like AFD, appears in the picture user_data() area of the video syntax. While the AFD gives a general view of the relationship between the coded frame and the geometry of the active video within it, bar_data() is able to indicate precisely the number of lines of black video at the top and bottom of a letterboxed image, or the number of black pixels at the left and right side of a pillar-boxed image.
For the ATSC system, AFD and/or bar_data() are included in video user data whenever the rectangular picture area containing useful information does not extend to the full height or width of the coded frame. Such data may optionally also be included in user data when the rectangular picture area containing useful information extends to the full height and width of the coded
The AFD and bar_data() are carried in the user data of the video Elementary Stream. After each sequence start (and repeat sequence start) the default aspect ratio of the area of interest is signalled by the sequence header and sequence display extension parameters. After introduction, each type of active format data remain in effect until the next sequence start or until another instance is introduced. Receivers are expected to interpret the absence of AFD and bar_data() in a sequence start to mean the active format is the same as the coded frame. Since it is not able to represent non-standard video aspect ratios, AFD may be only an approximation of the actual active video area. However when bar_data() is present, it should be assumed to be exact. If the bar_data() and the AFD are in conflict, the bar_data() should take precedence
Last edited by maicaw; 02-28-2006 at 03:11 PM.