Most modern imaging systems describe an image as a grid of very small but nonetheless square pixels. However, some imaging systems, especially those which must maintain compatibility with analog standard-definition, define image as a grid of rectangular pixels in which the width of the pixel is different from that of its height. To describe this proportion, we state it in form of Pixel Aspect Ratio.
Use of Pixel Aspect Ratio mostly involves pictures pertaining to standard-definition television. Most other imaging systems, including those complying with SMTPE standards and practices, use square pixels.
However, standard-definition television picture is essentially incompatible with digital image as it has well-defined lines but it does not have standardized or well-defined edges or well-defined picture elements similar to pixels. Furthermore, analog video systems such as NTSC 480i and PAL 576i, instead of employing progressively-displayed frames, employs fields or interlaced half-frames displayed in an interwoven manner to create motion.
However, the term Pixel Aspect Ratio was first coined when ITU-R BT.601-4 (commonly known as "Rec. 601") specified that standard-definition television pictures are made of lines which contain exactly 720 non-square pixels. ITU-R BT.601-4 did not defined the exact pixel aspect ratio but did provide enough information to calculate the exact pixel aspect ratio based on industry practices: The standard luma sampling rate of precisely 13.5Mhz. Based on this information:
SMPTE RP 187 further attempted to standardize the Pixel Aspect Ratio values for 480i and 576i. It designated 177:160 for 480i or 1035:1132 for 576i. However, due to significant difference with practices in effect by in industry and the computational load that they imposed upon the involved hardware, SMPTE RP 187 was simply ignored. SMPTE RP 187 information annex A.4 further suggested the use of 11:10 for 480i.
As of this writing, ITU-R BT.601-6, which is the latest edition or ITU-R BT.601, haven't explicitly specified the aspect ratio for any of standard-definition television pictures.
Unfortunately, not all the standard TV pictures are exactly 4:3: As stated above, in analog video, the center of picture is well-defined but the edges of the picture are not standardized. As a result, some analog devices (mostly PAL devices but also some NTSC devices) generated motion pictures that were horizontally (slightly) wider. This also proportionately applies for anamorphic widescreen (16:9) pictures. Therefore, in order to maintain a safe margin of error, ITU-R BT.601 required 16 more non-square pixels per each line (8 more at each edge), which may not be visible in all TVs, to be sampled to ensure that all video data near the margins are saved.
This requirement, however, had two implications:
Displaying an image with a certain pixel aspect ratio on a device whose pixel aspect ratio is different will cause the image to look unnaturally stretched or squished in on of the either horizontal or vertical directions. Picture of a circle, generated for a computer display would look like an ellipse on a standard-definition television. This issue is more evident on a wide-screen TVs.
Pixel Aspect Ratio must be taken into consideration by video editing software products which edit video files with non-square pixels, especially when mixing video clips with different Pixel Aspect Ratios. This would be the case when creating a video montage from various cameras employing different video standards. Special effects software products must also take the Pixel Aspect Ratio into consideration, since some special effects require to calculate the distances from a certain point so that they look visually correct. An example of such effects would be radial blur or motion blur.
Pixel Aspect Ratio is often confused with Picture Aspect Ratio. Picture Aspect Ratio is the ratio of the image width and height.Due to non-squareness of pixels in Standard-definition TV, there are two types of Picture Aspect Ratio: Storage Aspect Ratio (SAR) and Display Aspect Ratio (DAR). Storage Aspect Ratio is the ratio of the image width to height in pixels and can be easily calculated from video file. Display Aspect Ratio is the ratio of image width to height (in a unit of length such as centimeters or inches) when displayed on screen and is calculated from the combination of Pixel Aspect Ratio and Storage Aspect Ratio.
However, users who know the definition of these concepts may also get confused as well. Poorly-crafted user-interfaces or poorly-written documentations can easily cause such confusion.
Some video-editing software applications often ask users to specify an "Aspect Ratio" for their video file, presenting him or her with the choices of "4:3" and "16:9"! (Sometimes, these choices may be "PAL 4:3", "NTSC 4:3", "PAL 4:3" and "PAL 16:9"!) In such situations, the video editing program is implicitly asking the Pixel Aspect Ratio of the video file by asking for information about the Video System from which the video file in question is originated. The program will then use a table (similar to the one in Pixel Aspect Ratio of Common Video Formats section below) to determine the correct Pixel Aspect Ratio value.
The rule of thumb is: Video editing products never ask for Storage Aspect Ratio; they can simply retrieve picture dimensions and calculate it. Non-square-pixel–aware applications would often ask either Pixel Aspect Ratio or Display Aspect Ratio, from either of which they can calculate the other.
|Video System||Picture Dimensions||Pixel aspect ratio|
|Standard (4:3) NTSC (e.g. 480i)|| 720×480 |
| 10:11 || 10:11 || 8:9 |
|Anamorphic widescreen (16:9) NTSC|| 40:33 || 40:33 || 32:27 |
|Standard (4:3) PAL (e.g. 576i)|| 720×576 |
| 59:54 || 12:11 || 16:15 |
|Anamorphic widescreen (16:9) PAL|| 118:81 || 16:11 || 64:45 |
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