Pixel aspect ratio
Use of pixel aspect ratio mostly involves pictures pertaining to standard-definition television and some other exceptional cases.
Most other imaging systems, including those that comply with SMPTE standards and practices, use square pixels.
Actual displays do not generally have non-square pixels, though digital sensors might; they are rather a mathematical abstraction used in resampling images to convert between resolutions.
Digital video frames are generally defined as a grid of pixels used to present each sequential image.
However, there is not a standard-definition television standard that properly defines image edges or explicitly demands a certain number of picture elements per line.
Furthermore, analog video systems such as NTSC 480i and PAL 576i, instead of employing progressively displayed frames, employ fields or interlaced half-frames displayed in an interwoven manner to reduce flicker and double the image rate for smoother motion.
ITU-R BT.601 did not define 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+1⁄2 MHz.
Based on this information: SMPTE RP 187 further attempted to standardize the pixel aspect ratio values for 480i and 576i.
However, due to significant difference with practices in effect by industry and the computational load that they imposed upon the involved hardware, SMPTE RP 187 was simply ignored.
Therefore, the pixel aspect ratio of common standard television systems cannot be calculated based on edges of pictures.
In modern digital imaging systems and high-definition televisions, especially those that comply with SMPTE standards and practices, only square pixels are used for broadcast and display.
However, some formats (ex., HDV, DVCPRO HD) use non-square pixels internally for image storage, as a way to reduce the amount of data that must be processed, thus limiting the necessary transfer rates and maintaining compatibility with existing interfaces.
The pixel aspect ratio support is also required to display, without distortion, legacy digital images from computer standards and video-games what existed in the 80s.
The program then uses a table (similar to the one below) to determine the correct pixel aspect ratio value.
Generally speaking, to avoid confusion, it can be assumed that video editing products never ask for the storage aspect ratio as they can directly retrieve or calculate it.
Pixel aspect ratio values for common standard-definition video formats are listed below.
See references for sources giving both, and SDTV: Resolution for a table of storage, display and pixel aspect ratios.
