Aspect Ratio

1. of pictures. The ratio of length to height of pictures. All TV screens used to be 4:3, i.e. four units across to three units in height, but now all new models are widescreen, 16:9. Pictures presented this way are believed to absorb more of our attention and have obvious advantages in certain productions, such as sport. In the change towards 16:9 some in-between ratios were used for transmission, such as 14:9.

2. of pixels. The aspect ratio of the area of a picture described by one pixel. The ITU-R BT.601 digital coding standard for SD defines luminance pixels which are not square. In the 525/60 format there are 486 active lines each with 720 samples of which only 711 may be viewable due to blanking. Therefore the pixel aspect ratios on 4:3 and 16:9 screens are:

486/711 x 4/3 = 0.911 (tall)
487/711 x 16/9 = 1.218 (wide)

For the 625/50 formats there are 576 active lines each with 720 samples of which 702 are viewable so the pixel aspect ratios are:

576/702 x 4/3 = 1.094 (wide)
576/702 x 16/9 = 1.458 (wider)

All HD digital image standards define square pixels.

Account must be taken of pixel aspect ratios when, for example, executing DVE moves such as rotating a circle. The circle must always remain circular and not become elliptical. Another area where pixel aspect ratio is important is in the movement of images between platforms, such as computers and television systems. Computers generally use square pixels so their aspect ratio should be adjusted for SD television-based applications.

See also: ARC, Pixel