1. – of pictures. The ratio of length to height of pictures. Nearly all TV screens are currently 4:3, i.e. four units across to three units in height but there is a growing move towards 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 have been used, 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 defines luminance pixels which are not square. In the 525/60 format there are 486 active lines each with 720 samples of which 711 may be viewable due to blanking. Therefore the pixel aspect ratio on a 4:3 screen is:

486/711 x 4/3 = 0.911
(i.e. the pixels are 10 percent taller than they are wide)

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

576/702 x 4/3 = 1.094
(i.e. the pixels are 9% wider than they are tall)

The newer DTV image standards, and all those for HD, define square pixels.

Account must be taken of pixel aspect ratios – for example in executing a DVE move – when 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 computer platforms and television systems. Computers nearly always use square pixels so their aspect ratio must be adjusted to suit television. This change may not be real-time and its quality will depend on the processing used.