Aspect ratio

For standard definition (SD) television, the image is normally broadcast with an aspect ratio, i.e. the ratio of width-to-height of 4:3 although the widescreen 16:9 format is also used by several digital television broadcasters. For high definition (HD) television, 16:9 is the specified format.

Pixels and bandwidth

A pixel, short for ‘picture element’, is the smallest active element or dot in an image. A well-defined picture will in general have more pixels than a less-defined one. The resolution of a picture is defined by the number of pixels in the horizontal and vertical directions. The number of pixels in the vertical direction is set by the number of lines. However, the same cannot be said of the number of pixels per horizontal line. This has been internationally set to 720 pixels/line for both PAL and NTSC. This gives a resolution for SD television of 720 X 576. For HD television, the figures are higher, namely 1920 X 1080 or 1280 X 720 (PAL) and 720 X 480 (NTSC). Having determined the number of pixels per line, the total number of pixels in a single picture of a SD television may then be calculated as active pixels/line X active lines. This gives

720 X 576 = 414,720pixels(PAL)

720 X 480 = 345,600 pixels(NTSC)