576i

576i is a standard-definition video mode used in (former) PAL and SECAM countries. In digital applications it is usually referred to as "576i"; in analogue contexts it is often called "625 lines", and the aspect ratio is usually 4:3 unless otherwise specified. Its NTSC counterpart is 480i; these are the two common forms of standard-definition television.

The 576 identifies a vertical resolution of 576 lines, and the i identifies it as an interlaced resolution. The field rate, which is 50 Hz, is sometimes included when identifying the video mode, i.e. 576i50; another notation, endorsed by EBU/SMPTE, includes the frame rate, as in 576i/25.

Its basic parameters common to both analogue and digital implementations are: 576 scan lines or vertical pixels of picture content, 25 frames (giving 50 fields) per second.

In analogue 49 additional lines without image content are added to the displayed frame of 576 lines to allow time for older CRT circuits to retrace for the next frame, giving 625 lines per frame. Digital information not to be displayed as part of the image can be transmitted in the non-displayed lines; teletext and other services and test signals are often implemented.

Analogue television signals have no pixels; they are rastered in scan lines, but along each line the signal is continuous. In digital applications, the number of pixels per line is an arbitrary choice as long as it fulfils the sampling theorem. Values above about 500 pixels per line are enough for conventional broadcast television; DVB-T, DVD and DV allow better values such as 704 or 720.

The video format can be transported by major digital television formats, ATSC, DVB and ISDB, and on DVD, and it supports aspect ratios of standard 4:3 and anamorphic 16:9.