Interlace (scan)

Method of scanning lines down a screen (vertical refresh). It is still used in most of today’s television broadcasts but was originally designed to suit the needs of CRT displays and analog broadcasts. Interlace is indicated in television scan formats by an ‘I’ e.g. 1080I, etc. (though the use of ‘i’ is common). Each displayed picture comprises two interlaced fields: field two fills in between the lines of field one. One field displays odd lines, then the other shows even lines. For analog systems, this is the reason for having odd numbers of lines in a TV frame eg 525 and 625, so that each of the two fields contain a half-line, causing the constant vertical scan to place the lines of one field between those of the other.

The technique greatly improves the portrayal of motion and reduces picture flicker without having to increase the picture rate, and therefore the bandwidth or data rate. Disadvantages are that it reduces vertical definition of moving images by about 30% (see Interlace Factor) of the progressive scan definition and tends to cause some horizontal picture detail to ‘dither’ – causing a constant liveliness even in still pictures.

Interlaced video requires extra care for processing, such as in DVE picture manipulation of size, rotation, etc, as any movement between fields has to be detected if frame-based processing which can produce higher-quality results, is used. Also frame freezes and slow motion need ‘de-interlace’ processing.

There is continuing debate about the use of interlaced and progressive scans for digital television formats. This has intensified now that the increasingly popular panel displays all use progressive scans. Interestingly the latest television standard for ITU-R BT.2020, for 4K and 8K UHD only includes progressive scans.

See also: Interlace Factor, Progressive