Interlace

The relationship between the two fields of a television image.

Television or video images are scanned in a sequence of horizontal lines, beginning at the upper left corner, and reaching the bottom right corner at the end of the field. Thereupon the scan is returned to the upper left corner to begin the next field. As a consequence of the line structure, all television images are sampled vertically. Within a line, the signal may remain analog or be sampled digitally.

In most formats including NTSC, PAL and SECAM, these lines are interlaced 2:1 i.e. On the first scan lines 1,3,5,7,9, etc are scanned. This produces the first ‘field’. The second field is then made up of lines 2,4,6,8, etc down to the maximum line count (625 or 525). This reduces the flicker in the image.

https://www.nfsa.gov.au/sites/default/files/uploads/2010/09/01/fields.gif

The video raster is scanned over the face of the CRT by the electron gun tracing alternate scan lines in successive refresh cycles. The quality of interlaced video is lower than sequentially scanned (non-interlaced) video because only half of the lines are refreshed at a time and, interlaced video scans at a lower rate than non-interlaced video allowing for the manufacturer of less expensive video monitors. PAL video (standard TV in Australia ) uses interlaced video.

A display system where the even scanlines are refreshed in one vertical cycle (field), and the odd scanlines are refreshed in another vertical cycle. The advantage is that the bandwidth is roughly half that required for a non-interlaced system of the same resolution. This results in less costly hardware. It also may make it possible to display a resolution that would otherwise be impossible on given hardware. The disadvantage of an interlaced system is line twitter, especially when displaying objects that are only a single scan line high.

There is continuing debate about the use of interlaced or non-interlaced (progressive, as used for computer screens) scans for the new DTV formats.