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Definition: interlace


To illuminate a screen by displaying all odd lines in the frame first and then all even lines. Interlacing uses half frames per second (fields per second) rather than full frames per second.

The interlace method was developed for TV broadcasting because the allotted bandwidth for TV channels in the 1940s was not sufficient to transmit 60 full frames per second. It was decided that interlacing with 60 half frames was visually better than 30 non-interlaced full frames.

Interlace vs. Progressive Scan (I vs. P)
Interlaced screens display odd lines first: 1-3-5, etc.; then even lines: 2-4-6, etc. Non-interlaced "progressive scan" screens display lines consecutively: 1-2-3-4-5-6, etc.

All old tube TVs (CRTs) were interlaced. Very old CRT computer monitors were interlaced at their highest resolution and progressive at lower resolutions. Today, some digital TV standards are interlaced, such as the high-definition 1080i format, and 1080i material is commonly transmitted by the TV networks. HDTV sets support both interlace and progressive scan methods (see HDTV and DTV). See deinterlace, vertical scan frequency, 4K 3D TV and interlaced GIF.




The Difference
In one second, interlacing displays 30 sets of odd lines and 30 sets of even lines, resulting in 60 fields per second. For every half frame of interlaced scan, progressive scan displays a full frame. Modern DVDs and monitors convert the 60 fields in interlaced NTSC content to 60 frames by interpolating the missing lines.





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