Interlacing is a method used to display video images. An NTSC display contains 525 horizontal scan lines (vertical resolution) and uses a refresh rate of 60 Hz. Horizontal resolution is dependent on the CRT's ability to quickly display changes in the analog video signal. Horizontal resolution is expressed in terms of the number of narrow, vertical lines that can be visually resolved. A low-quality video tape may only have a horizontal resolution of 200 to 225 lines, while a high-quality image will be able to resolve over 400 lines. PAL and SECAM systems are similar in principle, but use more scan lines (625), a lower refresh rate (50 Hz), and improved colour. PAL and SECAM deliver better quality video than NTSC. An NTSC display has a frame rate of 30 Hz - the entire image changes every 1/30 of a second. Each frame is composed of two fields, which are updated every 1/60 of a second. The first field contains odd-numbered scan lines (1, 3, 5, 7, ..., 525) and the second field the even-numbered lines (2, 4, 6, ..., 524). The display is updated as follows: - Frame 1, field 1. - 1/60 second later: frame 1, field 2. - 1/60 second later: frame 2, field 1. - 1/60 second later: frame 2, field 2. - ... A non-interlaced video signal (sometimes called progressive scan) updates each frame every 1/60 of a second. This produces less flicker. Interlacing was a means of managing hardware limitations in early TV systems. Non-interlaced signals can be derived from interlaced signals by interpolating new video data. Digital televisions do not use analog scanning to generate images. They have a fixed number of pixels, which establish the basic image resolution. Video on these displays may be interlaced or non-interlaced, depending on the display format and image quality.

Interlaced scanning is a method of displaying an image on a cathode ray tube like a standard television (not an LCD or plasma screen.) An interlaced system alternates every sixtieth of a second between showing the odd and even horizontal scan lines. First it will show the odd lines, and then one sixtieth of s second later it will show the even lines, so every thirtieth of a second a complete image is created, giving a true speed of 30 frames per second.