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When referring to a computer monitor or other display, what do you mean by interlace or interlacing?

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This would be better asked on superuser.com – Kyle Trauberman Jan 11 '10 at 6:16
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migrated from stackoverflow.com Jan 11 '10 at 6:32

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3 Answers

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On a CRT monitor (before LCD that is) the image on the screen is drawn as lines across the screen in a scanning pattern from top to bottom.

If HALF the lines are draw in one 'scan', then the other half in a second scan, it's known as interlacing.

It has the advantage that the electronics can run at a slower speed, and the user's eye/brain combines the two images to give a single image.

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Yes, but it should not be forgotten, that this is for the price of lower actual refresh rate. For example, 50 Hz interlaced is flickering (only 25 full pictures per second are shown). That's why the later CRTs had 100Hz or more to eliminate flickering. – Martin Jan 11 '10 at 9:05
A very good point, Martin, I was thinking the same as I was typing the bit about the eye/brain... – pavium Jan 11 '10 at 11:33
@Martin At least as far back as the early 90s normal computer CRT refresh rates were non-interlaced. 60/75/85/100/120hz modes were NI unless explicitly marked. Some mostly cheaper monitors had an 85hz interlaced rate to claim a higher maximum resolution; but that was lying via the specsheet since 85hz interlaced on them was extremely flickery and unusable because, unlike a CRT TV (designed for 30hz interlaced), the phosphors on a CRT monitor faded too rapidly to maintain a steady image with only 42.5 refreshes/second. – Dan Neely Sep 28 '11 at 15:19
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up vote 1 down vote

Interlacing is a display technique that enables a monitor to provide more resolution inexpensively. With interlacing monitors, the electron guns draw only half the horizontal lines with each pass (for example, all odd lines on one pass and all even lines on the next pass). Because an interlacing monitor refreshes only half the lines at one time, it can display twice as many lines per refresh cycle, giving it greater resolution. Therefore, Interlacing provides the same resolution as non-interlacing, but does it less expensively.

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Interlace is a technique of improving the picture quality of a video signal primarily on CRT devices without consuming extra bandwidth. CRT production is declining and interlacing causes problems on certain display devices such as LCDs.

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