The Pause That Refreshes

Perusing a monitor's glossy spec sheet and seeing that it supports 1600- by 1200-pixel resolution and refresh rates as high as 120 Hz does not mean that you can get that high resolution at 120 Hz, or even at a solid 80-Hz refresh rate. The refresh rate that a monitor can support goes down as the resolution goes up. What you need, and what some spec sheets provide, is a list of screen resolutions with their maximum refresh rates. But without this list, you can still gauge a monitor's capabilities by examining two other frequency specifications.

While a monitor's maximum vertical-refresh frequency tells you little, the horizontal scanning freq uency (in kHz) and video bandwidth (in MHz) are reliable measures of what a monitor can do. Maximum horizontal scan rate describes the rate at which a monitor can write rows of pixels down the screen; video bandwidth is the rate (roughly) at which a monitor can pump pixels onto its screen surface. As the resolution goes up, a monitor has to write more pixels (and rows of pixels) with every vertical screen scan. The higher the vertical-refresh rate, the more times it has to paint that screenload of pixels per second. Monitor spec sheets don't always list supported video modes, but they always supply scanning-frequency ranges.

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Guideline Frequencies

Thumbnail specs for two high-resolution modes.

Mode                    Horiz. Scan Rate       Bandwidth

1280 X 1024 (75 Hz)          85 kHz             120 MHz
1600 X 1200 (75 Hz)          95 kHz             200 MHz