CRT Monitor Resolution and Refresh Rates (VSF)

The most important aspect of a monitor is that it should give a stable display at the chosen resolution and colour palette. A screen that shimmers or flickers, particularly when most of the picture is showing white (as in Windows), can cause itchy or painful eyes, headaches and migraines. It is also important that the performance characteristics of a monitor be carefully matched with those of the graphics card driving it. It’s no good having an extremely high performance graphics accelerator, capable of ultra high resolutions at high flicker-free refresh rates, if the monitor cannot lock onto the signal.

Monitor Resolution

Resolution is the number of pixels the graphics card is describing the desktop with, expressed as a horizontal by vertical figure. Standard VGA resolution is 640×480 pixels. This was pretty much obsolete by the beginning of the new millennium, when the commonest CRT monitor resolutions were SVGA and XGA – 800×600 and 1024×768 pixels respectively.

Monitor Refresh Rate

Refresh rate, or vertical scanning frequency, is measured in Hertz (Hz) and represents the number of frames displayed on the screen per second. Too few, and the eye will notice the intervals in between and perceive a flickering display. It is generally accepted – including by standards bodies such as VESA – that a monitor requires a refresh rate of 75Hz or above for a flicker-free display. A computer’s graphics circuitry creates a signal based on the Windows desktop resolution and refresh rate. This signal is known as the horizontal scanning frequency, (HSF) and is measured in KHz. A multi-scanning or autoscan monitor is capable of locking on to any signal which lies between a minimum and maximum HSF. If the signal falls out of the monitor’s range, it will not be displayed.

Calculating Maximum Refresh Rate

Where VSF = vertical scanning frequency (refresh rate) and HSF = horizontal scanning frequency the formula for calculating a CRT monitor’s maximum refresh rate is:

VSF = HSF / number of horizontal lines x 0.95

So, a monitor with a horizontal scanning frequency of 96kHz at a resolution of 1280×1024 would have a maximum refresh rate of:

VSF = 96,000 / 1024 x 0.95 = 89Hz.

If the same monitor were set to a resolution of 1600×1200, its maximum refresh rate would be:

VSF = 96,000 / 1200 x 0.95 = 76Hz.

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