At this point, another factor needs to be taken into consideration. The above calculation only shows us how much memory is required for a 24 bitimage at resolution of 1024 * 768. Once we have this image in memory, we need to convert it to an analogue signal and send it to the monitor. The speed at which we do this is affected by the "refresh rate", as the graphics card and the monitor must be synchronized when exchanging data. The refresh rate defines how quickly in hertz the graphics card needs to supply video data to the monitor, and the monitor needs to display it. With modern graphics cards, the monitor usually restricts the maximum refresh rate, however depending on the hardware in question either of the devices can be responsible for the restriction. When configuring a graphics card and monitor, the highest possible refresh rate should always be selected, up to a maximum of around 90 Hz. If the refresh rate is too low, an annoying flicker effect can be seen where the human eye is almost able to see the refresh occurring. This can cause eyestrain and headaches quite quickly. A refresh rate of above 90 Hz is usually indistinguishable from a lower rate, and just consumes processor time and bandwidth that could be better used elsewhere.
There is also an important distinction between LCD and CRT monitors. Whilst CRT monitors usually work in the 60 Hz to 110 Hz range, LCD panels operate at a refresh rate appropriate to the current screen resolution. The difference between these monitor types is discussed further in the Monitors chapter.
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