20 How do I calculate the minimum bandwidth required for a monitor?
Description
This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
20 How do I calculate the minimum bandwidth required for a monitor?
[Paraphrased from Richard Trueman (crtman@delphi.com) by Michael Scott (scott@bme.ri.ccf.org) ]
The bandwidth is a measure of the amount of data that a monitor can handle in one second. It is measured in MHz. The maximum bandwidth of a monitor should be matched as closely as possible to the dot clock of the video controller. If there is a mismatch, then capacity of either the controller or monitor may be wasted. It is not as serious for the monitor to lack video bandwidth as it is for a graphics controller to lack the dot clock rate needed for a given video mode. The maximum bandwidth of a monitor cannot be directly calculated without detailed timing information, but often this information is provided by the manufacturer. In fact, the exact bandwidth required in a monitor at a given pixel addressability and vertical refresh frequency is also dependent on internal timing of the monitor itself. To calculate an approximation of the required bandwidth for a given pixel addressability and vertical refresh frequency: [This approximation tends to overestimate the actual bandwidth frequency from my experience -Mike]
The bandwidth is dependent on the number of vertical and horizontal pixels and the vertical refresh rate. This approximation grossly simplifies the calculation:
Given that the vertical pixel addressability is Y, horizontal pixel addressability is X and refresh rate is R:
To account for the additional time required for the vertical blanking interval, Y is multiplied by 1.05. The additional time required for the horizontal blanking interval is about 30% of the scan time, so use 1.3X. Note that 30% is very conservative with most new monitors. In order to do an exact calculation, you would have to know the vertical and horizontal blanking intervals for the mode in question, as well as the horizontal scan frequency. So the resulting approximation is:
bandwidth = 1.05Y*1.3X*R
i.e. for 1280x1024 at 60 Hz,
approx. bandwidth required = 1.05*1024*1.3*1280*60 = 107 MHz
Clearly these are gross simplifications, so use this equation for approximations only.
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