Monitor timing formulae

[Gen Stonewall]

First of all, what is a timing formula? Second, what is the difference between General and Discrete timings, and why is selecting the right one important?

 

[Gosharkss]

There is a relatively simple way to calculate the maximum refresh rate of a monitor (simple for me, I've been doing this for 15 years). You need to know the following. The maximum horizontal scan frequency, and the resolution that you want to run. All manufacturers publish this data. The formula below will give you an approximate refresh rate, to calculate the exact refresh rate the math gets harder and you need to know more details about the monitor such as the exact horizontal and vertical blanking time.

(1 / Horiz Scan Freq) * (Number of Vertical lines + 50) = x
(1 / x) = Refresh rate

So in the case of a 121Khz monitor it has a (121,000) 121Khz horizontal maximum scan rate. Lets assume that you want to run 1600 x 1200 resolution. What is the theoretical maximum refresh rate?

(1 / 121000) = .000008264 * (1200 + 50) =1250
Note: Add 50 lines to the total number of line to account for vertical blanking time.

0.000008264 * 1250 = 0..010330579

1 / 0..010330579 = 96.8Hz maximum vertical refresh rate (Convert back to time base)

As long as the horizontal and vertical refresh rates are within the range of the monitor the monitor will display an image.

All this math leads to the following.

There are classes of monitors based on the horizontal scan frequency. Typically as the HSF goes up so does the price of the monitor.

I'm amazed at how many people purchase monitors that are overkill for the resolution or application they want to run. If you need to run 1856 x 1392 or even 2048 x 1536 spend the bucks and buy a high-end monitor. If you are going to run 1280 x 1024 up to 1600 x 1200 on a 21" monitor you can save $$$ by purchasing a monitor that is optimized for those resolutions. Most games for example run at lower resolutions. There is little benefit in purchasing a monitor with higher resolution capabilities than the one you plan to use.

The horizontal scan frequency is the key specification to look at.
70 - 85Khz monitor is optimized for 1024 x 768 at 85Hz
92 - 96Khz class is optimized for 1280 x 1024 at 85Hz
107 - 110Khz monitor is optimized for 1600 x 1200 at 85Hz
115 - 117Khz Monitor is optimized for 1856 x 1392 at 85Hz
121Khz is optimized for 1900 x 1200 Cinema screen

Good Luck

Jim Witkowski
Chief Hardware Engineer
Cornerstone / Monitorsdirect.com

 

[Gen Stonewall]

I'll definitely consider your response when I get my next monitor, but it seems that my question was on a different subject. I was wondering what General and Discrete timings mean (I can change them in my GeForce2 control program).

Now to connect the two issues... Is all of this only applicable to CRTs, that is, are monitor timings (what I can change in the control program) and the refresh rate formula you gave both not relevant to LCDs? Or is one relevant and not the other? Or are both relevant?

 

[ADxS]

The "timing" essentially establishes how much blank "border" there is, how its distributed "around" the active image, and what the refresh rate will be. Discrete timings are hard-coded standards that graphics chip vendors and monitor manufacturers agreed upon, back when 1280x1024 was about as high as resolutions could go. As the list of possible resolutions grew, a hard-coded list was impractical, so VESA agreed on a General Timing Formula instead, which most new cards and monitors adher to.

The distinction is pretty much irrelevant unless you're using a CRT.