Video Signals And Timing: Difference between revisions
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== Frequencies ==
When you increase the resolution, you will need to send more pixels to the display. If you would keep sending pixels at the same rate, the time to transmit one frame will go up, and consequently the amount of frames in a certain timespan will go down. Since a CRT displayed pixel only gives light for a short time before running out of energy, it needs to be repeatedly refreshed. If this is done fast enough (at about 60Hz, 60 times a second) the screen appears almost constant to the human eye. This improves further when the refresh rate goes up to a point where it doesn't matter to the human eye. However when it drops too much, the screen starts appearing flashing, causing headaches to the user. Hence, we need to keep the frequency at at least 60Hz for user's sanity, and below some other rate dictated by the monitor's capabilities. To make a full frame of pixels fit within one sixtieth of a second, we will have to adjust the speed at which these pixels are transmitted. This speed is called the pixel clock, or dot clock. For example, a VGA's dot clock is either 25MHz or 28MHz, corresponding to 25 million pixels per second or 28 million pixels per second, the latter one being only just enough to display a resolution of 720x480 at 60Hz (recall that the active display is only a part of the frame). Most higher resolution video can therefore use a wide range of dot clocks, well above 25Hz, with the current range allowing enough bandwith to easily exceed 1600x1200 at a 100Hz.
While the resolution is limited by the video card, In most non-VGA scenario's, it is the monitor that can not handle the speed of the signal. The monitor has a allowed vertical frequency (the amount of frames per second, usually listed in Hz), and horizontal frequency (listed in KHz). Some CRTs are fixed frequency, only allowing certain frequencies to be used both horizontally and vertically. Old VGA displays are infamous for burning out when you feed them a signal that doesn't exactly match these rates. While modern CRTs are mostly protected from bad signaling, you must know that you can break hardware in this fashion, and that you need to be careful.
== General Timing Formula ==
In order to cope with all the CRT antiquities, VESA has produced a set of equations that allows you to compute the various display settings you need, given the desired resolution. There are three separate sets of formulae: One to compute all settings from resolution and desired refresh rate, one taking resolution and horizontal frequency, and one taking resolution and dot clock.
== Brendan's Posts ==▼
Normally before setting a mode you would do the following:
* Compute the horizontal refresh rate and pixel clock from resolution and frequency
* Check the video card for the nearest pixel clock it can give you
* Compute all settings given the resolution and pixel clock.
* Check if the horizontal and vertical frequencies are within the monitor's allowed range.
If the frequencies happen to be out of range, you will have to adjust the refresh rate and or resolution. Since the refresh rate can go down a bit without giving issues, that's usually the first choice to fix. Hence:
* Clamp the horizontal frequency to its limit
* Compute pixel clock and vertical frequency from resolution and the new horizontal frequency
* Select the appropriate pixel clock the video card has (making sure you are rounding away from the limit, so round down if you use the maximum horizontal frequency)
* Compute the horizontal and vertical frequency given the new dot clock
* Check if the frequencies are within range.
The horizontal frequency should now be correct, you can decide whether or not to allow the new mode because the vertical rate has dropped below an acceptable rate, and you can inform the user that the mode is not supported.
▲== Brendan's Original Posts ==
>Any problems if I wikify this?
None at all - IMHO this information is required just to use VBE reliably; along with EDID (but you can find version 1.1 of the EDID data structures on Wikipedia) and details for the VBE DDC function (which VESA will let you have for free).
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