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Video Signals And Timing: Difference between revisions
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{{
To make the video card and monitor independent of each other, there is a standard in communication. This page describes the technical parts of that link with the necessary information to program a video card and have it render properly on the attached screen.
== Display Signal ==
There are
* Analog Red
* Analog Green
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The system is however not that simple. The first monitors were CRTs, which used magnetic fields to project electron beams onto a phosphoric layer, making them visible for a short period of time. The magnetic fields of a CRT had some inertia - they couldn't be set from a random location to another random location in the time available for one pixel. Hence the video signal has to have some gaps to cope with the time a monitor needs to alter the magnetic fields and point the electron beam back to the other side of the screen. In the meantime there could be no color signal, or you might have gotten stripes on the screen.
CRTs have improved a lot since then, and are now being superseded by the highly intelligent LCD display. The standard for signaling hasn't changed since. Since the original standard was made for CRTs, the rest of the document will implicitly assume an (old) CRT. LCD displays basically decode a CRT signal and then restructure it to fit their own screen grid under analog signaling methods. Similarly, in many other references you will see that the display unit of a video card is referred to as the "CRTC" or "CRT Controller"
== Display Composition ==▼
▲= Display Composition =
All frames of a video signal have a specific layout, and video cards have a semi-standard way of thinking about these signals. Basically, you will have to provide the video card with enough information to be able to derive all sizes present in the following diagram. [[VGA Hardware]] explains how you can give these sizes to a VGA compatible. For other hardware, you should check your card's documentation on how these values are stored within them.
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* where the overscan goes into active display, where the scanline or frame is completed and the next one is started.
Video cards usually gives you the following registers to program (both horizontally and
* Resolution (pixel size of active display)
* Total (total number of 'pixels' in a single run)
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* Sync start and end (or start and size) - marks the location of the synchronization pulse
== 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
▲= 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
▲= 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.
Normally before setting a mode you would do the following:
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The formula's provided by VESA use various scales of constants. Brendan's working on changing the scales to units. Right now, the scale space is still unknown.
=== Common parts of the GTF formulas ===
<tt>[[#H_PIXELS_RND|H_PIXELS_RND]] = ( ''ROUND'' ( [[#H_PIXELS|H_PIXELS]] / [[#CELL_GRAN_RND|CELL_GRAN_RND]] ) ) * [[#CELL_GRAN_RND|CELL_GRAN_RND]]
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:[[#RIGHT_MARGIN_PIXELS|RIGHT_MARGIN_PIXELS]] = 0<br />
}
'''/* use one of the GTF equations here */'''
[[#V_FRONT_PORCH|V_FRONT_PORCH]] = [[#MIN_PORCH_RND|MIN_PORCH_RND]] + [[#INTERLACE|INTERLACE]]<br />
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</tt>
=== GTF Using resolution and refresh rate ===
<tt>
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::( [[#TOTAL_ACTIVE_PIXELS|TOTAL_ACTIVE_PIXELS]] * [[#IDEAL_DUTY_CYCLE|IDEAL_DUTY_CYCLE]] / ( 100 - [[#IDEAL_DUTY_CYCLE|IDEAL_DUTY_CYCLE]] ) / ( 2 * [[#CELL_GRAN_RND|CELL_GRAN_RND]] ) )
:) ) * 2 * [[#CELL_GRAN_RND|CELL_GRAN_RND]]
[[#H_TOTAL|H_TOTAL]] = [[#TOTAL_ACTIVE_PIXELS|TOTAL_ACTIVE_PIXELS]] + [[#H_BLANK_PIXELS|H_BLANK_PIXELS]]
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</tt>
=== GTF Using resolution and pixel clock ===
<tt>
[[#PIXEL_FREQ|PIXEL_FREQ]] = [[#PIXEL_FREQ_REQUIRED|PIXEL_FREQ_REQUIRED]]<br />
[[#TOTAL_ACTIVE_PIXELS|TOTAL_ACTIVE_PIXELS]] = [[#H_PIXELS_RND|H_PIXELS_RND]] + [[#RIGHT_MARGIN_PIXELS|RIGHT_MARGIN_PIXELS]] + [[#LEFT_MARGIN_PIXELS|LEFT_MARGIN_PIXELS]]<br />
[[#IDEAL_H_PERIOD|IDEAL_H_PERIOD]] = ( ( [[#C_PRIME|C_PRIME]] - 100 ) + ( ''SQRT'' ( ( ( 100 - [[#C_PRIME|C_PRIME]] ) ^ 2 ) +
:( 0.4 * [[#M_PRIME|M_PRIME]] * ( [[#TOTAL_ACTIVE_PIXELS|TOTAL_ACTIVE_PIXELS]] + [[#RIGHT_MARGIN_PIXELS|RIGHT_MARGIN_PIXELS]] + [[#LEFT_MARGIN_PIXELS|LEFT_MARGIN_PIXELS]] ) / [[#PIXEL_FREQ|PIXEL_FREQ]] / 1000000 ) ) )
:) ) / 2 / [[#M_PRIME|M_PRIME]] * 1000<br />
[[#IDEAL_DUTY_CYCLE|IDEAL_DUTY_CYCLE]] = [[#C_PRIME|C_PRIME]] - ( [[#M_PRIME]] * [[#IDEAL_H_PERIOD|IDEAL_H_PERIOD]] / 1000 )<br />
[[#H_BLANK_PIXELS|H_BLANK_PIXELS]] = ( ''ROUND'' ( [[#TOTAL_ACTIVE_PIXELS|TOTAL_ACTIVE_PIXELS]] * [[#IDEAL_DUTY_CYCLE|IDEAL_DUTY_CYCLE]] / ( 100 - [[#IDEAL_DUTY_CYCLE|IDEAL_DUTY_CYCLE]] ) / ( 2 * [[#CELL_GRAN_RND|CELL_GRAN_RND]] ) ) ) * 2 * [[#CELL_GRAN_RND|CELL_GRAN_RND]]<br />
[[#H_TOTAL|H_TOTAL]] = [[#TOTAL_ACTIVE_PIXELS|TOTAL_ACTIVE_PIXELS]] + [[#H_BLANK_PIXELS|H_BLANK_PIXELS]]<br />
[[#H_FREQ|H_FREQ]] = [[#PIXEL_FREQ|PIXEL_FREQ]] / [[#H_TOTAL|H_TOTAL]]<br />
[[#H_PERIOD|H_PERIOD]] = 1 / [[#H_FREQ|H_FREQ]]<br />
[[#V_SYNC_AND_BACK_PORCH|V_SYNC_AND_BACK_PORCH]] = ''ROUND'' ( [[#MIN_V_SYNC_AND_BACK_PORCH|MIN_V_SYNC_AND_BACK_PORCH]] * [[#H_FREQ|H_FREQ]] / 1000000 )<br />
[[#V_BACK_PORCH|V_BACK_PORCH]] = [[#V_SYNC_AND_BACK_PORCH|V_SYNC_AND_BACK_PORCH]] - [[#V_SYNC_RND|V_SYNC_RND]]<br />
[[#TOTAL_V_LINES|TOTAL_V_LINES]] = [[#V_LINES_RND|V_LINES_RND]] + [[#TOP_MARGIN_LINES|TOP_MARGIN_LINES]] + [[#BOTTOM_MARGIN_LINES|BOTTOM_MARGIN_LINES]] + [[#INTERLACE|INTERLACE]] + [[#V_SYNC_AND_BACK_PORCH|V_SYNC_AND_BACK_PORCH]] + [[#MIN_PORCH_RND|MIN_PORCH_RND]]<br />
[[#V_FIELD_RATE|V_FIELD_RATE]] = [[#H_FREQ|H_FREQ]] / [[#TOTAL_V_LINES|TOTAL_V_LINES]]<br />
'''if''' ( [[#INTERLACE_REQUIRED|INTERLACE_REQUIRED]] == '''true''')
{
:[[#V_FRAME_RATE|V_FRAME_RATE]] = [[#V_FIELD_RATE|V_FIELD_RATE]] / 2
} '''else''' {
:[[#V_FRAME_RATE|V_FRAME_RATE]] = [[#V_FIELD_RATE|V_FIELD_RATE]]
}</tt>
=== GTF Using resolution and horizontal frequency ===
<tt>
[[#H_FREQ|H_FREQ]] = [[#H_FREQ_REQUIRED|H_FREQ_REQUIRED]]<br />
[[#V_SYNC_AND_BACK_PORCH|V_SYNC_AND_BACK_PORCH]] = ''ROUND'' ( [[#MIN_V_SYNC_AND_BACK_PORCH|MIN_V_SYNC_AND_BACK_PORCH]] * [[#H_FREQ|H_FREQ]] / 1000000 )<br />
[[#V_BACK_PORCH|V_BACK_PORCH]] = [[#V_SYNC_AND_BACK_PORCH|V_SYNC_AND_BACK_PORCH]] - [[#V_SYNC_RND|V_SYNC_RND]]<br />
[[#TOTAL_V_LINES|TOTAL_V_LINES]] = [[#V_LINES_RND|V_LINES_RND]] + [[#TOP_MARGIN_LINES|TOP_MARGIN_LINES]] + [[#BOTTOM_MARGIN_LINES|BOTTOM_MARGIN_LINES]] + [[#INTERLACE|INTERLACE]] + [[#V_SYNC_AND_BACK_PORCH|V_SYNC_AND_BACK_PORCH]] + [[#MIN_PORCH_RND|MIN_PORCH_RND]]<br />
[[#V_FIELD_RATE|V_FIELD_RATE]] = [[#H_FREQ|H_FREQ]] / [[#TOTAL_V_LINES|TOTAL_V_LINES]]<br />
'''if''' ( [[#INTERLACE_REQUIRED|INTERLACE_REQUIRED]] == '''true''') <br />
{
:[[#V_FRAME_RATE|V_FRAME_RATE]] = [[#V_FIELD_RATE|V_FIELD_RATE]] / 2
} '''else''' {
:[[#V_FRAME_RATE|V_FRAME_RATE]] = [[#V_FIELD_RATE|V_FIELD_RATE]]
}
[[#TOTAL_ACTIVE_PIXELS|TOTAL_ACTIVE_PIXELS]] = [[#H_PIXELS_RND|H_PIXELS_RND]] + [[#RIGHT_MARGIN_PIXELS|RIGHT_MARGIN_PIXELS]] + [[#LEFT_MARGIN_PIXELS|LEFT_MARGIN_PIXELS]]<br />
[[#IDEAL_DUTY_CYCLE|IDEAL_DUTY_CYCLE]] = [[#C_PRIME|C_PRIME]] - ( [[#M_PRIME|M_PRIME]] / [[#H_FREQ|H_FREQ]] )<br />
[[#H_BLANK_PIXELS|H_BLANK_PIXELS]] = ( ''ROUND'' ( [[#TOTAL_ACTIVE_PIXELS|TOTAL_ACTIVE_PIXELS]] * [[#IDEAL_DUTY_CYCLE|IDEAL_DUTY_CYCLE]] / ( 100 - [[#IDEAL_DUTY_CYCLE|IDEAL_DUTY_CYCLE]] ) / ( 2 * [[#CELL_GRAN_RND|CELL_GRAN_RND]] ) ) ) * 2 * [[#CELL_GRAN_RND|CELL_GRAN_RND]]<br />
[[#H_TOTAL|H_TOTAL]] = [[#TOTAL_ACTIVE_PIXELS|TOTAL_ACTIVE_PIXELS]] + [[#H_BLANK_PIXELS|H_BLANK_PIXELS]]<br />
[[#H_PERIOD|H_PERIOD]] = 1 / [[#H_FREQ|H_FREQ]]<br />
[[#PIXEL_FREQ|PIXEL_FREQ]] = [[#H_TOTAL|H_TOTAL]] * [[#H_FREQ|H_FREQ]]<br />
</tt>
=== Variable reference ===
These are the variables used in the equations above:
==== Hardware properties ====
These are constant for a given monitor. They describe the capabilities and limitations of a CRT.
{{Anchor|C_PRIME}}
''' C_PRIME '''
:Defaults to 40, the actual value can be computed from EDID data.
{{Anchor|M_PRIME}}
''' M_PRIME '''
:Defaults to 600, the actual value can be computed from EDID data.
{{Anchor|CELL_GRAN_RND}}
'''CELL_GRAN_RND'''
:Cell granularity - the amount of pixels the timings should be aligned to. For example, a VGA uses horizontal timings in multiples of the character clock, each character being 8 (graphics and some text modes) or 9 (most text modes) pixels wide. For a VGA graphics mode, you'll want to supply 8 here because of that. Depending on your specific hardware,
{{Anchor|MIN_PORCH_RND}}
'''MIN_PORCH_RND'''
:
{{Anchor|MIN_V_SYNC_AND_BACK_PORCH}}
'''MIN_V_SYNC_AND_BACK_PORCH'''
:The time in microseconds the monitor needs to detect the vertical synchronisation signal and subsequently retrace to the top-left corner of the screen. Defaults to 550.
{{Anchor|V_SYNC_RND}}
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: The percentage of a scanline that should have the synchronisation pulse active. Equals 8%
▲=== Video mode inputs ===
These form the request of the user/programmer.
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'''H_PIXELS '''
:Horizontal resolution of the display you want. For a 640x480 resolution, supply 640. Note that video cards will usually want this to be a multiple of eight because of [[#CELL_GRAN_RND|cell granularity]].
{{Anchor|PIXEL_FREQ_REQUIRED}}
'''PIXEL_FREQ_REQUIRED'''
:The speed of the dot clock (or pixel clock), in pixels per second. This is usually the same as the dot clock's frequency in Hz, but in some cases there's a factor two difference (2x the frequency when the clock is being doubled, half the frequency when pixels are emitted every other cycle) The dot clock is usually limited by the video card, as higher clocks are needed for higher framerates and resolutions.
{{Anchor|REFRESH_RATE_REQUIRED}}
'''REFRESH_RATE_REQUIRED '''
:The amount of fields per second in Hz. This is also the number of frames per second for non-interlaced modes, or twice the number of frames per second for interlaced modes.
{{Anchor|H_FREQ_REQUIRED}}
'''H_FREQ_REQUIRED'''
:The desired horizontal frequency in Hz. In the most common use, you'll want this to be the maximum horizontal frequency of the monitor, to find the best refresh rate possible that doesn't violate monitor limitations.
==== Outputs ====
The final results of the GTF formula. These correspond to the parameters of the VGA CRTC model.
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The remaining three components needed to generate a complete signal are inputs. For reference, these are: [[#H_PIXELS|H_PIXELS]], [[#V_LINES|V_LINES]] which form the resolution, and [[#V_SYNC_RND|V_SYNC_RND]], which is a constant for a given monitor.
==== Intermediates ====
Other results that are either byproducts, but may be useful for error and compatibility checking.
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{{Anchor|IDEAL_DUTY_CYCLE}}
'''IDEAL_DUTY_CYCLE'''
:
{{Anchor|IDEAL_H_PERIOD}}
'''IDEAL_H_PERIOD'''
:Optimal time to spend on a scanline. This can usually not be achieved since a scanline is a integer number of pixels.
{{Anchor|H_BLANK_PIXELS}}
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:The total amount of blank pixels horizontally (including horizontal sync)
=== Brendan's Sidenotes ===
Quoted:
I think part of the problem is that the original GTF formulas from VESA are copied from their spreadsheet, so that (for a silly example) instead of doing something like:
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frequency = 10000000
period = 1 / frequency seconds
They'll do:
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frequency_in_MHz = 10
period_in_ms = 1 / (frequency_in_MHz * 1000000) * 1000
And then simplify it to:
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frequency_in_MHz = 10
period_in_ms = 1 / frequency_in_MHz / 1000
And then they'll remove any indication of what they've done:
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frequency = 10
period = 1 / frequency / 1000
So that in the end, the formulas from VESA end up being a confusing mess where you're never too sure if there's a hidden scaling factor or not. Another simple example (that isn't made up) is "MARGINS_PERCENT", which is a value that would range from 0 to 50 that's typically divided by 100; and isn't a value that ranges from 0 to 0.5 that doesn't need scaling.
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Finally, all of these calculations can be simplified a lot if you assume that the "default GTF values" are being used. I want to provide simplified versions of the formulas, because most of the time you can use the default GTF values and can skip a lot of work.
== See Also ==
=== Forum ===
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* [http://www.cs.unc.edu/Research/stc/FAQs/Video/GTF_V1R1.xls http://www.cs.unc.edu/Research/stc/FAQs/Video/GTF_V1R1.xls] is one of the GTF spreadsheets you can find on the web.
* [http://en.wikipedia.org/wiki/VGA_connector VGA connector] on wikipedia
* [http://en.wikipedia.org/wiki/EDID EDID] on wikipedia
[[Category:Video]]
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