Video Signals And Timing: Difference between revisions

 

 

* Analog Red

* Analog Green

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.

 

 

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.

 

* where the overscan goes into active display, where the scanline or frame is completed and the next one is started.

 

* Resolution (pixel size of active display)

* Total (total number of 'pixels' in a single run)

* Sync start and end (or start and size) - marks the location of the synchronization pulse

 

 

 

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.

 

 

 

Normally before setting a mode you would do the following:

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.

 

 

<tt>[[#H_PIXELS_RND|H_PIXELS_RND]] = ( ''ROUND'' ( [[#H_PIXELS|H_PIXELS]] / [[#CELL_GRAN_RND|CELL_GRAN_RND]] ) ) * [[#CELL_GRAN_RND|CELL_GRAN_RND]]

}

 

 

[[#V_FRONT_PORCH|V_FRONT_PORCH]] = [[#MIN_PORCH_RND|MIN_PORCH_RND]] + [[#INTERLACE|INTERLACE]]<br />

</tt>

 

 

<tt>

::( [[#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]]

</tt>

 

 

 

These are the variables used in the equations above:

 

{{Anchor|C_PRIME}}

''' C_PRIME '''

 

{{Anchor|M_PRIME}}

''' M_PRIME '''

 

{{Anchor|CELL_GRAN_RND}}

'''CELL_GRAN_RND'''

 

{{Anchor|MIN_PORCH_RND}}

'''MIN_PORCH_RND'''

 

{{Anchor|MIN_V_SYNC_AND_BACK_PORCH}}

'''MIN_V_SYNC_AND_BACK_PORCH'''

 

{{Anchor|V_SYNC_RND}}

'''V_SYNC_RND'''

 

 

{{Anchor|MARGINS_REQUIRED}}

'''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|REFRESH_RATE_REQUIRED}}

'''REFRESH_RATE_REQUIRED '''

 

 

 

{{Anchor|PIXEL_FREQ}}

:The dot clock required (check units?)

 

 

{{Anchor|LEFT_MARGIN_PIXELS}}

:Amount of margin at the right

 

'''H_TOTAL'''

 

{{Anchor|TOTAL_V_LINES}}

'''TOTAL_V_LINES'''

:The amount of scanlines in a frame

 

{{Anchor|V_BACK_PORCH}}

'''V_BACK_PORCH'''

 

{{Anchor|V_LINES_RND}}

'''V_LINES_RND '''

{{Anchor|IDEAL_DUTY_CYCLE}}

'''IDEAL_DUTY_CYCLE'''

 

{{Anchor|H_BLANK_PIXELS}}

:The total amount of blank pixels horizontally (including horizontal sync)

 

 

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:

frequency = 10000000

period = 1 / frequency seconds

 

They'll do:

frequency_in_MHz = 10

period_in_ms = 1 / (frequency_in_MHz * 1000000) * 1000

 

And then simplify it to:

frequency_in_MHz = 10

period_in_ms = 1 / frequency_in_MHz / 1000

 

And then they'll remove any indication of what they've done:

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.

 

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.

 

[[Category:Video]]