help with VGA timings

HS & VS are not necessarly active low, it depends of the mode. Note that all recent monitor I tried absolutly didn't care about polarity and locked what ever I used ...

You have : * Front porch : Sync signal is not active, video signal is at blanking level * Sync : Sync signal is active, video signal is at blanking level * Back porch : Sync signal is not active, video signal is at blanking level * Active video : Sync signal is not active, video signal is whatever you need to display

and that's for both h & v sync.

The monitors try to "lock" onto the input signal ... depending on the monitor a particular signal might be recognized or not ... The only way to ensure it's gonna be recognized by all monitor is to use a "standard" timing. If it's just for experimentation, just try ... it might just work ...

Sylvain

Another good resource is

Paul

Jordi wrote:

On a sunny day (19 Feb 2006 06:02:13 -0800) it happened "Jordi" wrote in :

The horizontal timebase in the monitor will be triggered by the neg sync edge, and that is when it starts scanning. If you make backporch (blanking) longer, you will just get a vertical black bar on the screen.

Think of the 'monitor' as just 2 oscillators, one for H and one for V. These are triggered by H and V, and in fact H uses PLL and likely V too. So withing allowable syc range for a monitor you can change H and V frequency. So look at the back and front porch as just a percentage of the time a scanline takes. In fact these blanking areas are to prevent you to see non-linear effects in the scanning :-)

When the spot (still talking analog monitor) moves from left to right in that 25 uS its intensity is set by the voltage at the input. It has limited bandwidth though, and a limited number of pixels corresponding to a limited number of holes in the shadowmask, so you cannot feed it 800MHz and expect more detail ;-)

on

takes.

the scanning :-)

Something else to note - make sure that the video signal is properly blanked throughout the porch and sync periods. Failing to do this can confuse the auto-adjust process on LCD monitors.

bar on

scanline takes.

the scanning :-)

and

throughout the porch

LCD monitors.

Also, make sure you use "strong" drivers for the lines that goes directly to the VGA monitor. I had a "oscillating" image on a lcd monitor because my hsync & vsync line were 12mA SLOW drivers. Using 24ma and FAST made the problem go away ...

Sylvain

edge,

bar on

frequency.

scanline takes.

the scanning :-)

that 25 uS

and

throughout the porch

LCD monitors.

Also good to remember than the RGB input signals are terminated with 75 ohm resistors. So if you are building a R-2R DAC for each channel, you may have to take that into account.

Hi, thanks to all for the answers :) !!! (and sorry for my bad english :( )

I think I'm begining to understand how it works.

If the monitor uses PLL then the HSync frecuency determines how fast the monitor moves the beam.So if I increase the HSync frecuency the active time is shorter(and the back and front porch) but I can draw more lines per second so I can have better resolutions. I don't understand how resolution control works. The monitor has a fixed number of pixels(Let's supose a 1024x768 monitor). If I ouput 640x480 images and the monitor must draw 1024 pixels per line and 768 lines, then some pixels will be repeated. Isn't it?

Thanks in advance,

Jordi

You are partially correct. The HSync tells the monitor WHEN to go to the next line. Also the HSync frequencies are defined for each standard resolution. You must stay very close to them or else the monitor will reject your signal. The monitor will determine which resolution you are using by the frequency it "sees" on both the Vsync and Hsync inputs (also the sync pulse polarity changes too).

It is better to drop down to 640x480 to draw the 640x480 images. But if necessary you can use pixel scaling to scale the 640x480 image to fit a

-Isaac

On a sunny day (20 Feb 2006 11:55:08 -0800) it happened "Jordi" wrote in :

Yes in an analog monitor there will be an 'aliasing' effect. Some LCD monitors and TVs will use digital processing to recalculate the picture for the resolution these themselves have. In such a case the timing requirements for the input format may be more strict, like for example a specific TV standard.

In an analog monitor it is very possible to have 'half a pixel' on (if enough bandwidth):

-- -- -- -- -- -- -- -- -- shadow mask

--- --- --- --- --- --- --- video signal This 'half on' gives a brightness change at that pixel location.

The intensity of the ascanning electron beam is just modulated by the video.

In a LCD this is not the case, but the pixel brigtness can be corrected accordingly by processing.

Another good resource for standard video timing:

You can download the page and run it locally on your PC. It is often hard to get specifications for clocks per line, blanking intervals, sync time and polarity, etc. This page shows the values for most commonly used modes. The top drop-down menu allows you to select the known modes, or you can generate custom timing by entering data in the fields below.

Good luck, Gabor

Jan Panteltje wrote:

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