VGA Signals

Hi,

I have written VHDL code which generates standard VGA Timing signals; 640 x

480 resolution 60Hz refresh rate. My question is this; my test monitor is a SONY SDM-S93 which has a resolution of 1280 x 1024, the HSync, VSync and Blanking periods all meet with the specifications of the monitor. However how does the monitor know that i will only use 640 out of a possible 1280 dots per line, and 480 lines out of 1024? Do i have to send it some control info via the DC0-3 pins?

I understand that the monitor can display lower resolutions because this can be done inside windows! When selecting 640x480 resolution in windows the screen is stretched to fill the monitor; does this mean that the graphics card is always running at the 1280x1024 frequency and adds pixel data to fill the screen?

Thanks for the help.

Matt

Reply to
Matt North
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The monitor just sees the syncs and blank levels and get it right. He just match the time between two hync to it's full line. Idem for vsync.

No

No

Reply to
Sylvain Munaut

Does anybody know how this works for a LCD display?

Using my laptop as a reference system, if I select the 1024 x 768 I get a full screen display. If I select a lower resolution display, 800 x 600 I get a smaller size image inset on the display. Is this characteristic of LCD displays?

If somebody could point me to a resource that describe how to generate the signals necessary for LCD display I would greatly appreciate it.

Derek Simmons

Reply to
Derek Simmons

match the time between two hync to it's full line. Idem for vsync.

There is a small twist to this - the true VGA spec uses the polarities of the syncs to encode the resolutions, and some older fixed frequency monitors use this.

vsync hsync vert_size

  • + reserved
  • - 400 lines

- + 350 lines

- - 480 lines

A good way to think about this is that a monitor is ultimately an analog device - somewhere in it's guts is a pair of fancy ramp generators making high voltage sawtooth waves to sweep the electron beam(s) across the face of the tube. A fancy amplifier drives the electron beam(s) on and off.

The multisync monitors adjust the frequencies of the sawtooths to match the input hsync and vsync. The video amplifiers look at the blanking level and at analog inputs and amplify what's there. Whether the dot you send shows up on the screen depends on things like how fast the video amplifiers are, how well focused the beam is, how small of a dot the picture tube can actually resolve. It's perfectly possible to drive a "640x480" monitor with 1024x768 if you keep the horizontal and vertical frequencies in spec - individual pixels just may not show up all that well.

LCDs are a very different beast - they definitely have the concept of a pixel as a defined location on the display and if you don't send enough pixels, you don't fill up the screen.

Reply to
Andrew Dyer

well.

Probably wouldn't work. The monitor wouldn't like the extra lines. You could drive it 1024x480 though! Cheers, Syms.

Reply to
Symon

Some monitors simply detect the ratio between V-SYNC and H-SYNC. If there are 480 H-SYNC for each V-SYNC, it assume input is 640x480. If there are 600 H-SYNC for each V-SYNC, it assume input is 800x600. If there are 768 H-SYNC for each V-SYNC, it assume input is 1024x768, etc.

It also need to determine the refresh rate, this is done by checking the frequency of V-SYNC.

From these information it can then work out the exact time to sample each pixel.

Joe

Reply to
Joe

There are LCD monitors and LCD laptop displays. The difference is the interface. The LCD in the laptop uses an all digital interface so that the computer has full control over the operation of the LCD. The interface to a monitor is normally VGA like analog signal (unless you use a digital interface) so that the monitor has to receive an analog signal. This signal is converted back to digital by using a PLL to sync a clock to the pixel rate. This requires the monitor to be a bit smart about how it decides to select a pixel rate. So the two different display types have different capabilities and level of control.

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rickman

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