VGA Signals

[Matt North]

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

 

[Sylvain Munaut]

Matt North wrote:

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?

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.

Do i have to send it some control info via the DC0-3 pins?

No

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?

No

 

[Derek Simmons]

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

 

[Andrew Dyer]

On Tue, 03 Aug 2004 14:16:07 +0200, Sylvain Munaut wrote:

Matt North wrote:
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?

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.

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.

 

[Symon]

"Andrew Dyer" <andrew.spam.dyer@comcast.net> wrote in message
newsan.2004.08.04.04.53.31.481111@comcast.net...

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.

Probably wouldn't work. The monitor wouldn't like the extra lines. You could

drive it 1024x480 though!
Cheers, Syms.

 

[Joe]

Matt North wrote:

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

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

 

[rickman]

Derek Simmons wrote:

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.

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.

--

Rick "rickman" Collins

rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.

Arius - A Signal Processing Solutions Company
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