Re: TGA image too bright on TFT
From: Michael Kukat (michael_at_unixiron.org)
Date: 11/26/04
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Date: Fri, 26 Nov 2004 18:33:30 +0100
Hello,
On Thu, 25 Nov 2004 prep@prep.synonet.com wrote:
> 1V p-p output vs 0.7V p-p I suspect.
Nearly solved...
I just hooked up an oscilloscope to have a further look into R, G, H, V signals
in my special case. Here is the result:
Sync (H, V) is nearly the same on VGA and TGA output (including timing of FP,
BP and so), except that the TGA uses negative sync pulses and the VGA uses
positive pulses. Set the VGA to negative pulses -> no change on VGA, still
perfect picture.
So a difference from TGA to VESA standards is the sync polarity at
1280x1024@60Hz, which shouldn't confuse modern monitors, as they just look at
the timing and don't decide the resolution and frequencies by looking at the
polarity.
Now for the most interesting part: The video outputs.
First, TGA sends out the same signal waveform on red and green, so no
sync-on-green is active. Good. Talking about video channel in general now.
The VGA uses 0.0V (0 IRE) for black and about 0.8V for white. A bit above the
specs, but okay, i don't have a problem with the VGA.
The TGA uses 0.0V (0 IRE) for blanking and about 0.05V (7.5 IRE) for black.
0.7V is white. So the signal has a blank pedestal, which seems to confuse the
auto calibration of the TFT screens. This was one of my first thoughts, so i
modified XFree for this, but as my measurement shows, there are some parameters
which don't come through to the TGA from the driver, meaning that turning off
blank pedestal and switching sync polarity just doesn't work.
"It's all a matter of the driver"
Besides this... these are the differences of the signal. So, if someone wants
to hook up a TFT screen on some machine with TGA(2) inside, the TFT must be
able to handle a black level of 7.5 IRE, mine doesn't seem to be able to do so.
If running XFree86, there might be a solution soon, i'll keep on hacking :)
Just to have this fed back to the group, as my google results didn't bring out
useful results, maybe the next one has more luck and finds this.
...Michael
-- http://www.unixiron.org/ Home Powered by: (Net|Open|Free)BSD IRIX NonStop-UX Solaris AIX HP-UX Tru64 MUNIX Ultrix VMS SINIX Dolphin_Unix OpenStep MacOS A/UX
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