Coming back to those pesky RGsB signals, it’s time for the heavy artillery!
I managed to borrow a digital scope from a friend for a weekend so we’re back in business baby!
Of course you remember my first success on showing for the first time a ghostly greenish-tinting zombie like XP background on the on-board display of my BMW. Well, that was obviously not what I had in mind, so I have to really understand how the signals work there.
I am sure this little guy will be useful.
Please keep in mind that I am not an electronics professional, just a mere DIY enthusiast.
Well, after a quick setup of the probes I’ve got my fist images:
Analyzing the signals and making various calculations of timings and voltages I have finally been able to understand the problem. The best explanation is the next image:
BMW display image superimposed to the green signal
You can clearly see now that the voltage of the signal is not what can be expected. It has a shift (or delta, or bias, I don’t know the proper term in English) of about .5v that leaves the sync signals to lower voltages and elevates the white level to about 1.47V. This shift is present on all channels.
That’s why my first image was so bad.
Than, with the aid of the scope and PowerStrip I tried various settings for my video card to get a better image. I eventually got rid of the rolling effect and I minimized the greenish tint to some extent but the quality of the signal did not get even close to that of the original.
I first built a testing image and I started analysing:
The second image you see is the green channel of the ATI card with the sync multiplexed. You can see it is completely different from BMW RGsB. The sync is positive in the picture. If I set it to negative, it screws everything up.
The third image is the green channel of the Matrox card with the sync multiplexed. It has a little resemblance to the BMW signal except for the voltages. The green is ok but the red and blue channels are unchanged, the same as ATI and completely different from BMW.
So, it looks like the end of the road here. I understand that a simple solution is not possible. This is also where my electronics knowledge ends. Of course a device can be made to shift the voltages of the Matrox to correct levels, and with the help from PowerStrip I am sure a good image can be obtained.
But I don’t have the time or the knowledge to design/manufacture/test such a device.
More important, I have to be realistic here. If such a device could actually work flawless, it is entirely based on an old graphic card, subjected to automotive environment. You see where I am getting? If the video card does work perfectly, it is still an old PCI card, that demands an old computer, a PIII maybe as a base, subjected to automotive environment. If it work perfectly, well… let’s not forget that the actual BMW display is 400×240 so much of the detail of the original image is lost. Of course it would be better than composite but not as good as it could be. A lot of IFs there!
Well, if you are interested in what CAN really be done, you can visit the web page of Bengt-Olof Swing, another BMW CarPC enthusiast which has achieved marvels by replacing the original BMW monitor with a high-resolution one. He was a major source of inspiration for me a lot of times, thank you and good work, Bengt-Ol’ Swing!
Anyway, what he has done is way beyond my abilities.
You can also visit jlitko’s page. He’s went the risky way, with a PIII and a Matrox. He managed to build a pretty decent machine but I don’t know the status of his CarPC now…
Well, taking all that into consideration, I am afraid the old composite video input I so strongly despise is the only practical solution. I must admit it is hard for me to let go this quest for perfect RGsB but I agree there is a difference between what CAN be done and what IT’S WORTH to be done.
So, I have decided, composite video input it is! This little side quest is over. Back to my main project!
January 29th, 2009 at 6:45
Excellent findings……………….
not such a good result though……….
shame……………
July 7th, 2009 at 3:04
How soon will you update your blog? I’m interested in reading some more information on this issue.