CF 631 ghosting problem

[PeterSteele]

I've implemented some scrolling code so that lines longer than can fit on the 631 scroll to the left at a specified rate. On the PC on which I did the development the scrolling effect looks very good, behaving exactly as we wanted. However, when we tested the software on other PCs with the 631 installed we are seeing a very obvious ghosting effect.

Basically, when the existing text is erased in preparation for the new text to be displayed (shifted of course by one character) there is a momentary black image of the original text. This ghost fades to the normal blue of the screen but the effect is very obvious. We see this even when we just clear the screen--the original text doesn't simply disappear but instead turns briefly black and then the screen goes blue.

On my original development PC this effect is hardly noticely, but on other PCs it is very obvious. The PCs are not all identical so there is clearly something different about my PC compared to the others. Does anyone have an explanation for this ghosting effect?

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[CF Tech]

Did you try adjusting the contrast while it is scrolling? That can make some difference.

 

[PeterSteele]

I have not tried that but will give it a try. One thing we noted is that the chipset used on my development PC is different than the chipsets used on the other systems. That clearly is playing a role.

 

[PeterSteele]

I tried adjusting the contrast and the results are quite surprizing. The manner in how the contrast changes are represented on my development PC compared to how it appears on the other PCs in which we have these panels installed is quite different. On my PC, when the contrast is lowered the white letters become dimmer and dimmer until they are barely visible. On another PC I tested as the contrast is lowered the letters become dimmer but the black ghosts remain. The net effect is that when the white letters are completely gone the black ghosts in fact become the real letters. This effect is only when scrolling is happening though. When no scrolling is taking place when the contrast is set really low, no letters are visible. If a "clear screen" command is sent, these invisible letters turn black briefly.

I ran the same tests on a third PC with yet a different chipset and the behavior is different again. Why do the USB chipsets play such a role in how the LCD panels bahave?

 

[CF Tech]

I think if you measured the voltage supplied to the module, you might see that it is different for the different computers.

You should be able to tweak the conrast on the display to compensate for the different voltages though.

If you vary the contrast too much, the display will "invert".

 

[PeterSteele]

We tried various contrast levels but none got rid of the ghosting effect. The only solution we can think of is to avoid using the scrolling we've implemented. The 631 just doesn't handle that kind of thing well it seems...

 

[CF Tech]

The response times of the STN fluid is a bit on the slow side for doing scrolling.

About all you can do is slow down the rate.

 

[PeterSteele]

The response times of the STN fluid is a bit on the slow side for doing scrolling.

About all you can do is slow down the rate.

Slowing down the rate has zero effect with this particular problem. The black ghosting is still present regardless...

 

[CF Tech]

Is there a way you can post a video or pic of the problem?

Your original post says "when the existing text is erased in preparation for the new text to be displayed". Do you send a clear or a bunch of spaces between frames? If so, it would be better to just over-write the previous frame with the new frame instead of clearing in between.

 

[PeterSteele]

I'll see if I can get a video or pic. I only have the camera on my cell phone today and I'm not sure if that will give very good results.

As for the implementation, I simply redraw the characters, shifted by one character. Wherever a space occurs in the text, the black character is particularly obvious. It's a pretty weird effect though. When the contrast is set so low that the white characters disappear entirely, the black ghosts can still be seen marching across the screen, at least that's the behaviour on two of the systems. On my original development system, the scrolling works exactly like I want it. It has a different USB chipset though than what we'll be using on our real deployed hardware. Our real systems have a different motherboard and we're stuck with the chipset this motherboard uses.

 

[CF Tech]

Moved to a ticket.