VGA console/problem with trails when scrolling

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Vitis
Posts: 24
Joined: Thu Jul 10, 2014 7:00 pm

VGA console/problem with trails when scrolling

Post by Vitis »

I've been working on a microcontroller-based VGA video interface inspired by the Uzebox. It uses an 8-bit, resistor-based DAC for the RGB lines, similar to the one the Uzebox uses but without the color helper hardware, because it's VGA. (The project is just for fun and learning.) I have created some assembly-based tile-rendering video modes with vertical and horizontal scrolling. My problem is that when I scroll, there are trails behind the tiles, especially behind tiles with a dark border. The faster the scrolling, the longer the trails. The trails are even displayed during relatively slow scrolling, like Super Mario walking pace and they become very long and look like smudged ink when the scrolling is at Super Mario running pace and faster. I tried to take some pictures and video to demonstrate, but the effect is very difficult to capture with the low quality camera I have. I have seen demo videos of other VGA console projects that use resistor-based DACs, like the bitBox, but I've never seen this problem in those videos. I think the problem is with noise and slow response time of my DAC, but I'm not even sure about that. I've considered putting some kind of low pass filters on the DAC, but I haven't done the math on that yet to see if it would even be feasible. I thought I would consult with the Uzebox community first to see if anyone had any advice or suggestions. Any ideas?
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martinsustek
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Location: Ostrava, Czech Republic

Re: VGA console/problem with trails when scrolling

Post by martinsustek »

Have you tried it on different kind of monitor?

From the principle of analog signal: If it was problem of slow DAC it will be seen as horizontally blurred vertical edges even on static objects. Problem with moving objects can be because of slow LCD response or some kind of digital processing in the display electronic.
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Vitis
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Re: VGA console/problem with trails when scrolling

Post by Vitis »

Unfortunately, I only have an old LCD monitor to test it on. The static images are really good. There is only a little horizontal-axis jitter on the vertical borders between pixels on a static image, and you have to look very closely to see it. I need to test it on different LCD screens and possibly an old CRT monitor.
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Vitis
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Joined: Thu Jul 10, 2014 7:00 pm

Re: VGA console/problem with trails when scrolling

Post by Vitis »

I just tried it out on a couple of different LCD screens. On the older LCD, the image is really bright, but the scrolling is bad due to trails. When I tried it on a newer LCD monitor, the scrolling trail problem goes away. On the newer monitor, I can even scroll at very fast speeds without noticeable trails, but the image is much dimmer. I also tried it on an LCD flat screen TV; the trailing problem is not evident on the TV, but the image is somewhat dimmer than the old LCD monitor. Also, I noticed that I need to extend the horizontal back porch significantly in order to even get the image to render properly on the TV.

So in summary, the trailing problem seems to be related to the slow response time of the LCD screen, not the DAC. However, I have not yet figured out the dimness problem. I'm thinking it has to do with the resistor values for the DAC. Perhaps there is too much impedance and not enough current flowing through to keep the RGB voltages at their intended levels after passing through the connector cable wires. I thought about using potentiometers on each color line so that the voltage range can be adjusted if the image appears too dim on certain monitors. I still haven't tested it on a CRT monitor yet. I think I could probably buy one for around $20-30.
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