Video driver thoughts

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stephen_usher
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Re: Video driver thoughts

Post by stephen_usher »

I know this is about a composite video issue, but it does seem possibly similar...
http://www.notanon.com/page/2/

Could the DAC's output be too weak to drive the video directly?
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Re: Video driver thoughts

Post by exxos »

stephen_usher wrote: Thu Nov 29, 2018 5:57 pm Is it just me or does that look like an analogue issue on the output? Almost as if it's a "ringing" feedback?

The fact that it takes a while from the top of the screen for the white to fade up to white in the scan lines makes me think that there's a power issue of some kind where something has to charge up before "white" can be displayed, which takes a few scan lines to do.
I have added and removed capacitors and resistors from the output, hasn't made any difference.. If it was ringing or drive issues, then it should have changed something lease..

stephen_usher wrote: Thu Nov 29, 2018 6:56 pm I know this is about a composite video issue, but it does seem possibly similar...
http://www.notanon.com/page/2/

Could the DAC's output be too weak to drive the video directly?


I will take a look at the link in a moment, but don't forget this DAC is the exact one used in the Falcon which works really well..
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Re: Video driver thoughts

Post by exxos »

I looked at the RGB outputs, and I can see them switching as a square waves, but oddly I do not see any voltage steps... Could be as I am on desktop and its probably "full on green" for example.. but maybe someone else can look at that on the scope as well ?
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Re: Video driver thoughts

Post by PhilC »

I did similar earlier and the RGB was a square wave of about 1.7V. Didn't check much of anything else as I was just getting the board to work with the shifter problems I mentioned earlier.
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Re: Video driver thoughts

Post by exxos »

I still do not really get why the initial white screen is totally black, other than when the first text comes up..

It does almost seem like a power issue where it can only manage to draw the white areas when there is 50% black (text) with it also.

123.JPG
123.JPG (12.53 KiB) Viewed 4016 times
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Re: Video driver thoughts

Post by stephen_usher »

Can you add transistors controlled by the DAC output but driving a higher power source? I'm guessing that those using a breadboard could test this.
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Re: Video driver thoughts

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stephen_usher wrote: Thu Nov 29, 2018 7:53 pm Can you add transistors controlled by the DAC output but driving a higher power source? I'm guessing that those using a breadboard could test this.
Theres transistors already on the MB, I did wonder that, but probably end up a lot of messing about :( If we heading towards buffering the DAC, then might as well just stick with the original RGB circuit :roll: .
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Re: Video driver thoughts

Post by sandord »

exxos wrote: Thu Nov 29, 2018 7:45 pm It does almost seem like a power issue where it can only manage to draw the white areas when there is 50% black (text) with it also.
123.JPG
Makes sense, like the signal takes a dive when going over a certain threshold. You could try to amplify this by displaying a picture of the right half of a pyramid in 2D. You know, a line running from the top left of the screen to the bottom right and the screen one side of the line being black. This causes the amount of black increasing gradually by each line.
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Re: Video driver thoughts

Post by exxos »

sandord wrote: Thu Nov 29, 2018 8:02 pm Makes sense, like the signal takes a dive when going over a certain threshold. You could try to amplify this by displaying a picture of the right half of a pyramid in 2D. You know, a line running from the top left of the screen to the bottom right and the screen one side of the line being black. This causes the amount of black increasing gradually by each line.
Did you see the diagnostic cart tests Icky did ?
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Re: Video driver thoughts

Post by sandord »

exxos wrote: Thu Nov 29, 2018 8:09 pm Did you see the diagnostic cart tests Icky did ?
Yeah. Though not quite what I mean, still interesting.

Here's what I meant, perhaps someone can try it (I wish I could do it :cry:)

Untitled.gif
Untitled.gif (1.71 KiB) Viewed 3994 times

By the way, has the DAC board been tried in an original ST? (maybe I missed it...)
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