Video driver thoughts

[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?

[exxos]

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..

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..

[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 ?

[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.

[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.

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[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.

[exxos]

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 .

[sandord]

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.

[exxos]

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 ?

[sandord]

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 )

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