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Video driver thoughts

Posted: Thu Sep 27, 2018 9:33 pm
by exxos
I thought about this before basically I think the conclusion was that it was hard to get any simple video DACs now..I do have a huge stack of the Falcon ones though..

It is a six bit input, so likely the STF would have to tie 2 inputs together.. But it would probably work.. I don't think there is any advantage to this other than being less parts. I guess I could do a small adapter PCB to plug into the video buffer area to try it out one day..

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EDIT:

There are ones like this...

https://www.mouser.co.uk/new/Analog-Dev ... v7125-dac/

But really huge overkill for this stuff... we only really need 3 bit inputs, 4 tops, these are 8 bit..

Re: Video driver thoughts

Posted: Thu Sep 27, 2018 9:56 pm
by rpineau
You also need the dot clock (ICLK).. I'm not sure that's the 32MHz clock there as on the Falcon it's generated by the VIDEL.

Re: Video driver thoughts

Posted: Thu Sep 27, 2018 10:09 pm
by exxos
rpineau wrote:
Thu Sep 27, 2018 9:56 pm
You also need the dot clock (ICLK).. I'm not sure that's the 32MHz clock there as on the Falcon it's generated by the VIDEL.
Yeah, can use any clock on this board.. Though not sure why its there really in the first place, unless its a sampling clock.. in which case just run as fast as possible... I not looked at the datasheet for it yet.

Re: Video driver thoughts

Posted: Fri Sep 28, 2018 8:45 am
by Stimpy
I used a falcon video DAC when I did my ST, as it was what i had kicking about. It worked off 32MHz in all resolutions. I just used the MSBs and tied lower bits to ground like any DAC or ADC. I do remember it getting pretty hot though, it was mounted with kynar wire to stripboard so no real thermal stuff and driving RGB down 50 or 75 ohm loads is pretty hard going. Must be better solutions, maybe that is just HDMI....

Re: Video driver thoughts

Posted: Fri Oct 19, 2018 12:53 pm
by exxos
Another thing the video suffers is when the floppy drive is accessed, it causes a momentary drop in voltage which you can see on the monitor as the white border dimming.. Of course a lot of this is down to the power supply.. Could actually be improved and I have done just that the test before in the past..

I took this reading off the remake board just..

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Of course I'm only powering it my bench power supply isn't exactly ideal to start with.. Shows the issue clearly..

Fix I did previously was to cut the 5V from the shifter and RGB drivers, and run them by their own regulator powered from the 12 V rail. This helps considerably..

I talked about not needing 12 V on the remake board in the future, but if we seemed it was still going to have 12 V on the power supply then the next revision of motherboard can run the video circuit its own 5 V regulator.

I have noticed a little bit of wavy lines video of the remake board, this is probably down to the missing capacitors on the RGB lines.. So likely those will be added on the next board..

Of course we still have the option of using the video DAC from the Falcon... But this needs to be tested first... So likely what I will have to do is design small shifter adapter board to use this DAC and see how well it performs..


EDIT:
After upping the brightness on the RGB drivers, I don't think I even noticed this problem now.. But still my thoughts still remain about improving the circuit..

Re: Video driver thoughts

Posted: Fri Oct 19, 2018 9:11 pm
by exxos
exxos has been at the lib editor again (everyone should be afraid and run and hide)


Basically idea is a shifter break out board. This board plugs into the shifter socket, then the shifter plugs into this board.

The RGB outputs from the shifter (digital) Go to the video DAC. The output of the DAC will need 3 jumper wires going to the video socket.. This can be done relatively easily as there are 33R resistors which can be lifted to isolate the original RGB circuit and patching in the new one.

Technically the lower un-used bits on the DAC can probably have some jumpers for a crude brightness select.

Still have some more wiring up to do yet though..

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Re: Video driver thoughts

Posted: Fri Oct 19, 2018 9:14 pm
by Icky
exxos wrote:
Fri Oct 19, 2018 9:11 pm
exxos has been at the lib editor again (everyone should be afraid and run and hide)
Piggy back board time again :D

Re: Video driver thoughts

Posted: Fri Oct 19, 2018 9:16 pm
by exxos
Icky wrote:
Fri Oct 19, 2018 9:14 pm
Piggy back board time again :D
Gotta keep you all busy ;)

Re: Video driver thoughts

Posted: Fri Oct 19, 2018 9:21 pm
by rpineau
the unused input (R,G,B) should be grounded or you'll get weird interferences in the colors :)

Re: Video driver thoughts

Posted: Fri Oct 19, 2018 9:24 pm
by exxos
rpineau wrote:
Fri Oct 19, 2018 9:21 pm
the unused input (R,G,B) should be grounded or you'll get weird interferences in the colors :)
yep, just doing that now, putting jumper on input for a brightness control.. see what effect it has :)