Sidequest: VGA scan doubler

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Smonson
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Re: Sidequest: VGA scan doubler

Post by Smonson » Thu Jul 30, 2020 11:02 am

nikoskon wrote:
Thu Jul 30, 2020 3:55 am
Thats what i have currently, OSSC outputting to monitor. It does work really well in medium resolution, but haven't been able to get high resolution out of it yet. Don't get the option to enable it when in GEM.

Is there a way to get the high resolution mode out of the Atari with the OSSC? although i've noticed (reading through some threads in various forums), that colour is not an option in high res :(
It's because the ST was designed to use a different monitor for high-res - one that synched at 31KHz - than the colour ones, which synch at 15KHz. With that in mind, the monitors use different connection arrangements in the cable. That's why the option for high-res is greyed out in the menu - it hasn't detected the high-res monitor being plugged in.

The good news is you don't NEED the scan converter for high res - the high-res signal is compatible with almost any VGA monitor, you just need to obtain or build the right type of cable.

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Re: Sidequest: VGA scan doubler

Post by nikoskon » Fri Jul 31, 2020 4:18 am

stephen_usher wrote:
Thu Jul 30, 2020 8:58 am
You’ll need an adapter to do high res as it needs to pull a line up on the connector at the very least. There are ST-to-VGA adapters which will do this.
Thanks mate, i'll have a look for one.

Cheers :D

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Re: Sidequest: VGA scan doubler

Post by PaulJ » Fri Jul 31, 2020 4:25 pm

nikoskon wrote:
Thu Jul 30, 2020 3:55 am

It does work really well in medium resolution, but haven't been able to get high resolution out of it yet. Don't get the option to enable it when in GEM.

Is there a way to get the high resolution mode out of the Atari with the OSSC? although i've noticed (reading through some threads in various forums), that colour is not an option in high res :(
I had the same trouble, upgraded to latest firmware and just played with setting. On the gray background there are very slight vertical bars which are not noticeable unless you look very closely. When something is displayed its pretty much perfect. The older firmware wasn't usable in highres for me.

IMG_2545.jpg
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IMG_2546.jpg
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Re: Sidequest: VGA scan doubler

Post by Smonson » Sun Aug 02, 2020 11:14 am

Yesterday I hooked up the blank connection, but it didn't solve the remaining issue.

The issue is that when the screen has a dark background, such as black, it's got an interesting pattern of distortion on it: a gradient of brightness which alternates in intensity and also gets brighter towards the bottom of the screen. The darker the background is, the more noticeable the effect:
vidout10.jpg
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After connecting blank, there was no difference. To experiment, I adjusted the blank timing so that it covered all 4 edges of the screen (this way I know it's in effect for all the blanking areas that it should be switched on for). With that setup running, the borders are definitely DARK black to the eye:
vidout11.jpg
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When the background is light, you can't see the effect.
vidout12.jpg
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I'm not sure what's going on here. A strange thing is that the effect seems to alternate between ST scanlines. That means it's drawing two doubled lines at a lower brightness level, then two lines at the brighter level, then repeating. The only difference between the brighter and darker sections is that the FPGA is reading the scanline data from different SRAMs. After leaving the SRAMs the FPGA sends the data to the RGB line drivers, which are two 74AHCT245 bus transceivers. Maybe one SRAM draws a lot more current than the other, affecting other parts of the circuit? Even if that's the case, it doesn't explain the gradually-increasing brightness gradient.

Having no 15KHz monitor I can't test my ST to see if this kind of thing was happening when it was stock. I have never done any recapping or adjustments to the video on this machine. I also can't test it on my H4 machine, because I have to leave the monitor socket unpopulated in order to use the HDMI mod.

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Re: Sidequest: VGA scan doubler

Post by exxos » Sun Aug 02, 2020 11:19 am

You sure its just not a bad solder joint on the sram ?
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Re: Sidequest: VGA scan doubler

Post by Smonson » Sun Aug 02, 2020 11:45 am

exxos wrote:
Sun Aug 02, 2020 11:19 am
You sure its just not a bad solder joint on the sram ?
I'm not sure of anything! But even if it is, after passing through the FPGA and then the bus driver (which has Schmitt triggers), as I understand it should be back to a clean signal again.

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Re: Sidequest: VGA scan doubler

Post by exxos » Sun Aug 02, 2020 11:50 am

Have you done the usual , just touch various places on the pins on every chip to see if anything changes. Fingers make great diagnostic tools :lol:
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Re: Sidequest: VGA scan doubler

Post by Smonson » Sun Aug 02, 2020 12:04 pm

exxos wrote:
Sun Aug 02, 2020 11:50 am
Have you done the usual , just touch various places on the pins on every chip to see if anything changes. Fingers make great diagnostic tools :lol:
Sure did! But no change in the picture. I'm 90% of the way through building a second one, so if the problem is bad solder joints, hopefully the second one will have a totally different set of bad solder joints :)

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Re: Sidequest: VGA scan doubler

Post by sandord » Sun Aug 02, 2020 3:24 pm

exxos wrote:
Sun Aug 02, 2020 11:50 am
Have you done the usual , just touch various places on the pins on every chip to see if anything changes. Fingers make great diagnostic tools :lol:
Better not apply this method when live testing PSUs though :shock:

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Re: Sidequest: VGA scan doubler

Post by Darklord » Mon Aug 03, 2020 2:56 pm

sandord wrote:
Sun Aug 02, 2020 3:24 pm
Better not apply this method when live testing PSUs though :shock:
I'll *never* forget the first time I reached across a "live" 1040ST I was
working on and touched the P/S with the bottom of my forearm! :o

Forget cattle prods or shock therapy! :)
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