Sidequest: VGA scan doubler

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Smonson
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Re: Sidequest: VGA scan doubler

Post by Smonson » Wed Jun 17, 2020 6:07 am

Awesome, thanks for the feedback guys! The quality of the colour image through the same cable is amazing, so I'm going to rule out my cable as the problem, and suspect that the FPGA itself (i.e. my code) is doing something wrong.

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Re: Sidequest: VGA scan doubler

Post by Smonson » Wed Jun 17, 2020 12:33 pm

Bad solder joint in one of my bodge wires apparently. Sorry for the pointless questions :)
mono3.png
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Re: Sidequest: VGA scan doubler

Post by Aeberbach » Fri Jun 19, 2020 12:42 am

kludge asked "could this work on Amiga"?

Not really. I had a look at Atari schematics and it looks like 3 lines from the shifter for each color. The Amiga 1200 has 8 lines for each color coming out of the Lisa chip (which is why Indivision and other scan doublers connect over the top of the Lisa, to grab those signals). A BT or ADV101 video DAC is used to make the analog VGA signal from that input, both more and less complicated than the Atari discrete approach. I think a 500 has 4, a 600 might have 5? Point is it's a different number of bits per color. The only way a video converter would really work on both machines is to use ADC to take the actual analog RGB voltages from the output port and then recreate the signal through a desired output - just like the OSSC does.

What could work on an Amiga is to get Chucky to rework the ReAmiga PCB - remove the 23-pin video connector, the BT101 and instead add an FPGA with HDMI or VGA output. The current ReAmiga has a VGA output but it is the same signal as comes from the 23-pin Commodore video port.

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Re: Sidequest: VGA scan doubler

Post by Smonson » Fri Jun 19, 2020 10:05 am

Aeberbach wrote:
Fri Jun 19, 2020 12:42 am
kludge asked "could this work on Amiga"?

Not really. I had a look at Atari schematics and it looks like 3 lines from the shifter for each color. The Amiga 1200 has 8 lines for each color coming out of the Lisa chip (which is why Indivision and other scan doublers connect over the top of the Lisa, to grab those signals). A BT or ADV101 video DAC is used to make the analog VGA signal from that input, both more and less complicated than the Atari discrete approach. I think a 500 has 4, a 600 might have 5? Point is it's a different number of bits per color. The only way a video converter would really work on both machines is to use ADC to take the actual analog RGB voltages from the output port and then recreate the signal through a desired output - just like the OSSC does.

What could work on an Amiga is to get Chucky to rework the ReAmiga PCB - remove the 23-pin video connector, the BT101 and instead add an FPGA with HDMI or VGA output. The current ReAmiga has a VGA output but it is the same signal as comes from the 23-pin Commodore video port.
You're absolutely right about all the bit depths. I'm using 9 bits per pixel to encode the ST's colour information, but the RAMs are 16 bits wide, so they could hold 4 bits per colour. The simplest way to support the Amiga colour depth would just be to double the RAMs and have 4 instead of 2. That takes care of the digital side, anyway. As you say, you'd want a video DAC instead of the R-2R ladders as well. Or, even better, inject the rescanned data back into the Amiga. I believe the Lisa is a PLCC, so you could theoretically solder such a mod into the board in place of the PLCC socket. A bit more messing around than doing it with a DIP40.

I don't have an Amiga otherwise I'd volunteer :D

I'm also in favour of the HDMI approach but keeping the price down could be a challenge. After struggling with the HDMI mod for some time, I started this project in parallel because I believe it's going to be more easily achievable, stable, reliable, and cheaper.

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Re: Sidequest: VGA scan doubler

Post by Aeberbach » Sat Jun 20, 2020 3:53 am

I just started FPGA dev for this reason... was hoping to pick some good screen resolutions and then to be able to sample Amiga video and scale it into those resolutions. 1280x1024 SXGA first, maybe. I like VGA out for selfish reasons (and it's easy compared to HDMI) but there are tons of monitors and even TVs that support it.

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Re: Sidequest: VGA scan doubler

Post by Smonson » Sat Jun 20, 2020 6:36 am

As previously mentioned, I wanted to try running the video out through the original ST monitor port. I've just completed that test.

First, one track needs to be cut to separate the HSYNC signal from the Glue into HSYNC_IN (from Glue) and HSYNC_OUT (to the monitor). These signals have flying wires attached. On my dev machine I have the SMT chipset. Here's how that looks on the front of the board:
vidout1.png
vidout1.png (2.19 MiB) Viewed 1656 times
And the cut track on the back:
vidout2.png
vidout2.png (1.61 MiB) Viewed 1656 times
Next, to disable the ST's BLANK generation, I removed R153 (1K ohm) and tied the base of Q13 to +5V through a new 1K resistor:
vidout3.png
vidout3.png (2.25 MiB) Viewed 1656 times
I did some fragile rewiring on my mod board to connect the RGB outputs from the FPGA straight to the shifter socket's RGB pins. These are only operating at 3V instead of 5, but it's good enough to get an image out of it.
vidout4.png
vidout4.png (1.41 MiB) Viewed 1656 times
The mod board installed in the shifter socket. I have the flying wires for HSYNC_IN and HSYNC_OUT terminated with header pin sockets. VSYNC is also connected but not used for anything for this mod (it was used on the HDMI one).
vidout5.png
vidout5.png (2.02 MiB) Viewed 1656 times
After powering up, the picture is not too bad.
vidout6.png
vidout6.png (1.25 MiB) Viewed 1656 times
I have some vertical lines, but that may be partially blamed on my ST-to-VGA cable which is kind of a mess. It looks better in real life than it does in the photo. You can notice a decrease in quality here compared with the signal coming straight from the mod board, which is probably to be expected considering the long signal path through the ST. I also have not done any recapping on this machine which I know can improve the video quality. Also, the fact that it's generated at only 3V is probably responsible for lower signal-to-noise ratio. The quality of the picture in mono is expected to be the same as seen here in medium.

Given this successful test, the next revision of the board will work this way and not have a VGA-out capability. The three DACs and the pin header use up a fair bit of board space and soldering time (30 resistors, 3 transistors). Chopping holes in cases to fit DE15 connectors doesn't fill me with joy either :lol:

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Re: Sidequest: VGA scan doubler

Post by Smonson » Sat Jun 20, 2020 6:41 am

Test render of the next board. It's considerably smaller.
vidout7.png
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Re: Sidequest: VGA scan doubler

Post by Icky » Sat Jun 20, 2020 8:34 am

:coolpics: - nice looking board.

Great to see the progress here @Smonson

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Re: Sidequest: VGA scan doubler

Post by Smonson » Sat Jun 20, 2020 9:11 am

Thanks Icky!

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Re: Sidequest: VGA scan doubler

Post by troed » Sat Jun 20, 2020 9:30 am

Wow this is really cool. I foresee a bunch of us needing to help others with the installation, but being able to reuse existing cables and switches will be great!

/Troed

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