Sidequest: VGA scan doubler

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Smonson
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Sidequest: VGA scan doubler

Post by Smonson » Sun Apr 07, 2019 12:46 pm

Hello, me again!

So, I like to have several projects going at once, as you have probably guessed from my short attention span.

I wanted to get started with small and very cheap FPGAs (as opposed to the huge and expensive Cyclones) so I picked up some MAX-Vs in my last Mouser order. These guys go all the way down to $1.32 each.

So I decided to do a VGA scandoubler that reads the RGB values straight out of the shifter into a couple of SRAMs. I started several months ago and today came to a point where it's worth discussing. Here's what it looks like plugged in:
doubler.jpg
doubler.jpg (87.78 KiB) Viewed 926 times
The MAX-V is at the bottom centre. The two SRAMs on the right alternate between reading and writing. The addresses on both are generated by ripple counters (above the SRAMs). At the top are the three DACs for RGB, and the VGA pin header. I had to do a couple of fix-up wires, which is fun on an EQFP-64 with 0.4mm pin spacing.

Unfortunately the picture isn't great right out of the gate. It looks like it could be because of address lines changing in the middle of reads or writes, and thus reading or writing to/from the wrong address, but I haven't been able to narrow it down any further than that so far. Because I took the photo on my phone, it's got that annoying moire pattern on the pic, but you can see the vertical green lines on the right half for example. The dodgy pixels are also quite evident in the menu bar due to the presence of lots of text.

Anyway, apart from the "reflections", the image is quite stable and crisp, and the colours look right. So that's something I guess!

I'm probably violating the timing requirements for VGA, but you can't have everything.
doubler2.jpg
doubler2.jpg (78.83 KiB) Viewed 926 times

troed
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Re: Sidequest: VGA scan doubler

Post by troed » Sun Apr 07, 2019 12:51 pm

A cheapish scan doubler will do wonders for the ST community - awesome!. Everyone with a socketed Shifter can plug'n'play it right out the gate I assume?

/Troed

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Smonson
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Re: Sidequest: VGA scan doubler

Post by Smonson » Sun Apr 07, 2019 1:17 pm

Yep, assuming that they have access to a monitor that can display a 50hz image via the VGA input. I'm assuming that many TVs would fall into that category but I've only tried one so far. Monitors are less likely to work. 😊

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Smonson
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Re: Sidequest: VGA scan doubler

Post by Smonson » Sun Apr 07, 2019 1:20 pm

...actually one thing I forgot to mention. It does require a connection to HSYNC, which means soldering to a via near the GLUE. So it's not totally plug-and-play unfortunately.

Without HSYNC, it can't tell where to begin the scanlines.

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PhilC
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Re: Sidequest: VGA scan doubler

Post by PhilC » Sun Apr 07, 2019 2:29 pm

Another interesting project, well done.
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kludge
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Re: Sidequest: VGA scan doubler

Post by kludge » Sun Apr 07, 2019 2:40 pm

As I don’t know shit about Atari this might be a stupid question, but would this be possible to adapt to the Amiga? What are the input signals?

A cheap-ish scandoubler that attaches to the DB23 RGB Port of the Amiga would be a welcome addition to the community.

Sorry I’m advance if this is way off topic.
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exxos
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Re: Sidequest: VGA scan doubler

Post by exxos » Sun Apr 07, 2019 3:12 pm

Super nice well done!

I assume your using the 32mhz shifter clock to sync the ripple counters to the video sampling ?
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rubber_jonnie
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Re: Sidequest: VGA scan doubler

Post by rubber_jonnie » Sun Apr 07, 2019 4:34 pm

Wow, that's pretty cool 😁
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Icky
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Re: Sidequest: VGA scan doubler

Post by Icky » Sun Apr 07, 2019 5:03 pm

Nice one Smonson :thumbup:

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Smonson
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Re: Sidequest: VGA scan doubler

Post by Smonson » Sun Apr 07, 2019 11:21 pm

kludge wrote:
Sun Apr 07, 2019 2:40 pm
As I don’t know shit about Atari this might be a stupid question, but would this be possible to adapt to the Amiga? What are the input signals?
Maybe. I'm using the 32MHz master clock, the digital RGB signals (3 bits each) out of the shifter, and HSYNC. VSYNC might also be needed for better TV compatability. I don't know how easy it is to get hold of those in the Amiga's hardware because I don't own one at this time.

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