Sidequest: VGA scan doubler

[atari030]

That looks the goods, it really does the speccy pic justice.

[Smonson]

I wanted to add a scanline effect to the 'doubler, since a lot of people want to recreate the authentic mid-80s feel when they look at their Atari screens. Having anticipated doing this, I put 2 jumpers on the board, with one being for the scanline effect, and one spare one.

For the HDMI mod, it's an easy effect because we have 24-bit colour output and so I just scaled the intensity of the second line by 75%, and that looks great. For this mod however, since the video is running through the ST's original video hardware, we only have the normal ST palette of 9-bit colour to work with. Therefore I went with a different approach of just subtracting 1 from each of the colour intensities, which are normally 0-7.

That does have a side effect of slightly increasing the contrast, because anything that was only an intensity of 1 gets reduced to 0 for the second scanline, so the average brightness of a pixel of intensity 1 becomes 0.5 in the output (a reduction of 50%) while a pixel of brightness 7 will be paired with a brightness 6 on the next line, averaging 6.5 which is only a reduction of 7%.

A result of that contrast increase could mean that the colour of objects onscreen is affected, especially at very dark levels. Something with colour value 221 gets averaged with 110 on the 2nd line which means the red and green channels get boosted a little bit on the 2nd scanline, but the blue doesn't get anything. As a person with red/green colour blindness, I'm not a very good judge of how bad that actually looks in real life!

Here's the final effect looks on the desktop:

vidout17.jpg

And in QFG1:

vidout16.jpg

I think this is about the best that can be achieved while sending the video out through the ST's original video circuit. Let me know what you think!

[sandord]

I think it looks fantastic!

And what you're describing as a possible colour deviation, I don't think it'll be much of a problem since this is it doesn't actually change the balance between the primary colours. It's how making shades on the ST has always worked and that's why I think it'll look natural.

Take this gradient of #766->#000 for example:

#766
#655
#544
#433
#322
#211
#100
#000

Untitled.png

I don't think there's a way of getting a more accurate gradient of #766->#000 than this and this is exactly what your scan doubler seems to be doing.

[Smonson]

Thanks @sandord! Graphing it like that makes it a lot easier to see the progression of colour, and it's not as bad as it could be :)

I think that's the last thing I was going to add to the FPGA code... I guess that makes it a final prototype.

[Atarian Computing]

Volunteering to test a prototype if possible :geek:

[Smonson]

Volunteering to test a prototype if possible :geek:

Well I'm only planning to make a few of these, since it takes a long time to build them by hand. I'll keep you on the list :)

[Smonson]

For the next board version I've made a bloody big hole through the board so it can fit around the electrolytic that's in the way. That should let it sit at the proper height. This is the same place on both my STs, and I hope it's in the same place on every other revision too!

"If it's stupid and it works, it's not stupid"

vidout14.jpg

[Aeberbach]

"If it's stupid and it works, it's not stupid"

Been watching The Mighty Jingles?

[Smonson]

The other jumper connection that was spare will be to disable the mod and return the ST to its original workings (albeit with a longer analogue signal path). It'll still fit into the FPGA. Basically it just works the same as the 71.2Hz mode and passes the hsync and vsync through untouched, but with the addition of passing the RGB pins through as well. That should hopefully make people feel safer about cutting that one track!

[Icky]

If you have an H5 @exxos added jumpers to accommodate this board so no track cutting necessary on the H5 :)