Internal scan doubler idea

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Petari
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Joined: Tue Nov 28, 2017 1:32 pm

Internal scan doubler idea

Post by Petari » Fri Aug 03, 2018 5:20 pm

I spotted on this today: http://atari-forum.com/viewtopic.php?f=15&t=34108
Indeed interesting, although I wouldn't say that it is secret mode, rather is side effect of not harmonized HW registers in ST.
So, possible to have 160x400 px in 16 colors, 320x400 in 4 colors, and even 1280x200 in monochrome . Little weird resolutions, indeed :D Sadly only on ST. Or maybe not ... See below. STE will rather show darkness.
What would be more useful, I think is 320x200 at 71 Hz - so ST can go straight on most of LCD in color mode. In that case we using czietz's trick and line scandoubler. That will need very little SRAM - for 1 line only, so 1-2 KB is enough. It reads 2 lines of 160 px in row, and then outputs it 2x in row at double speed - what doubles hor. and halves vert. res. Same stays for med. res. - then of course 4 colors and 640 px horizontally.
STE - I see on schematic that BLANK signal goes to GST shifter, so it can be disconnected and manipulated. That will need some experimenting first, indeed. Goal is to prevent permanent blanking (darkening) in this mixed mono/color mode.
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Cyprian
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Re: Internal scan doubler idea

Post by Cyprian » Fri Aug 03, 2018 6:47 pm

cool idea.
If Im not wrong, we discussed about that sometimes ago on AF.
that (Dual-ported RAM?) memory would be connected to SHIFTERs RGB lines.
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Smonson
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Re: Internal scan doubler idea

Post by Smonson » Wed Aug 29, 2018 1:45 pm

Petari wrote:
Fri Aug 03, 2018 5:20 pm
That will need very little SRAM - for 1 line only, so 1-2 KB is enough. It reads 2 lines of 160 px in row, and then outputs it 2x in row at double speed - what doubles hor. and halves vert. res. Same stays for med. res. - then of course 4 colors and 640 px horizontally.
I recommend using 2x 2KB SRAMs instead of a dual-ported RAM - you can write to one device while the other is being read, then exchange them at the end of each scanline. This is the method I used for the scandoubler in my FPGA project. You would need to use either tristate bus transceivers or else interleave read and write access on alternate clock cycles... or something :)

The SRAMs probably need to be 2x 8-bit bus devices to get the 9-12 bits for the palette info.

Petari
Posts: 527
Joined: Tue Nov 28, 2017 1:32 pm

Re: Internal scan doubler idea

Post by Petari » Wed Aug 29, 2018 5:58 pm

Smonson wrote:
Wed Aug 29, 2018 1:45 pm
..
I recommend using 2x 2KB SRAMs instead of a dual-ported RAM - you can write to one device while the other is being read, then exchange them at the end of each scanline. This is the method I used for the scandoubler in my FPGA project. You would need to use either tristate bus transceivers or else interleave read and write access on alternate clock cycles... or something :)
The SRAMs probably need to be 2x 8-bit bus devices to get the 9-12 bits for the palette info.
That's seems as good idea - will be cheaper for sure. 9 bits on ST, 12 bits on STE - so, surely need 16 bit data width.
There is 2 kind of people: one thinking about moving to Mars after here becomes too bad, the others thinking about how to keep this planet habitable.

Gargoyle
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Re: Internal scan doubler idea

Post by Gargoyle » Wed Aug 29, 2018 9:25 pm

It was a really interesting discovery, Troed managed to do a hardware mod to his ST and double the width too, as well as the height with this mode for medium res, so he achieved 640 x 400, 4 colours.

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