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Analogue Mega Sg (FPGA Genesis/MD; April 2019) Sega CD Support, adapters for SMS, SG-1000, & GG

#16 User is offline Black Squirrel 

Posted 19 October 2018 - 10:26 AM

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No he's right, an actual, full-fat proper Mega Drive clone should have its cons and well as its pros. Though I don't know if their methods of direct capture produce a different image to what would be rendered on a CRT.


I'm not actually sure what those dots are but a quick skim of YouTube suggests they don't appear in the PAL version? Not that I'm pretending to understand what it is and why it appears. Or if indeed it's even a feature shared across different revisions of the hardware.



I could really do with a crash course on this subject so I can add things to the wiki. How all these Sega consoles are meant to appear across multiple televisions and video cables.

#17 User is offline Flygon 

Posted 19 October 2018 - 10:26 AM

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Overscan borders actually get graphics rendered into them on Overdrive 2, so there's that. :v:


Edit for Black Squirrel's benefit: Those are CRAM dots. They're drawn up because of where the game is writing palette entries during that period of the vblank.
I presume PAL software would deliberately perform those CRAM writes later, because otherwise they'd show up inside the 50Hz border. 60Hz seems to have its border cropped out by most TVs anyway.
This post has been edited by Flygon: 19 October 2018 - 10:29 AM

#18 User is offline MarkeyJester 

Posted 19 October 2018 - 03:02 PM

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View PostBlack Squirrel, on 19 October 2018 - 10:26 AM, said:

I'm not actually sure what those dots are but a quick skim of YouTube suggests they don't appear in the PAL version? Not that I'm pretending to understand what it is and why it appears. Or if indeed it's even a feature shared across different revisions of the hardware.

They do actually appear on PAL machines too, the cause is (as Flygon mentioned) writing to CRAM during active display. Although the VDP triggers V-blank on the 68k, the VDP is still writing backdrop colour to display (the boarder colour on the bottom of the screen). So even though the 68k has interrupted into the V-blank routine, the VDP is still writing colour data as it's actually still during display, so any CRAM writes still cause dots to appear during the boarder. The reason you don't see PAL games display it, is because software deliberately delays the CRAM write for a short while to give the TV time to get into the "real" V-blank (when the VDP is not actually writing the backdrop colour).

The same software generally (and intentionally) do not give NTSC this same delay simply because NTSC has less time during V-blank to transfer data, although you could delay until the TV actually reaches real V-blank, it would be a heavy loss of time for NTSC, you can only transfer a maximum of about $1D30 (or less if the H/V resolution is shorter) approx bytes via DMA during V-blank, so time is heavily critical, waiting for real V-blank would be foolish on NTSC hence why they didn't do that. For PAL machines, you have roughly $3914 (or less if the H/V resolution is shorter) approx bytes via DMA, so if you can transfer the data in NTSC within V-blank fine, you'll definitely be able to transfer the same data on PAL systems even with deliberately delaying for real V-blank. This is why PAL machines get the delay treatment, and NTSC machines don't, it is software controlled though.

As for why the dots appear at all, I do not actually know the exact science, but from what I can work out, during H-blank the VDP is calculating the scanlines of colours to display on the next active display, in order to do that, it needs to read from CRAM during H-blank, but if you are writing new colours into CRAM during H-blank (real TV/VDP H-blank) while the VDP is reading from CRAM, the mechanism (or address) the VDP is meant to be reading from is probably being directed to the FIFO and reading the last value in there as opposed to the CRAM address (again, I don't have any hard evidence so don't take what I say as definite), it's similar to performing a DMA copy of data (copy, not transfer) where the copy destination is the sprite table, the caché doesn't update properly, and instead the VDP ends up copying data from the FIFO instead of the sprite table address in VRAM. These could be related.

I have noticed VSRAM to have a similar issue if you are to write too much data into VSRAM during H-blank, it can cause the -1 V-scroll to screw up (not in the common AND bitwise way that it normally does).

EDIT: I'm wondering if it's to do with the fact that CRAM and VSRAM are internally within the VDP, you don't get this issue with VRAM writes which is external.
This post has been edited by MarkeyJester: 19 October 2018 - 03:20 PM

#19 User is offline Black Squirrel 

Posted 19 October 2018 - 03:51 PM

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Sweet - thanks for that


Turns out Hivebrain did mention "CRAM dots" very briefly but there's no explanation of what (or "where") they are (I think I originally thought they might be these "dots"). Not his fault though - not sure anyone ever asked until now :v:

#20 User is offline MarkeyJester 

Posted 19 October 2018 - 04:15 PM

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View PostBlack Squirrel, on 19 October 2018 - 03:51 PM, said:

(I think I originally thought they might be these "dots").

Actually, no, you are exactly right, the link regarding the water dots is exactly the same the dots you see on the bottom of NTSC games, it's the exact same issue.

The VDP displaying the boarder is the same as the VDP displaying the graphics during tile display, even though the VDP doesn't display graphics in the boarder, it is still reading from CRAM in order to draw that boarder colour while it's waiting for the TV to reach "real" V-blank, and writing to CRAM during H-blank is overwriting the storage colour. It is the exact same dots.

#21 User is offline ICEknight 

Posted 19 October 2018 - 05:44 PM

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View PostFlygon, on 19 October 2018 - 10:26 AM, said:

Overscan borders actually get graphics rendered into them on Overdrive 2, so there's that. :v:

For reference:


(Happens at around 4:10)
This post has been edited by ICEknight: 19 October 2018 - 05:45 PM

#22 User is offline Gryson 

Posted 19 October 2018 - 06:55 PM

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View PostICEknight, on 18 October 2018 - 08:26 PM, said:

View PostAshura96, on 17 October 2018 - 08:10 AM, said:

View PostICEknight, on 16 October 2018 - 04:56 PM, said:

Doesn't look so accurate when it's missing some borders on the sides.

EDIT: For reference:


Also, it seems to be missing those moving dots in the lower border?


Why would you want overscan borders when you will have multiple resolution options on the system itself?

"Why would you want" is irrelevant when we're speaking of accuracy.


There is a lot of marketing hype coming from Analogue - I guess what you're taking issue with is the statement "Total Accuracy."

Byuu has an excellent reply to this kind of marketing hype: https://byuu.org/articles/fpgas-aren't-magic/

In the end, Analogue is absolutely not worried about accuracy, despite what the marketing says. They're not releasing their source code, they're not devoted to preservation, they're not MAME. They're a business trying to make a profit - so they sure as hell don't want overscan borders.

#23 User is offline Overlord 

Posted 21 October 2018 - 05:01 PM

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Frankly I'm interested in this just for the Game Gear compatibility. Mega Game Gear, a reality at last =P

#24 User is offline saintminya 

Posted 22 October 2018 - 02:12 PM

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@Overlord, I'm in the same in the boat. I was interested at the notion of Master System support since I don't own the console or a powerbase converter. Now I am all about it, my Gamegear needs a replacement screen, but I'd much rather play my collection on a TV.

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