Corrupted PS1 Polygon Graphics on PS2

Discussion in 'Repair, Restoration, Conservation and Preservation' started by Aurshur, Dec 20, 2018.

  1. Aurshur

    Aurshur Member

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    Hi everyone,

    I have a Japanese SCPH39000 unit that plays PS2 games without issue however when I pop in an original Japanese PS1 disc I'm greeted with this:
    [​IMG]
    [​IMG]
    [​IMG]

    The odd thing is it exclusively affects polygons, 2d titles perform perfectly:
    [​IMG]

    Any idea what the issue is?
     
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  2. PixelButts

    PixelButts Site Soldier

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    That admittedly looks cool and I want to know what's wrong too.
    @sp193 @TriMesh any ideas what's happening here?
     
  3. la-li-lu-le-lo

    la-li-lu-le-lo ラリルレロ

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    A problem with the GTE? Just a guess, but that would explain why it only affects 3D graphics and only PS1 games. Beyond that, I can't speculate.
     
  4. master991

    master991 Enthusiastic Member

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    PS2, if I remember correctly uses psx cpu and ram from retrocompatibility. Maybe one of those are damaged...
     
  5. Aurshur

    Aurshur Member

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    Yeah, I've basically resigned myself to the fact it is an issue that I won't be able to fix. The sad part is that I bought the 39000 because they're supposed to be more reliable!.
     
  6. FromAllAspects

    FromAllAspects PlayStation Collector

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    Regardless of reliability, it can happen to anything! Hahaha. For me, the most reliable PlayStation 2 I've had so far is the SCPH-30002.

    Kind regards,

    FromAllAspects
     
  7. Aurshur

    Aurshur Member

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    Yeah my PAL launch model is still kicking!, it needs a replacement laser but otherwise it hasn't skipped a beat. It isn't really an option for me though due to the fact I wish to play NTSC PS1 games and the subtle timing issues wreak havoc with my OSSC.
     
  8. sp193

    sp193 Site Soldier

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    If that is a chipped console and you have FMCB, you can use PS1VModeNeg to change the video mode to NTSC.
     
  9. smf

    smf mamedev

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    It uses GTE to display the boot logo? The GPU is emulated using the EE+GS, there could be some weird hardware problem.
     
  10. Aurshur

    Aurshur Member

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    No it's not a chipped console its a stock Japanese 39000, although I did swap in a PAL power board (from a 39002) to enable it to run off mains power here in Australia. I tried PS1VModeNeg just for the hell of it to no avail.

    I've since put the original power board back in and the PAL one back into my donor console. I'll happily knock it all back together if there are any other tests you guys can think of.
     
  11. wisi

    wisi Rising Member

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    This is indeed an interesting question, given that AFAIK the logo is 3D polygons and is not transformed (rotated, scaled, etc.), (or is it?). But then again, when the actual rendering to a 2D image is done, GTE instructions may be used. I am not familiar with the PS1 3D rendering, but if 3D vertex coordinates are sent to the PS1 GPU (in this case the EE emulating it), then it might be an IOP-EE synchronization issue that is causing this - perhaps due to a broken connection on some of the lines. Note however, that for what I know, all the IOP-EE SBUS lines are used for both PS1 and PS2 mode, so if the cause was there, then PS2 games and maybe even booting the console would fail as well (with the exception of an interrupt line maybe).
    There is even the possibility that there are some test-points on the IOP ASIC related to the GTE, which got short-circuited, resulting in incorrect calculations (but this is far-fetched).
    So what was already suggested - a fault in the GTE - seems to be the most likely reason. This can be checked with a test program, but I don't know if such exists (somebody at psxdev.net or PCSX2 coders might know of such). And there is also the matter that the faulty calculation does not always happen - it seems some vertices are at their correct positions, moreover the vertices that are incorrectly transformed are not far from their correct positions, so it is almost as if the calculation is wrong by just a few bits - otherwise we would be seeing polygons stretching in all directions. So it may be a low-order line from some of the internal GTE buses that got disconnected and keeps at a constant potential. When the coordinates to be processed have that bit in the same level as the line, the calculations end-up correct, and when the (input or output) value has this bit different, it is forced to the state of that line, resulting in an incorrect output value.
    But this is just a speculation, and either way, there is nothing that can be done to fix it (AFAIK).
     
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  12. Aurshur

    Aurshur Member

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    I knew the machine was toast the moment I saw the problem, I've lurked on here for years so I thought a few people would get a kick out of it. Thanks for the write up!
     
  13. bart_simpson

    bart_simpson Dauntless Member

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    bad solder joint via ee or gs
     
  14. SONIC3D

    SONIC3D Spirited Member

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    It seems vertex buffer content is error thus the vertices position is affected.....May be caused by memory problem....
     
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  15. sp193

    sp193 Site Soldier

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    I was referring to your PAL console, as it seemed to be a working set. PS1VModeNeg is meant to change the video mode, so a PAL console's PS driver would operate in NTSC mode. The only limitation is that the game compatibility list is not present, so it may have slightly lower compatibility than on a true NTSC console.

    Consoles older than G-chassis are also unable to output true NTSC or vice-versa, due to some components being installed for either video mode (i.e. clock input is 54MHz for NTSC, while it is marginally lower on PAL sets). This is related to video encoding properties, not about running a NTSC game with a PAL signal.
     
    Last edited: Dec 21, 2018
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  16. Aurshur

    Aurshur Member

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    Ah my mistake.

    Yeah, I've instead resolved to pick up a 39002 as my understanding from your explanation is that because the timing comes from a clock synthesiser it can be properly adjusted through PS1VModeNeg, unlike on my 35002 which would likely cause issues with my OSSC due to the timing differences from the hardware clock.

    Lastly, would this setup play nice with a Modbo 4.0?, I'm interested in PSone imports and backups so I'd ideally like a setup where I could load out of region discs but still run homebrew elfs (PS1VModeNeg) at the same time.
     
  17. rama

    rama Gutsy Member

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    It's an interesting and rare glitch for sure.
    I'll go with a minute fault on the GTE silicon, maybe a permanently stuck gate on an instruction decoder or so.
    Definitely not external, so fixing it would mean replacing the IOP.

    There is some homebrew that can test a real PSX's GTE. I don't know if it'll work on a PS2 though.
    https://psx.amidog.se/doku.php?id=psx:download:gte
    https://github.com/ogamespec/pops-gte/tree/master/GTETest
     
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  18. TriMesh

    TriMesh Site Supporter 2013-2017

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    Seems unlikely - the same RAM is used for the IOP and PS compatibility, and since PS2 mode apparently works fine it's hard to see of how the RAM could fail in a way that only affected PS mode. Unless there is more stuff in there I don't know about, I would have to go with it being a problem with the GTE. Having said that, I don't know for sure if the GTE is used for rendering the boot logo, although since it's stored in .TMD format it seems likely.
     

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