N64 – Inconsistent V Backporch

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  • #27322
    Jademalo
    Participant

    This is a really weird issue, and fairly rare to actually come across. When it does crop up however, it’s a genuine annoyance.

    My understanding is that for the N64, the top is cut off by one pixel, and the bottom is cut off by two. This means when capturing 640×480, you’re expected to crop two off the top and one off the bottom.

    For most instances, this seems to fairly consistently require a V Backporch of 15.
    However, not all the time. An issue seems to crop up occasionally in games that will swap between 480i and 240p. Sometimes when swapping back to 240p from 480i, there is a chance that the image will shift up by one pixel, meaning that a V backporch of 14 is required. This interestingly puts the top line of the output right on line 1 of the OSSC capture, so it’s definitely moving up and not just specific to the game.
    Even less frequently, there’s a chance it will shift it up by *two* pixels, resulting in a backporch of 13 being needed and the top line actually being cut off entirely.

    I have absolutely no idea what is causing this, nor how to solve it. It’s a bit of an issue though, since it means I can’t reliably crop out the black unrendered area.

    Any ideas?

    #27324
    nmalinoski
    Participant

    Is this something that happens consistently with just one or two games, or is it something that happens reliably with all games you’ve tried that have 240p/480i mode switching?

    #27327
    Jademalo
    Participant

    Inconsistently with everything I’ve tried.

    I’ve had it happen with the 240p test suite a couple of times, but not always. I’ve had it happen with Vigilante 8: 2nd Offense more reliably, but it’s still extremely weird.

    As an example, try this;
    Load up Ocarina of Time, and crop the top and bottom exactly. There should be one line off the top and two lines off the bottom.
    Load up Vigilante 8: 2nd Offense
    Go into a basic match, and pause
    Change between High resolution and low resolution graphics

    Low resolution will sometimes have 3 black lines at the bottom, and sometimes have 4. It will also be right up against the top, without any extra room which shouldn’t be possible with the N64 hardware.

    For whatever reason, it’s randomly being moved up or down in absolute space, it’s definitely not just a game cropping differently.

    #27330
    nmalinoski
    Participant

    Which RGB mod are you using?

    #27331
    Jademalo
    Participant

    Errm, it’s an older THS7314 amp one I think. I was planning on getting it swapped out for one of the THS7316 ones eventually.

    I believe it uses Csync too rather than composite video sync.

    #27359
    paulb_nl
    Participant

    Even less frequently, there’s a chance it will shift it up by *two* pixels, resulting in a backporch of 13 being needed and the top line actually being cut off entirely.

    So you mean in this case the top line is not visible with any backporch?

    I think this is probably a N64 hardware quirk. Does the vertical position change when you are just switching inputs on the OSSC?

    #27361
    Jademalo
    Participant

    Sorry I phrased that a bit unclearly. From what I can tell –

    With backporch 15, there’s normally one black line on top and two black lines on the bottom. Sometimes the image can be moved up one, and sometimes moved up two. When it moves up two, there are four black lines on the bottom and the top line gets chopped off.

    With backporch 13, there’s normally three black lines on top and no black lines on the bottom. Sometimes it moves up one, and sometimes moves up two. When it moves up two, it shows the full resolution and there’s one line top and two lines bottom.

    It’s the offset needed that seems inconsistent, it’s very weird.

    #27374
    marqs
    Participant

    Does the offset remain if you power-cycle OSSC?

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