SNES losing sync in 60Hz only

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  • #20191
    Bananmos
    Participant

    Hi,

    I got my OSSC after getting an old Samsung LE46M87BD TV set for free from work to replace my smaller Sony KDL-40BX400 TV, and realised the Samsung wouldn’t take 240p. The OSSC did manage to do 2x mode on the Samsung, but fared even better than the Sony. So in the end, I decided to keep the smaller Sony and donate the Samsung to a friend with a tiny TV to reclaim living room space.

    However, there was one bug that I had with the Sony TV running on the OSSC: When playing my SNES (2chip model, using a simple 50/60Hz switch) at 60Hz, the TV would occasionally lose the picture. This was really annoying, but after reading about how that simple mod can have problems like that because it will output 59.53Hz rather than 60.08Hz, I blamed it on this. The fact that my RGB-modded NES outputs 60.08Hz and run on the OSSC without any problems made me even more convinced that this had to be the reason. And as I already had a 1chip on my wishlist to get rid of that ugly blur, I was convinced this would fix everything.

    Well, fast-forward to now, and I have bought a 1chip and installed a SuperCIC board. And the issue in 60Hz mode remains. If anything, it’s even worse than on the 2chip. Bummer…. looks like I might have to retire the Sony TV after all, if I want to use the OSSC with it. 🙁

    I know there’s a mod board in works to give better compatibility with NES/SNES video signals: https://videogameperfection.com/forums/topic/nes-snes-240p-audio-video-stability-issues-read-before-posting/

    …but I’m not convinced this jitter is the reason for the picture losses at 60Hz, or if I might just spend more time and money on trying to mod away an issue that is not the root cause. My doubts come from the fact that the OSSC is not showing any problems whatsoever with my RGB-modded NES, which also outputs 60.08Hz, according to the OSSC display. This is really surprising, because as far as I know, that extra pixel difference is the same for both the NES and the SNES. So I’m really surprised one would work but not the other…

    Does anyone have any other ideas for how to deal with loss of picture on the SNES?

    #20192
    Bananmos
    Participant

    So according to marqs (the mod board author in the thread at https://shmups.system11.org/viewtopic.php?uid=11668&f=6&t=61285&start=0), there is one difference with the SNES compared to the NES:
    “The short scanline actually occurs deeper in vblank”

    But it seems odd that this would be the cause of the SNES@60Hz not working on my TV while the NES works perfectly. So I’m still not convinced this is the problem in my case.

    Nevertheless, another thought I had regarding the odd/even frames problem… as the OSSC does a bit of buffering already, shouldn’t it be possible for it to buffer just enough to be able to smooth out the odd/even frames into frames with consistent timing? If the OSSC can generate output sync at 2x the pixel clock rate of the NES/SNES PPU, a frame_clocks+2,frame_clocks,frame_clocks+2,… sequence at the input should just equal frame_clocks+1 at the output.

    I know this might complicate the FPGA code in order to support just two very quirky systems and may not be top of the priority list for new OSSC firmware. But if possible, it seems like a better alternative than asking people to do yet another complicated hardware mod to their SNES/NES consoles…

    #20199
    paulb_nl
    Participant

    The effect of the short scanline on the NES is that the top of the screen wobbles a bit. If the short scanline occurs on a different line on the SNES then it could easily affect the sync instead and lose sync.

    Does the OSSC lose sync though when you lose the picture? The 1-CHIP has an issue when you use a composite video sync or luma sync RGB cable where the OSSC loses sync with some games. In that case you need to increase Analog sync Vth.

    If you have changed H-PLL Pre-Coast/Post-Coast from the defaults 1 & 0 that will cause the OSSC to lose sync with SNES on 240p60.

    To test if it is the sync jitter you can use a 480i game or use the 240p test suite and change the video output to 480i. The SNES does not have sync jitter in 60i and 50p.

    #20223
    Bananmos
    Participant

    Hi, and thanks for your response.

    The H-PLL Pre-Coast/Post-Coast are set to 1 & 0, and I’ve tried to increase Analog sync Vth until it loses sync, with no noticeable improvement in picture loss.

    > Does the OSSC lose sync though when you lose the picture?

    Hmm, doesn’t look like it. The main menu keeps displaying “AV1 RGBS 262p / 15.74kHz 60.08Hz” even when the picture disappears.

    I tried your recommendation of 480i, but actually made Super Mario World run in interlaced mode on my SNES Powerpak, using the game genie codes from http://www.hdretrovision.com/blog/2016/12/29/super-mario-world-in-480i

    But I’m a little bit unsure of what settings the OSSC should use for this kind of test? To start with, there doesn’t seem to be any way to keep the 5x scaling when running SMW with those Game Genie codes. Instead, the OSSC will automatically switch to a 480 mode, which is dependent on the “480i/576i proc” setting.

    And I get the following results for the different settings
    “Passthru”: Picture is stable, and my HDTV says it is “480i”. I get no loss of picture
    “Line2x (bob)” Picture is looking very flickery and shimmering, but I guess that’s as expected from an interlaced signal? HDTV says it is “480p”. I get no loss of picture.
    “Line3x (laced)”: HDTV won’t accept it – Just says “out of range”
    “Line4x (bob)”: HDTV won’t accept it – Just says “out of range”

    So I guess I’ve done the supposed test then? But I am a little bit surprised that the OSSC forces a line-doubling mode here, rather than staying at the 5x setting I chose for 240p. But I guess it’s as designed.

    #20235
    paulb_nl
    Participant

    The OSSC can’t just use 5x for 480i because that would result in something like 2160i/2400i. SNES 480i output has 262.5 lines vs 262 for 240p.

    You have picture dropouts on every lineX mode right? Because you didn’t mention it only happens on a specific mode. If you have picture dropouts on 240p lineX2 and not on 480i lineX2 then its most likely caused by the sync jitter.

    You could try to set Allow H-PLL 2x to off. At least it reduces the wobbly lines at the top of the screen with the NES. Maybe it will help with your picture dropouts.

    #20308
    Bananmos
    Participant

    Ah, of course… I forgot about the half-scanline in interlaced mode. Silly of me!

    In theory, I suppose the OSSC *could* buffer half a scanline and have the output frame rate be 265.5/262 of the original, in order to support higher scaling for interlaced… sort of how the Hidef-NES manages to compensate for the original signal’s line rate being different to the output. But that would be a lot to ask for a mode that’s going to look pretty rubbish no matter what… 🙂

    Unfortunately, changing H-PLL 2x setting didn’t help either. But I’m now pretty convinced it must the shorter scanline causing all this, and will patiently wait for Marq’s hardware mod to be available.

    But thanks for all the advice Paul!

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