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Line5x and aspect ratios?

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This topic contains 5 replies, has 3 voices, and was last updated by  Harrumph 3 weeks ago.

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  • #16651

    tppytel
    Participant

    Hi all! New here… been gaming since the days of the 2600 and Apple IIe, but haven’t run original hardware for old titles since the LCD era rolled in. I have a 4K LG C7 that accepts line3x/4x/5x output for 240p and I’m trying to understand what’s happening behind the scenes.

    1) Can someone elaborate just a little on the wiki’s statement that line5x “uses full horizontal sample rate without pixel multiplication, resulting to 1536×1080/1200 output (4:3 aspect)”? Neither 1536/1080 nor 1536/1200 are equal to 4/3. I get that analog video doesn’t have a “horizontal resolution” per se. As I understand it, the game graphics have an internal resolution and the console pumps those digital values out at a fixed rate to the DAC so that one digital line of pixels corresponds to one analog line at 15.734 kHz. And then there’s some allowance for the electron gun to return horizontally, so it’s not quite a “fixed rate” exactly, but fixed while the gun is firing at least. Is that rate the “sample rate” in the wiki? Regardless, I don’t understand where that 1536 number is coming from.

    2) I get that the cropping in line5x 1920×1080 mode is done to match HD resolution. I assume the OSSC is simply throwing away 12 original lines on the top and bottom (12 x 5 = 60, 60 x 2 = 120). I notice in that mode that my scanlines are exactly the same height as the image lines. But in 1920×1200 mode, my scan lines are slightly narrower than the image lines. Is that due to the hardware scaling my 4K display is doing to bring 1200 lines to 2160 lines, or is the OSSC narrowing the scan lines intentionally to make the numbers work out? I guess I’m wondering whether 1920×1200 mode is more accurate than 1920×1080 mode, apart from the relative height difference between the image lines and scanlines.

    3) Measuring the image in either 1920×1080 or 1920×1200 mode with a measuring tape, I seem to have an aspect ratio of just a little bit greater than 4/3. Is this expected?

    Thanks for any insight you can provide. I’m going to have a lot of fun playing with this new toy.

    –Todd

    edit: I’m testing with my old PSOne (the late, mini model) if that matters. Figured that would be an easy place to start.

    • This topic was modified 3 weeks, 1 day ago by  tppytel.
    #16658

    Harrumph
    Participant

    1. It reflects the ratio of horizontal active to total line length. E.g. 256 width consoles (NES/SNES/some PS1 modes) have 256 active out of 341 horizontal total = 0.75, 320 width consoles (Mega Drive, Saturn, some PS1 modes) have 320/427 = 0.75. In other words, 3/4 of the horizontal signal is actual content, the rest is blanking. OSSC preserves this: Line x5 in OSSC is 1536/2046 = 0.75.

    2. The OSSC is not narrowing/removing scanlines. In Lx5 mode you will have 2 darkened lines out of 5 total. Any variation from this ratio is due to the upscaling of your screen.

    3. Not sure about PS1. Afaik, most consoles have a slightly narrower aspect, if you count strictly the visible area. E.g. SNES 256/224 * 8/7 = 1.306

    #16664

    paulb_nl
    Participant

    1536 is wide enough for NES/SNES but it crops other sources like N64. I set H.active to 1600 for line5X mode.

    #16693

    tppytel
    Participant

    Thanks for the replies. Those and some more Googling led me to some helpful reading. I understand now how the dot clock and the horizontal scan rate get you a 341 pixel line, of which 3/4 of the line (256 pixels) is signal. So it’s the 75% (the percentage of signal in each line) that’s staying constant. Got it.

    I’m still a bit confused about the 1536, though. I found the Optimal Timings page on the wiki and I see that it’s coming from 256 x 6 = 1536. And 1536/1200 ~= 1.28. So is the x6 multiplier just the integer multiplier that gets us closest to 4:3 aspect? So that way we can scale up the native 256×240 without doing any interpolation?

    #16707

    tppytel
    Participant

    Also, is there any expectation that the 256×240 optimal sampling mode should work for PS1? I tested a bit with SOTN and couldn’t get a clean image despite playing around with the Sync LPF and Sampling Phase settings. From what I’ve read, the PS1 in 256×240 mode has the same dot clock and other characteristics as an NES or SNES, but maybe I’m missing something. I’m not unhappy with the generic 4:3 setting. Looks terrific.

    #16713

    Harrumph
    Participant

    3. again, forgot to mention of course the 1920×1080 mode will be wide since it crops the image to display 1536×1080 area = 1.42. Dunno what’s up with your 1920×1200 mode though, probably some scaling on your TV.
    In relation to what paulb_nl wrote, you can basically set H Active to anything that works for your monitor, most seem to ignore this value for scaling/AR adjustment anyway (so it wouldn’t affect the scaling of NES/SNES games anyway), and 1600 is perhaps especially appropriate for 1200 height mode.

    Regarding your PS1, I don’t remember but might be one of those where you need to adjust the sample rate up or down by one (because dot clock is not exactly 341, just close). Alternatively, adjusting video LPF could probably help. Really doubt sync LPF would have any effect at all.

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