Dreamcast – VGA or DTV 480p confusion

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  • #8738
    marqs
    Participant

    TVs normally scale Rec. 601 720×480 either to 4:3 (standard) or 16:9 (anamorphic widescreen) based on which one user selects as aspect ratio. As show in the previous picture, DC outputs only 640×480 subsection (which is designed to fill whole 4:3 screen) within that area so it won’t result to correct aspect with modern displays by default. However, recent OSSC fws have advanced timing tweaking option which can reduce the width of signal marked as active from 720 to 640, so monitors and compatible TVs can display only that area when scaling to 4:3 or 16:9 (widescreen games), resulting to exactly correct aspect ratio. Note that the mode is not same as VESA 640×480@60Hz which has different sampling rate.

    #8741
    RoundCircleS
    Participant

    Ok I am learning that within the 480p DV standard, the 2:3 ratio is mainly a pixel aspect ratio for conversion and storage from 16:9 to 4:3, so that the 16:9 image changed to 2:3 pixel aspect ratio and then stored in 4:3 will show up on a 16:9 screen properly. Not sure if that can be used somehow going the other way, but I think that’s what I was trying to determine when I came across the thread.

    marqs
    As show in the previous picture, DC outputs only 640×480 subsection (which is designed to fill whole 4:3 screen) within that area so it won’t result to correct aspect with modern displays by default.

    Ok let me know if this is correct. Sounds like those black bars (that mark where to stop stretching the image on a 640×480 3:4 set) are put there by the DC, but do not affect the pixel aspect ratio coming out of the DC by being there. Is it correct to think that the DC hangs those black sidebars over the 720×480 rendered graphics and then sends all of that to the TV as 720x480p without changing the pixel aspect ratio, and then the 3:4 TV squishes it, black bars and all, and that is the first time the pixel aspect ratio gets affected? The chart is helpful, but I found myself wondering about pixel aspect ratio, what does the DC use initially, and does that change internally before it leaves the system via VGA-out, and what is it when it leaves the system? It seemed like if it is changing before it leaves and then the TV is affecting it again, that it would have been a lot tougher to resolve the issues.

    marqs

    However, recent OSSC fws have advanced timing tweaking option which can reduce the width of signal marked as active from 720 to 640, so monitors and compatible TVs can display only that area when scaling to 4:3 or 16:9 (widescreen games), resulting to exactly correct aspect ratio.

    Sounds good, wouldn’t mind some more discussion about my above questions anyway.

    With my possible setup below and reading about the OSSC, I see to set 480p line x2 to off for basic 480p passthrough, does using the DTV-480p setting or using the dvi-d-to-hdmi cable somehow cause the output to change to 480i?
    ( DC <> VGA cable <> OSSC <> dvid-to-hdmi cable <<still 480p here?>> TV )

    I pretty much play three games, none burned, on my NTSC system. NTSC Shenmue, NTSC F355 Challenge, and UK-PAL Shenmue2 using a reindeer boot disk that I made to get it started. I know reindeer disk won’t display through VGA.

    Has anyone tested these three games to see if the 16:9 aspect ratio setting works on all of them? I know that these should all boot into VGA, but do I have to worry about PAL format with Shenmue 2 conflicting with the the DV standard that I need to set for widescreen? The game appears to run OK on my TV through S-Video, so I am assuming that the game runs in “PAL-60” then.

    #8742
    paulb_nl
    Participant

    Is it correct to think that the DC hangs those black sidebars over the 720×480 rendered graphics and then sends all of that to the TV as 720x480p without changing the pixel aspect ratio, and then the 3:4 TV squishes it, black bars and all, and that is the first time the pixel aspect ratio gets affected?

    You are correct except that it renders at 640×480 and pads it to 720×480 with black bars.

    With my possible setup below and reading about the OSSC, I see to set 480p line x2 to off for basic 480p passthrough, does using the DTV-480p setting or using the dvi-d-to-hdmi cable somehow cause the output to change to 480i?
    ( DC <> VGA cable <> OSSC <> dvid-to-hdmi cable <<still 480p here?>> TV )

    The OSSC will only output 480i on 480i passthrough mode. The DTV-480p setting is just for sampling the correct amount of horizontal pixels.

    I know that these should all boot into VGA, but do I have to worry about PAL format with Shenmue 2 conflicting with the the DV standard that I need to set for widescreen?

    What do you mean with the DV standard that you need to set for widescreen? You just need to set your tv to 16:9 with a widescreen game.

    #8760
    marqs
    Participant

    Sounds like those black bars (that mark where to stop stretching the image on a 640×480 3:4 set) are put there by the DC, but do not affect the pixel aspect ratio coming out of the DC by being there. Is it correct to think that the DC hangs those black sidebars over the 720×480 rendered graphics and then sends all of that to the TV as 720x480p without changing the pixel aspect ratio, and then the 3:4 TV squishes it, black bars and all, and that is the first time the pixel aspect ratio gets affected?

    I’m no expert of DC internals, but the following is how I’ve understood the chain works:

    * DC internal framebuffer is typically set at 640×480, and content rendered there is in 4:3 (1:1 PAR).
    * PowerVR utilizes fixed 27MHz pixel clock, and with default register settings draws 525 scanlines of 858 dots each (=59.94Hz) in VGA mode.
    * Border/background color can be drawn on ~720 dots, while rendered image is placed in the centermost 640 dots. Active content is drawn on 480 lines (240 for non-VGA 60Hz output) – CRT TVs typically display only ~224 lines due to overscan, resulting to top and bottom getting cropped.

    In analog video, all the receving end sees is 525 scanlines of ~32us each – there is no indication on aspect ratio (WSS does not apply here) or whether actual source picture is 640×480, 720×480 or something else. A digitizer can either assume that the signal follows some standard (e.g. CEA or VESA) or try auto-adjusting correct size, both which are not guaranteed to give correct result (especially with DC).

    #8761
    RoundCircleS
    Participant

    paul_bnl
    The OSSC will only output 480i on 480i passthrough mode.

    Understood.

    paul_bnl
    The DTV-480p setting is just for sampling the correct amount of horizontal pixels.

    Somehow I started to think that using that setting, or perhaps using a dvi to hdmi cable, would cause the 480p signal to change to 480i. I think I confused myself while reading about the various video standards, possibly reading about DTV standards or something. My main concern is that I still have 480p getting to the TV with my proposed setup, and the OSSC set to DTV-480p.

    RoundCircleS
    Is it correct to think that the DC hangs those black sidebars over the 720×480 rendered graphics and then sends all of that to the TV as 720x480p without changing the pixel aspect ratio, and then the 3:4 TV squishes it, black bars and all, and that is the first time the pixel aspect ratio gets affected?

    paulb_nl
    You are correct except that it renders at 640×480 and pads it to 720×480 with black bars.

    Ok, let me see if I am getting some of this. Using a hypothetical setup of DC> vga cable > 3:4 CRT TV. The original image is stored in 720×480 of some pixel aspect ratio, the DC takes that info and changes it when it renders it in 640×480 (does that change the original pixel aspect ratio?), then it places the bars on either side of that, and that whole thing is output to the TV as a 720×480 image of some pixel aspect ratio, and then the TV changes the pixel aspect ratio at that point when it displays all of that in 3:4 640×480? So the pixel aspect ratio possibly changes at least twice (and maybe more) by the time it is actually displayed and before the end-user changes it again? I guess I am wanting to know what the pixel aspect ratios are within the DC, and exactly how may times they might change, and what it is by the time the VGA signal is exiting the DC.

    paulb_nl
    What do you mean with the DV standard that you need to set for widescreen? You just need to set your tv to 16:9 with a widescreen game.

    Sorry, not sure what got lost, I was asking the question specifically in terms of using the OSSC in the setup. DC > vga cable > OSSC DTV 480p > dvi-hdmi cable > my TV, set to 16:9. Just looking for clarification if the OSSC definitely helps fix the aspect ratio problems with those specific games.
    Also I was asking if I would have any issues that would affect the aspect ratio if I were running Shenmue2 UK-PAL on that same setup.

    (Also just to clarify, none of those games are what I would call widescreen games. although F-355 does have an in-game widescreen option, it also shows up incorrectly in 3:4 and 16:9 on my tv in S-Video, and from what I understand, that in-game widescreen option disappears once the DC goes to VGA output. I have not tested that but my VGA cable should be here any day now and that is what I expect to happen there.)

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