400p mode for retro PC

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This topic contains 6 replies, has 4 voices, and was last updated by  James-F 2 weeks, 1 day ago.

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    320×200 (doubled to 640×400) mode is the default for 99.99% of all DOS games and BIOS screens (to this day) and it will be great to have a 400p mode in the OSSC for retro PC gamers like me and thousands others.
    Currently the OSSC recognizes a 640×400 70Hz VGA signal from a 386/486/Pentium as 384p which is not correct but still fully functional.
    It was by sheer luck (or huge tolerance) the OSSC recognized VGA signal as 384p.

    I already have a post about this topic but not as a feature request.

    OSSC + Pentium 233 MMX (VGA)




    I second this request; also, as mentioned in the linked topic, a 720×400 mode would be nice.



    Could this be coupled with an option to stick this in a 640×480 or DTV 720×480 frame for use on pickier monitors and TVs?



    It is very unlikely that a TV will receive a 70Hz signal at any resolution but 640×400 or 720×400, unless its scaler is more like a monitor than a TV with plenty of VESA modes.

    To stick 640×400 in a 640×480 frame, the output mode for 400p should be in passthrough, but the image is blurry like plugging the PC straight to the VGA port on the LCD monitor.
    We want Line2x for crisp and clear DOS image on an LCD.

    In line2x mode I had to change the active vertical resolution to 480 from 384 so my monitor reads it as 1280×864 70Hz, only then the 4:3 option is selectable in the monitor, but on 1280×720 and 1280×800 it is always Wide.
    As you can understand the current implementation of 384p is definitely not designed for a DOS PCs but manageable with advanced tweaking.

    Ideally, 640/720 x400 frame in 640×480 doubled to 1280×960 resolution would be great, but 960 active requires at least 1000 lines total vertical resolution which 449p (400p) does not provide.



    Yes, 640×400 could be added on the mode list. The parameters on this page look correct (mainly: 449 lines, 800 dots per line) so I’ll use those. Separating it from 720×400 would be hard as both are 449p, but it should quite easy to adapt from 640×400 parameters (h_total -> 900, h_active -> 720) e.g. using a different profile when necessary. I’d assume most people prefer using DOS PC with a monitor, so there’s better chance of 70Hz compatibility than when using a TV.



    Thanks for your consideration!



    Thanks marqs!

    These settings work well and ‘to-spec’ for 640×400:
    H.samplerate 800
    H.synclen 96
    H.backporch 47 640
    V.synclen 2
    V.backporch 34 400

    Default for 400p should be passthrough to let the monitor handle the signal as if it was through VGA.

    Line2x with upsample2x in results in 1280×800 image but I cannot force it to 4:3, nor can I force passthrough to 4:3 on both my PC monitor.

    I tried to aim at XGA+ of 1152×864 which is perfect 4:3 and VESA standard all monitors support.
    I fiddled with the Advanced setting till I got a very nice 4:3 image in 1152×864 resolution on both my monitors.

    H. Samplerate : 700 (this gives correct phase and pixel aspect [no “waves”]).
    H. Synclength : 62
    H. backporch : 54
    H. Active : 576
    V. Synclength : 2
    V. Backporch : 18
    V. Active : 432

    It is important that Upsample2x is On to smooth the image.
    This 432 (1152×864) mode is preferred for Line2x+Upsample2x in 640×400 and looks fantastic on a PC monitor.
    Of course it will require further tweaking of the advanced setting as not all old school VGA/SVGA cards are the same.

    Like with 240p resolutions, it will be useful to separate Advanced modes for passthrough and Line2x of 400p, as currently both passthrough and line2x for 384p are on single advanced setting list.

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