Reply To: 400p mode for retro PC

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#27188
James-F
Participant

@marqs
In Output Options the 384p proccssing setting controls 400p too, can you please display “384p/400p proc” so people know it also controls the standard 720×400 DOS mode?
Also, can you please add a Line3x in this mode for 1200p output with 720×400 input, for sharper image?

Maybe a separate 400p would be useful since since PC CRT are becoming rarer and more people using the OSSC for retro computers but no clear 400p settings in the menu?

BTW, advanced timing should be renamed from 640×400 to 720×400 (70Hz), since it is indeed the default DOS resolution, and that’s what my Pentium outputs.

Here is what I found makes a pixel perfect image with my two Pentiums (different video cards):
H.Samplerate: 900
H.Synclength 96
H.backporch 57
H.Active 728
V.Synclength 2
V.backporch 34
V.Active 400

900 Samplerate is the important part, while 730 active resolution is to accommodate slight horizontal shift between 720×400 (80×25 text mode) and 320×200 (most games) which the video cards output as the same resolution and CRT monitors and OSSC treat as the same resolution.
Then just tweak the backporch till both modes do not clip of the edge of the screen.

I think the OSSC needs some more love towards the 400p computer mode.

EDIT:
Just to add, Line2x is not enough Samplerate to produce correct pixel geometry and 4:3 aspect ratio.
I think Line3x is important to have in 400p to increase samplerate for less distorted pixels, and all 1080p PC monitors support 1200p so that should not be a problem.