Line2x and Ppsample2x should always be enabled.
These settings work well and ‘to-spec’ for 720×400:
Default for 400p should be passthrough to let the monitor handle the signal as if it was through VGA.
But most LCD monitors will not display this mode as 4:3.
Adjust H.backporch to center the image.
For 4:3 image try this setting:
I tried to aim at 1066×800 which is perfect 4:3.
I fiddled with the Advanced setting till I got a very nice 4:3 image in 1066×800 resolution on both my monitors.
H. Samplerate : 700 (this gives correct sampling [no “waves”]).
H. Synclength : 64
H. backporch : 52
H. Active : 570
V. Synclength : 2
V. Backporch : 34
V. Active : 400
It is important that Upsample2x is On to smooth the image.
The second mode is preferred for Line2x+Upsample2x in 720×400 and maintains 4:3 aspect ratio.
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.