I see that Shadow over Mystara was converted from 304x224@59.637406 to 304x40@59.940, any reason to this ? Because of the TV or GreenAntz ?
That's probably because he's using the "ntsc" monitor preset in GM. I bet the board can handle the "arcade_15" preset just fine.
Calamity is 100% correct - though in this case I'm actually using two "crt_range" presets, with both NTSC and PAL available (in this case it chose a mode/timing from NTSC crt_range). It is just a default/conservative setup of mine. I have tested other monitor presets with GreenAntz before and it works just as well with the generic_15 and arcade_15, and can handle difficult timings like ~55hz.
GreenAntz itself doesn't care one way or the other as it does nothing to your video mode timings. Transcoding the colour space doesn't touch these, nor does the composite sync generation (which just takes the timings given to it). It introduces no new limitations and can handle any "normal" frequencies/video modes you throw at it. You have the same flexibility you'd expect if using RGB.
Hmmm, maybe i should install some other monitor presets and take some pics to demonstate this.
A caveat to that is GreenAntz will probably not work with all modern HDTVs. This is not GreenAntz fault - Some HDTVs do not display lowres modes like 240p properly (or at all) and it is a common problem for console retrogaming in particular. Only worth mentioning here because some of these HDTVs have component inputs intended for a Wii or DVD/Bluray players. Console players get around this with scalers that multiply the video mode lines to effectively make it 480p/720p/1080p. You could use a scaler with GreenAntz to get around this issue, just like you would with a console.