@reengine: The machine has no composite out, but the video chip (YAMAHA V6355) has it, but is disconnected. All test i did with that pin was a not-so-clear picture. Maybe the PAL tv set didn't help. But this effect is a different trick.
All test to "race the beam" failed also on line basis (changing 16 color per line is too slow also for 80186, and i need to use many OUTs... maybe change only 4 color could be achieved... but per line)
The machine has 16kb video memory (repeated four times from B000 to BFFF), and i activated 160x200x16 mode that is proprietary of then YAMAHA chip but in common with i.e. IBM PCJR. That video mode was in pre-production specification but was dropped in the final (maybe because after the PCJR market failure?). No trace also in the BIOS. Fiddling with registers finally open that mode!
@Scali: The Olivetti PC1 is connected to the RGB (analog) to the TV set via SCART, but I build a cable C/Amiga 1084s to display RGB (very nice). RGBI digital did not permit to redefine colors: the output is the same 160x200 but the 16 color are from standard CGA/EGA set. The video chip can freely redefine the 16 colors with a 3 bit for each R/G/B channel. 9 bit RGB is like MSX palette, limited but better than fixed EGA. Similar to MSX the chip is capable of 512x192 pixel and a whooping 640x204 resolution (but sadly, the two resolutions are monochrome!). The chip is capable also of 640x200x16 but in a different configuration (require extra circuits and video RAM).
Redefining the palette each frame should indeed be possible (out of retrace time), but I have to build a special video encoder. Also time is critical.
For reaching 25fps I had to load directly to video memory the byte flux already interlaced and exactly of 16384 byte (instead of 16000). Also because we have the video RAM repeated 4 times I loaded 4 frame each INT13 call. (loading 128 consecutive sectors from hard disk). Not much time left to make other tricks...
@Trixter:Yea I have the detail and the port you have to write. However the hack would work only on YAMAHA V6355D. M24 has a 640x400 mode (16 color with dedicated expansion card), some of the bios call vestigia are found in PC1's BIOS but is a complete different beast.
I have another machine Olivetti M200, also with NEC V40 but has a VLSI chip (for what I see) that use 640x400 (and a neat 8x16 character set!) and maybe similar to M24. If I found something I'll tell you.