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Finally implemented character rounding:
Old

New

This also includes a few fixes to the roms, note corrected 'D'.

I'm using a teletext engineering test image for testing and are still some obscure control characters not being handled correctly. Hope to get it submitted for this months release though.

Nice! I hope it gets submitted for 0.167.

Completed my saa5050 updates, here's some screenshots to compare old/new with the real thing.
old

new

real

All control codes are now handled correctly.
It is still not perfect though, if you look closely you'll notice the bottom scanline is missing from each character row, most noticeable on y and g. This is not a regression as the old screenshot shows this too. I'll get back to it when I understand more of video handling.

So, I broke malzak and p2000t with this update. I did test them before submitting but obviously not with what actually got submitted.

This has been quite puzzling but I think I now have a conclusion

The saa5050 outputs characters of 20 scanlines but the mc6845 has R9 set to 18 and so outputs 19 scanlines (I had fudged saa5050 to accommodate this and so broke malzak etc). This explains why I'm missing the bottom scanline in the above screenshots.

I've now been reliably informed the bbc actually uses the hd6845 http://www.cpcwiki.eu/imgs/c/c0/Hd6845.hitachi.pdf and on page 90:
So in interlace mode it would output 20 scanlines when R9 is 18.

I see our implementation of hd6845 doesn't implement this so will try to fix this.

I also noticed we have a MCFG_MC6845_INTERLACE_ADJUST only used in apricot. Is this a workaround for my issue and the apricot should actually use a fixed hd6845?

Probably related yes. Without the hack the apricot displays 350 instead of 400 lines.

I know this is old, was digging into SAA5050 behaviour in the P2000 computer.
When I run the test image into the real computer with real SAA5050, all colour control codes are displayed as white box, as they are high value (129-135), same for the boxes between the divide symbol, those are black on white background. To get the image as seen here, you have to use the low ones (1-7), and replace the box value (255) with the non inverted box (127). Also in your rendition "Conceil Flash * Box Steady Gone ?" is visible, on a real SAA5050 only "* Box" is visible and blinks, and "Steady" is continuously visible.
Another minor difference is the 2 lines of 4 "separated" magenta pixels left of Test Page are in reality 2 lines of 6 pixels in magenta.
Back to the rounding. On a CRT the rounding is hardly visible. The image is closer to the original non-rounded one. I have the feeling the odd and even scanlines are roughly at the same spot, and thus more blurring together than above eachother. So the interpolation gives some fuzz to the end of the scanline. A character is clearly visible as 9 scanlines in reality.

Even though those screenshots are a good few years old now I don't think much (if anything) has changed since then. The current behaviour on a BBC Micro is correct, for normal use case.

I've never seen an actual P2000T video output for comparison, online videos seem to be all emulated. The current SAA5050 behaviour assumes an interlaced display and applies character rounding, maybe the P2000T is not interlaced? Can you supply some photographic evidence to work with?

Adapting the SAA5050 to work with both interlaced and non-interlaced (non rounding) displays is on my TODO list, but is kinda dependant on MC6845 improvements for what I want to achieve.

Did you see this pull request? https://github.com/mamedev/mame/pull/7577