ChinnyVision – Ep 224 – Southern Belle – BBC Micro, Sinclair Spectrum

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  1. I had a BBC back in the day, and I have never had much of a chance to discuss it, so bear with me whilst I hold forth on the subject of screen modes for a moment… 😉

    Firstly, CPC and BBC micros are extremely similar when it comes to screen modes, as they both use a 6845CRTC (Cathode Ray Tube Controller), the most obvious differences being the screen modes have different numbers (BBC has Mode 0 for high res 2 colour, Mode 1 for medium res 4 colour and Mode 2 for low res "16" colour, as well as some others the CPC doesn't have by default), and the fact the CPC (iirc) only has 200 lines per screen by default compared to the BBC's 256 (saving the CPC 4k of RAM). Obviously, the CPC has a far superior pallete, but that's not handled by the 6845, so we'll ignore that for now.

    The long and short of that is that you should be able to recognise some changes of modes as you would on the CPC, however the BBC has other modes (such as medium res 2 colour) which would have the same size pixels and be tougher to spot.

    I would concur that some kind of mode/pallete swaps are happening here . I'd guess the top of the screen is mode 1 (med res, 4 col), and the rest of the screen is mode 4 (med res, 2 col), with a pallete swap before the bottom line to change black to red. This would probably bring the memory requirements down from 20K for a full mode 1 screen to around 13K, assuming mode 1 occupies 20% of the screen.

    I have to wonder though…as the BBC had no horizontal scan interrupt, and its timers were quite low down the interrupt list, its difficult to make the mode/pallete change occur on the same scanline every frame, nevermind in the horizontal blanking interval. This generally leads to obvious flicker on certain lines as the mode/pallete change happens at slightly different times on everyframe.

    This is usually masked by having several blank lines around where the change occurs, displaying a colour which is the same in both modes/pallete, however I don't see these lines on this screen (there appears to be a 1 pixel white line between the blue bar under the station name and the black line around the train window – in my experience this is not enough to cater for the BBC's ragged timing…).

    So we have a somewhat interesting choice here, either;
    A) The whole screen is, in fact, in mode 1 (4 col), but for some (stylistic?) reasons the colours in the bottom of the display have been limited. This seems somewhat pointless, and as we don't have a nice red firebox like the speccy version, seems unlikely.
    B) The authors of this program spent far too much time getting that timing rock solid (and I for one would love to know how they did it, too). However, as it saves about 6k over a mode 1 20k screen, most authors would of just chosen mode 4 (same res, 2 col) and called it a day, surely?
    C) The authors are dark and evil magicians who stumbled across the secret and kept it to themselves (as I don't recall any other games with 6845 changes this solid). I suspect they did this solely to annoy me 35 years later 😉

    Excellent video as always Mr. Chinny, do keep up the good work!

  2. Damn what are the chances! I'm playing this right now on the Amstrad CPC. I remember this being given away free with one of the early regular AA covertapes. I think I might just be getting the hang of stopping at the stations. Now just got to do something about timing and not emergency breaking at the signals!

  3. It's funny that a steam train sim could be so oddly engrossing. I never would have through they'd have tried to go with vector 3D, but there you go.

    It's certainly sated any curiosity in checking it out for myself that's for sure. An excellent amount of work to get this up!

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