From https://en.wikipedia.org/wiki/BBC_Micro "the height of the graphics display was reduced to 200 scan lines to suit NTSC TVs". But NTSC is supposed to have 241 visible scan lines per half frame. Why wouldn't you want to make the graphics display vertical resolution 240 instead of 200?

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    Interesting. Timing reasons, I expect (240 lines uses almost all of the available frame time), but as the BBC Micro isn't one of those 8-bit machines that have the CPU and graphics contending for the memory bus, it's not the obvious timing reason. – hobbs Jan 24 '17 at 6:27

While nominally 241 scan lines were visible in the sense they contained video information, all TV sets hid a varying amount of scan lines on top and bottom (and left and right) by overscan and by the bezel in front of the screen.

So with a vertical resolution of 240, on most TV sets parts at the top and bottom would not be seen. While this doesn't matter much for movies, it's not a good thing if you want to do text editing.

This is also the reason while basically all homecomputers and game consoles had some sort of border (which often could be colored) around the center part of the image that carried information: It was to make sure this central part would be visible on all TV sets.

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    Right. Most TV-based home computers were restricted to about 200 pixels vertical resolutions due to this reason. – tofro Jan 24 '17 at 8:00
  • Okay, so if you look at the Commodore 64, 200 vertical resolution and the border wastes a large part of the screen on every screenshot you see, you're saying it varied with the TV, and the designers decided to heavily overshoot in the direction of making sure no part of the image would be lost on any TV, even if that meant wasting much of the screen on most TVs? – rwallace Jan 24 '17 at 8:03
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    @rwallace: Yes, the designers overshoot in the direction of making sure no part of the image would be lost (also taking all other considerations that determine the video image, like aspect etc., into account). You can see the same phenomenon in PC graphics cards that had analog TV output, though sometimes the amount of overscan was adjustable. – dirkt Jan 24 '17 at 10:12
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    Similarly Teletext was also designed for a 24x40 character grid. – Nick Westgate Jan 24 '17 at 10:38
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    @rwallace, to give you an idea of the importance of the border, look at a screenshot of the C64 startup screen. Then consider that the television I had mine hooked up to almost entirely hid the top border. – Mark Jan 24 '17 at 21:49

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