6

Computers that used TV sets for display, needed to worry about the title safe area. By consensus, on NTSC sets, it was 200 scan lines, or 192 if you wanted to play it really safe.

What was the equivalent on PAL sets? The BBC Micro could display 256 scan lines; did that ever run into overscan?

11

According to EBU R95, the title-safe area for 576i format (corresponding to PAL SDTV) is 258 lines tall in each field. This is just large enough to accommodate the 256 lines per field that the BBC Micro uses. This is probably not a coincidence, as the BBC Micro was in part designed so that the BBC itself could use the micro for generating titles and graphics for broadcast.

However, by default the display area of the BBC Micro tended to be misaligned by about one line of text (8 graphics lines per field). This could be corrected using the *TV command, which altered the width of the blanking intervals at the top and bottom of the screen. The BBC Master allowed *TV settings (among others) to be saved in NVRAM, but this was not present on the original machines.

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  • 1
    I also uncovered one measured anecdote — martin.hinner.info/vga/pal.html — in which the author was able to count 266 lines on his particular screen. Perhaps also interesting: 258 lines is 82.56% of the total, whereas in NTSC world 200 lines is only around 76.19% of the total. – Tommy Jan 3 at 23:54
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    Actually, those percentages are fatuous as I’ve not allowed for the retrace period being an analogue fact that doesn’t care about line resolution. That should eat a smaller percentage of a PAL field, as PAL fields take longer. – Tommy Jan 4 at 0:26
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    I fought for months to get a nice display output on my BBC Master before eventually figuring out how to tweak those settings and installing a battery for the NVRAM. Now it looks great on my Sony Trinitron PAL monitor. – Brian H Jan 4 at 14:03
  • Several of my TVs from the early 80s have/had v-pos and v-size adjustment on the outside, so I could see everything including the teletext data. – Richard Broadhurst Jan 10 at 15:47
4

I can answer half of that only: the BBC did often run into trouble, to the point that they cite the *TV command as early as Page 17 of the user guide:

If the picture on your television screen is either too far up or too far down the screen, you can move the whole display with the command *TV.

*TV 255 will move down one line
*TV 254 will move down two lines
*TV 1 will move up one line
*TV 2 will move up two lines

The movements come into effect next time you press BREAK or change mode.

*TV also controls the interlace of the television display. See chapter 43.

They felt the need to drop that in at the end of the second chapter, 'Commands', which first introduces directly the idea of typing a command in order to get the computer to do something.

Having felt the need to mention that use right up front, they don't bother defining *TV in detail until Page 419.

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