BBC B screen position

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JasonStonier
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BBC B screen position

Post by JasonStonier »

I feel slightly stupid asking this question - but I've been trying to sort this out for days and I can't do it.

My beeb is connected to a widescreen LCD TV with an RGB to SCART cable. All working nicely, except the top...probably...6 pixels is off the top of the screen. This has no operational effect I've yet found but it's annoying when half the top line is missing, especially with Tricky's Menu (which uses the whole of the screen).

TV settings make no difference - stretch, zoom, position, fit to screen, 16:9, 4:3 - that top half line is always missing.

Does anyone have any pointers? Thanks.
Last edited by JasonStonier on Wed Nov 25, 2020 9:54 am, edited 1 time in total.
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BigEd
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Re: BBC B screen size

Post by BigEd »

Try *tv 255 and then MODE 7?
The *TV command allows the vertical position of the screen to be altered and interlace to be switched on or off. The first parameter causes the vertical position to be altered; a value of 0 causes no change, a value of 1 would cause the screen to be moved up one line and a value of 255 would cause the screen to be moved down one line. The second parameter should be 0 or 1, a value of 0 causes interlace to be enabled and a value of 1 causes interlace to be switched off. Any change of interlace or screen position will only come into effect at the next mode change and will remain until a further *TV command or a hard reset. Interlace cannot be turned off in mode 7.
(From p20 of the Advanced User Guide (see also here maybe))
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tricky
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Re: BBC B screen size

Post by tricky »

I can't remember if my menu resets it though :oops:
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JasonStonier
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Re: BBC B screen size

Post by JasonStonier »

It doesn’t :D

So, this trick worked, thanks! Obviously it’s not permanent, but it’ll do nicely until I work out why it’s happening.

I guess for things I use a lot I can just add it to the !Boot.
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BigEd
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Re: BBC B screen size

Post by BigEd »

Phew!
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JasonStonier
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Re: BBC B screen size

Post by JasonStonier »

(I’d still love to understand if it’s the beeb hardware, or the TV, or what...)
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tricky
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Re: BBC B screen position

Post by tricky »

I don't think that the actual position was specified (but I'm happy to be enlightened), only that the visible range can extend do. That is all I could fins for horizontal anyway.
*TV 255 was fairly common bitd. My TV was tight on the top with 0 and tight on the bottom with 255, so I didn't bother, other than *TV xxx,1 to turn off interlace for the graphics modes.
The other micros didn't have this problem as they had much smaller display areas that didn't get anywhere near the sides.
I would try searching for an engineering menu option for your TV.
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Re: BBC B screen size

Post by Coeus »

This sounds related to over-scan, or to put it another way, although the TV standard at the time the BBC micro was produced was described as 625 line, the way TV sets were typically set up some of these lines were not displayed on the screen.

Possibly the difference is more than many people remember because, when writing a emulator, if one displays every scan-line generated by the (emulated) hardware and not just the ones that have something on them, the border seems absurdly big to the point that B-Em has an option to trim the borders down in size.

The BBC video modes all have a vertical resolution of 256 pixels. Add interlace where the same set of 256 lines are mapped to alternately the odd and even TV scan lines and this uses 512 out of the 625 lines and it seems this is very close to the number of lines that would actually be on-screen on a typical TV.

Of course BITD the vertical size control was analogue and simply controlled the Y direction gain. It is is probably not so simple with a modern LCD TV.
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JasonStonier
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Re: BBC B screen position

Post by JasonStonier »

tricky wrote:
Wed Nov 25, 2020 4:54 pm
I would try searching for an engineering menu option for your TV.
Yeah, been there, done that. I even cracked and got the manual out (that's how bad it got). It seems there are advanced options for every input apart from SCART.

To be fair, this is all your fault. I'd never have noticed if it wasn't for your menu :wink:
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*TAPE
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Re: BBC B screen position

Post by *TAPE »

Its likely its the telly causing the problem.

Digital TVs are made with firmware for digital transmissions. Displaying PAL is an after thought, so the implementation is likely something the manufacturer found on Stackoverflow!

That is, IF the process is something like....
1) trim PAL overscan
2) send output to display firmware
....the problem arises when the display firmware also trims a few pixels from all edges of the digital signal. (Even thought there is no need to do that with signals originated in the digital domain)

Why would the manufacturer even bother to check whether analogue pictures are being over cropped?! Who even uses analogue any more?


I had precisely this problem with a high spec PC I built and then connected to a domestic telly. Panasonic, FTR.
....it cropped the Windows task bar, and the Start button was no where to be seen.

I found the fix buried in the TV menus....
Menu -> Setup -> Other settings -> 16:9 overscan -> Off
Problem -> Fixed
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