Maximum overscan screen size

discuss bbc micro and electron emulators (including mame) here!
Post Reply
garfield
Posts: 443
Joined: Mon Jan 03, 2005 1:38 am
Contact:

Maximum overscan screen size

Post by garfield »

Hello stardotters,

A two-part question:
1. What is the maximum overscan screen size that current BBC Micro emulators support?
2. What are typical "real world" overscan screen sizes that make sense with 1980s authentic display hardware (i.e. typical Ferguson/Philips/Sony tellys back in the day and things like Microvitec monitors).

( For part one, I am going on the assumption that the maximum size should approximate a PAL displayable field 768x288 pixels. Let us keep it in the realm of pixels for now, and ignore the complexities of interlaced frames and the analogue nature of rasterisation. )


I am presuming that this means that a BBC Micro emulator should be able to display this theoretical maximum screen size of approximately 96 CRTC characters wide by 36 rows high. This works out at about 27KB and in terms of a "MODE 1"-like screen would be 384x288. Are there any example programs out there that test the waters of BeebEm, or B2 or any other emulator in this regard? Or do they run into trouble with assumptions on stuff like the sync positions etc? :?:

...

For part two, would 80s contemporary TVs/monitors "throw a fit" if such a large screen was attempted to be displayed? Or would it display fine (despite maybe some cropping due the TV/monitor bezel obscuring the edges of the CRT tube) ? By fine here, I mean that a stable image is shown and no sync or rolling trouble is encountered... :?:
Coeus
Posts: 2149
Joined: Mon Jul 25, 2016 12:05 pm
Contact:

Re: Maximum overscan screen size

Post by Coeus »

garfield wrote:
Mon May 03, 2021 8:10 pm
1. What is the maximum overscan screen size that current BBC Micro emulators support?
B-Em now dynamically adjusts the size of window it asks for according to what border option you have chosen but if you choose "full borders" which gives the biggest gap between the edge of the drawn on area of the standard screen modes and the edge of the window, the size is 832x308. The vertical direction is doubled, either by interfacing, line doubling, or scaling depending on what you choose.
garfield wrote:
Mon May 03, 2021 8:10 pm
For part two, would 80s contemporary TVs/monitors "throw a fit" if such a large screen was attempted to be displayed? Or would it display fine (despite maybe some cropping due the TV/monitor bezel obscuring the edges of the CRT tube) ? By fine here, I mean that a stable image is shown and no sync or rolling trouble is encountered... :?:
Remember that many sets had h-size and v-size controls. That means that, even BITD, many people didn't operate with the large borders that the definition of the standard modes would seem to suggest, i.e. people would adjust the size controls to get the useful part of the picture to occupy as much of the CRT as possible, except that going right into the corners would expose any imperfections in pincushion compensation.

That's why b-em offers the three border options because "full" which looks huge on a modern PC is not necessarily any more authentic than a slimmer border.

So I suspect, as you make the displayed part of the line bigger, it will extend to the edges of the screen and, once it has gone off the edge, you would be able to get it back on screen again by reducing h-size and likewise in the v direction. Eventually, though, I would expect it to lose sync. But there are some people with a much more detailed knowledge so hang around.
Last edited by Coeus on Mon May 03, 2021 8:47 pm, edited 1 time in total.
tom_seddon
Posts: 471
Joined: Tue Aug 30, 2005 12:42 am
Contact:

Re: Maximum overscan screen size

Post by tom_seddon »

b2 displays 736x288 - 92 x 2 MHz CRTC columns wide. This is wide enough to display Boffin (90 CRTC columns wide); that filled the entire width of the CRT I was testing with at the time. I figured anything significantly wider would be unlilkely.

(I don't remember the exact details, but my slightly unclear notes from the time about why 736 rather than 720: https://github.com/tom-seddon/b2/blob/a ... conf.h#L95 - suspect I just didn't quite get the TV emulation correct. This hasn't been a big issue, but now that I'm thinking about it, it is kind of annoying. So I may revisit this in future.)

I should do another round of testing with this, really, as since then I've somehow accumulated 4 CRTs...

--Tom
User avatar
Richard Russell
Posts: 2206
Joined: Sun Feb 27, 2011 10:35 am
Location: Downham Market, Norfolk
Contact:

Re: Maximum overscan screen size

Post by Richard Russell »

garfield wrote:
Mon May 03, 2021 8:10 pm
What are typical "real world" overscan screen sizes that make sense with 1980s authentic display hardware (i.e. typical Ferguson/Philips/Sony tellys back in the day and things like Microvitec monitors).
The maximum area that any standard CRT TV or monitor would be expected to display (because broadcast 625-line television never contains picture content outside that area) is 52 μs x 576 lines, or with a 16 MHz pixel clock (e.g. MODE 0) that's 832 x 576 non-square pixels.

How much more a CRT TV might be able to display by reducing the width and height controls will depend on how fast the horizontal and vertical flyback are, and that is likely to have been quite variable. In any case the linearity would almost certainly suffer at the extreme ends of the scan.
I am going on the assumption that the maximum size should approximate a PAL displayable field 768x288 pixels. Let us keep it in the realm of pixels for now
You can only refer to "pixels" if you make an assumption about the pixel-clock rate, and of course analogue TV has no such concept. Your figure of 768 seemingly corresponds to the square pixel clock rate of about 14.77 MHz, but since that's not a clock rate used by the BBC Micro its relevance is questionable.

The pixel clocks used by the BBC Micro are 4 MHz (MODEs 2 & 5), 8 MHz (MODEs 1, 4 & 6) 12 MHz (MODE 7) and 16 MHz (MODEs 0 & 3).
I am suffering from 'cognitive decline' and depression. If you have a comment about the style or tone of this message please report it to the moderators by clicking the exclamation mark icon, rather than complaining on the public forum.
User avatar
tricky
Posts: 5509
Joined: Tue Jun 21, 2011 9:25 am
Contact:

Re: Maximum overscan screen size

Post by tricky »

Richard is the most knowledgeable person about this that I know ;)
garfield wrote:
Mon May 03, 2021 8:10 pm
...
2. What are typical "real world" overscan screen sizes that make sense with 1980s authentic display hardware (i.e. typical Ferguson/Philips/Sony tellys back in the day and things like Microvitec monitors).
...
When I was doing my Pac Land demo, I checked four of my 14" portable CRTs and found that in MODE 1, at the widest part, 384 pixels were visible on one and 386 on the other three. Vertically all were between 33 and 34 character rows. I have seen CRTs that only show 31.5 vertically, but not checked their horizontal visible time.

On b-em, I had assumed that full borders was fixed to the sizes that Richard specified, normal was roughly what I found my CRTs to be, that is some kind of average visible and small was just what was visible (displayed by the 6845) at the time. Should I take it that this is wrong?
User avatar
1024MAK
Posts: 10710
Joined: Mon Apr 18, 2011 5:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Maximum overscan screen size

Post by 1024MAK »

As sold, CRT TVs are deliberately set up so that a television picture will completely fill the displayable area of the screen. So even with a normal TV picture, you don’t get to see the edges of the transmitted picture.

Details on how a set should be set up are here.

This is one of the reasons that most home computers have a border around the main part of the picture area (the others being that it helps to keep the video circuitry cost lower and allows some flexibility between 625 line and 525 line TV standards).

Also keep in mind that the user controls (if any) on CRT TVs and monitors only allow a limited range of adjustment to the horizontal and vertical size. Even the techinican presets inside the set will have a limited range.

Mark
User avatar
Richard Russell
Posts: 2206
Joined: Sun Feb 27, 2011 10:35 am
Location: Downham Market, Norfolk
Contact:

Re: Maximum overscan screen size

Post by Richard Russell »

1024MAK wrote:
Tue May 04, 2021 12:48 pm
Details on how a set should be set up are here.
That's somewhat misleading since it stems from the late 1960s when colour CRTs (and their masking faceplates) had very curved sides, and were often more like a 5:4 aspect ratio than 4:3. As CRT technology and scan stability improved, the visible screen became more like a true 4:3 rectangle (think Sony Trinitron), and I for one always adjusted our TVs so that the picture edges, as indicated by the arrowheads, could be fully seen.
I am suffering from 'cognitive decline' and depression. If you have a comment about the style or tone of this message please report it to the moderators by clicking the exclamation mark icon, rather than complaining on the public forum.
Post Reply

Return to “8-bit acorn emulators”