Connecting BBC B to VGA Flatscreen Monitor.

[brunel]

I have been asked as usual to help a friend connect his BBC B to VGA monitor.

He is using an adaptor:http://snipurl.com/26k9o6

This does function but there is far too much colour.

As I have never seen a BBC B before, I need some advice.

Is there anything relatively simple that can either be adjusted or fitted to adjust the level?

[julie_m]

Hmm, interesting device ..... not far short of an Aurora even!

The BBC's RGB output gives 5V into a TTL load (and only composite sync, but most inputs will accept composite sync on the H SYNC input and a steady level on the V SYNC). Proper VGA is 1V into a 75Ω load.

So I'd start by adding 270Ω or 330Ω resistors in series with each of the three colour signals.

[cmjones01]

What do you mean by 'far too much colour'? The colours produced by a BBC Micro are all fully saturated. They're as bright as a TV or monitor can show, and there are only eight of them: black, red, green, yellow, blue, magenta, cyan and white. A BBC Micro does not do subtle colours. However, they should at least be clear and sharp.

Does the description mean that the colours are smeared and blurred? If so, I'm not surprised. The output from a BBC Micro's composite video connection doesn't have the greatest colour quality, and it looks like the adapter shown only takes a composite input, not an RGB one.

For the best image quality, you need to find some way of taking the BBC's RGB output and converting that to VGA. I don't know of a suitable box off the top of my head, but I'm sure they exist.

I hope this helps
Chris

[brunel]

Thanks for the suggestions.

I have just been informed that this is a late model MK IV, that does have composite video O/P, this is what is being used.

[brunel]

Also... I have looked at the adaptor in more detail, especially the adjustment menu.

Saturation can be adjusted, along with bright, hue, contrast.

Will post back if this solves the issue.

[McMurdo]

Apparently the BBC BMk4 did suffer from smeared colour if the composite video out is used. The original composite video socket was monochrome only and a link fitted to the later boards 'mixes' the chroma into it in a crude manner.

So you are taking an already compromised signal at 15.625KHz/50Hz and upscaling it to VGA, I woudn't expect perfect results!!

I studied on the BBC and we either had the Philips 12in orange & green screen monitors on the composite mono phono socket out, or else the RGB TTL + CSync Sanyo and Microvitec 14in colour monitors on the DIN socket.

Given the option i would connect it to a domestic TV using the digital DIN RGB TTL via resistor attenuators into the analogue RGB pins of the socket.

A screenshot would be nice to see if you can get it working.

[cmjones01]

Thinking about it, it may not be too hard to bodge the Beeb to provide an S-Video output - luminance from the composite socket (colour link removed) and chrominance from the colour link inside the machine. That might provide a worthwhile improvement in quality, since much of the Beeb's display is sharp edges and narrow lines.

If there's a problem with the colour, though, that's a separate issue and I agree that a screen shot is the best way of diagnosing what's going on.

Chris

[Dave Moll]

Certainly with my original BBC, I found that the composite colour signal obtained by making the link was far inferior to the output of the RF modulator, let alone the RGB output.

As an aside, I found that I had to attenuate the RGB to feed it into a SCART socket, so the same may be true of the signal being converted to VGA.

Unless the aim is to meet the challenge of this technology, my advice would be to use (attenuated) RGB into SCART.

[arjoll]

I suspect the best option would be something like this. Alternatively I know in the vintage computer community there are some highly regarded early NEC Multisync monitors that can go down low enough for the likes of CGA - I'm guessing the BBC is similar in its RGB output?

I've got a couple of computers with TTL RGB outputs - Sord M23's and a Apple IIe with what appears to be a relatively rare RGB card. I'm also on the lookout for a BBC, so I suspect at some point I may be tempted with something like the above! Unless there's an easy way to convert RGB to component - but then the only TV here with component in is a 29" Sony CRT so it might be a smidge big!

[brunel]

With a bit of adjustment of the adaptor, it now appears to produce acceptable results.

A compromise will have to be accepted in order to run on VGA.

As an aside, this adaptor gives perfect results when playing a VCR or DVD player to the same monitor.

[cmjones01]

Originally Posted by arjoll

I suspect the best option would be something like this. Alternatively I know in the vintage computer community there are some highly regarded early NEC Multisync monitors that can go down low enough for the likes of CGA - I'm guessing the BBC is similar in its RGB output?

Yes, it is, though its timings are PAL 50Hz field and 15625Hz line rather than CGA's NTSC-derived 60Hz/15750Hz. An early Multisync monitor should be able to sync to the Beeb's output, though such monitors are a rare breed these days. The Beeb's output is also composite sync where CGA is separate H and V, but a crude sync separator does the job. Somewhere I've got a box I made which allows a BBC's RGB output to be connected to a CGA monitor, and it only contains a transistor and some resistors and capacitors.

Originally Posted by arjoll

I've got a couple of computers with TTL RGB outputs - Sord M23's and a Apple IIe with what appears to be a relatively rare RGB card. I'm also on the lookout for a BBC, so I suspect at some point I may be tempted with something like the above! Unless there's an easy way to convert RGB to component - but then the only TV here with component in is a 29" Sony CRT so it might be a smidge big!

RGB to component is generally a straightforward matrixing job, so you might be able to enjoy big-screen BBC (or Sord, or Apple)!

Incidentally, CPC here supply all sorts of useful converter boxes. Component to RGB:
http://cpc.farnell.com/_/c2rgb/compo...tor/dp/AV17829
and RGB to VGA/XGA:
http://cpc.farnell.com/cyp/cm345s-pr...ter/dp/AV10386

which would probably do better job than the OP's composite converter.

Chris

[woodchips]

Hello, I have suddenly a need to display mono composite video and the monitors seem to have disappeared. I was intrigued by the link Arjoll suggested to the early NEC multisync monitors, can you provide an example please? I used the NEC monitors 25 years ago, the multisync 2 was much better than the multisync 1, and the Sony 1420 was better than both. Bob

[cmjones01]

I don't think those NEC or Sony multisync monitors of the late 80s/early 90s would be much help for displaying composite video, not without some extra electronics, anyway. All the ones I've seen don't have sync separators and expect separate R/G/B/Sync signals. The sync may be composite H and V or separate.

If you have such a monitor and want to connect (monochrome) composite video to it, the simplest way is probably to rig up a sync separator using something like the trusty LM1881, then feed your composite video into that and one or more of the RGB inputs on the monitor. The sync separator's output can then feed the sync input on the monitor. Many of these monitors had separate BNC sockets for each signal to make life easier

Chris

[neon indicator]

Most Multisync monitors do 480p minimum, not the 576i needed for BBC RGB or Composite, as that's about 1/2 the line frequency.

You can use a suitable cable for Amiga, Atari, Acorn, BBC to SCART (RGB or Composite) and B&W on S-Video (sometimes colour with Composite to S-Video adaptor).

"multisync" usually referred to ability to do VGA (about 32kHz Line rate as it's progressive 60Hz) and SVGA (About 40kHz). Some could do EGA and CGA also (all progressive 60Hz) Some did 1024 x 768i 43Hz, which is Interlace, not progressive, but about same line rate (about 33kHz) as VGA

AFAIK all BBC machines did 625 line (i.e. 576i, 15.625KHz line rate) so won't work with most "multisync" at all, even if you interface the LM1881 to VGA H & V and tie RGB together to get white.

[julie_m]

I thought CGA was 60 fps interlaced, for compatibility with NTSC TV signals?

Any monitor that can sync to that, should be able to sync to a 625/50 interlaced signal.

Not to be confused with the later, similar-resolution, progressive-scan mode, MCGA.

[cmjones01]

CGA is, I think, strictly speaking 262 lines 60fps, making its timings compatible with NTSC.

To neon_indicator: the multisync monitors we're discussing here are the type which do sync down to 15kHz. They were very popular about 20 years ago as some machines (especially the Amiga and Archimedes) made the transition from TV-type outputs to higher resolutions. NEC and Eizo were the big names I remember. Some time in the mid 90s the term 'multisync' ceased to mean the ability to sync down to TV rates and, as you say, around 30kHz became the minimum line frequency for such monitors.

Chris

[neon indicator]

CGA does more lines in Mono, must be Interlaced. OK. Hence The actual IBM EGA screen I have doesn't work on CGA even with adaptor. EGA = Progressive and CGA = Interlaced.

I think you are right though, that CGA is basically NTSC, hence a strange palette for graphics due to some early composite version card.

Yes. I have ONCE seen a Multisync that would do 15kHz. The one I have in the Attic doesn't. Perhaps on later models they dropped CGA compatibility.

I used to own a company repairing and maintaining Apple II, Research Machines, BBC, Acorn Etc for Schools in UK. I think it's 25 to 30 years ago the Multisync could do 15kHz. I'm thinking that by 20 years ago they did 30KHz minimum.

[cmjones01]

Originally Posted by neon indicator

CGA does more lines in Mono, must be Interlaced. OK. Hence The actual IBM EGA screen I have doesn't work on CGA even with adaptor. EGA = Progressive and CGA = Interlaced.

No, both CGA and EGA are progressive scan. CGA uses 15.75kHz horizontal scan rate, EGA uses 15.75kHz and 24ish kHz.

I think the system with more lines in mono you're thinking of is Hercules/MDA - 348 active picture lines, and a horizontal scan rate of 18-19kHz. It was once fairly common to have both Hercules and CGA in the same machine running both at once - Hercules for mono text, CGA for colour graphics. A friend of mine had his CGA card connected to his TV via an RGB SCART connector.

I think we're quite a long way from the original topic now, though...

[neon indicator]

No, I wasn't thinking of Hercules... But Looking up documentation I see the vertical is still only 200 lines, but I was wrong, it's Progressive 15kHz, not Interlace.

Monochrome Hires CGA mode.

BIOS Mode 6 sets up the 640×200 graphics mode. This mode disables the composite color burst signal by default.

[arjoll]

The Multisync monitors I was referring to are the old ones that can do everything from CGA up to VGA. They are sought after by users on the likes of The Vintage Compute forums. A quick google brings up the Multisync 3D as a possibility, compatible with MDA/CGA/PGC/EGA TTL RGB as well as analogue VGA.

There's a table here which lists scan rates supported by a number of monitors. That might help. At the moment I don't have a BBC (one is on my wish list), so I'm more interested in something that will work with my Apple IIe RAMworks RGB card and my Sord M23!