Unstable display on issue 7 beeb

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mlouka
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Unstable display on issue 7 beeb

Postby mlouka » Thu Nov 02, 2017 10:18 pm

Hi,

Hello all. Greetings from an ex-pat half-Norwegian brit living in Norway. I'm in the process of resurrecting an old beeb and a master 128 and ping-ponging between various hardware problems with both. I've got furthest with the beeb, having reconditioned the PSU and traced problems with the tube interface and starting up to a dodgy keyboard ribbon....So now it works, loads my old programs from disks, as well as a TuboMMC, and I can even run the tube-enhanced version of Elite on it using a B2P-6502 second processor. I've got one last problem with the beeb that I am struggling to solve so am hoping that someone here might be able to assist me in locating the source of the problem.

The problem is that the display is a little erratic. Not in the sense of flickering, shuddering, or anything like that but in clarity. It intermittently -- every few seconds, switches between a clear image to a slightly fuzzy one, and it's progressively worse at the top of the screen to the bottom. It looks little like very other line is shifted by about a pixel or two. Sometimes it can get "stuck" in a correct or good or bad state for more than a few seconds. Unsurprisingly, then, this most of a problem in mode 0 where the first few lines of text look very fuzzy -- it's not so bad in Mode 2 or 5. I've attached a photo of the TV screen.

I suspect that this is something to do with scanlines in the analogue signal. What has surprised me a litte is that I am seeing the same behaviour from both the antenna and the RGB outputs. I initially thought that it might be an issue with the TV but the signal from the same outputs from the Master 128 is perfect so the problem does not appear to be the TV or the RGB to SCART cable.

Does anyone have a clue where to start looking here? I guess this might be down to a dodgy resistor or solder somewhere in the output circuitry? I've got a multimeter and a soldering iron but not an oscilloscope, although we may have one at work that I can borrow if necessary.

Thanks in advance for any advice -- it would be great to get this beeb working 100% The biggest problem I have left with the master 128 is that three of the keys on the keyboard don't work but that should be relatively easy to fix...

Michael.

IMG_0050.JPG
BBC Master 128, BBC Model B i7, Watford Electronics Solderless Sideways ROM board, PMS B2P-6502 2nd proc., PiTubeDirect (internal in Master, external for Beeb), RetroClinic Multi-OS Selector, Sundby 256k RAM/ROM card, MMFS, Cumana disc drives ....

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daveejhitchins
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Re: Unstable display on issue 7 beeb

Postby daveejhitchins » Fri Nov 03, 2017 7:54 am

Welcome to the Forum, Michael . . . Enjoy . . .

I'm sure you'll get lots of advice here. You'll have to keep everybody up-to-date with progress on your display issue and anything you find with your Master. Good luck with progress . . .

Dave H :D
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Products: ARA II, ABR, ATI, AP6, MGC, AP5 . . .
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vanekp
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Re: Unstable display on issue 7 beeb

Postby vanekp » Fri Nov 03, 2017 11:02 am

The display looks very odd, although you say it works fine with your master it still looks like a problem with the tv itself you don't have a diffrent display that you can try it on to see if it does the same or not.
Peter.

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mlouka
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Re: Unstable display on issue 7 beeb

Postby mlouka » Fri Nov 03, 2017 12:13 pm

vanekp wrote:The display looks very odd, although you say it works fine with your master it still looks like a problem with the tv itself you don't have a diffrent display that you can try it on to see if it does the same or not.
Peter.


Thanks for responding. I can try on a couple of other TVs. I have a 12" CRT-based TV that I ultimately intend to use as monitor but haven't been using it for testing. I can dig that out too. I have tried turning off all picture-processing on the plasma-TV that I have been using for testing but that has no effect.

Michael.
BBC Master 128, BBC Model B i7, Watford Electronics Solderless Sideways ROM board, PMS B2P-6502 2nd proc., PiTubeDirect (internal in Master, external for Beeb), RetroClinic Multi-OS Selector, Sundby 256k RAM/ROM card, MMFS, Cumana disc drives ....

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1024MAK
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Re: Unstable display on issue 7 beeb

Postby 1024MAK » Fri Nov 03, 2017 1:10 pm

There are two major chips that form (part of) the video system in a BBC Micro.

The Video ULA/Video Proc (IC6) provides division of the master clock (square-wave) to provide various clock signals for the system, including the clock to the 6845 (IC2).

The 6845 (IC2) provides all the video timing and the address generator for selecting the picture information from the RAM. But the only element of the display that it directly produces, is the cursor.

For MODEs 0 to 6, the Video ULA/Video Proc (IC6) takes the data provided by the RAM (with the address from the 6845) and converts it from an eight bit data byte to a digital logic level three line colour signal (RGB).

However, in MODE 7 (the Teletext mode), the Video ULA/Video Proc (IC6) receives a digital RGB signal from IC5 (SA5050). It then outputs digital RGB on it's outputs, as described earlier.

The digital RGB outputs from IC6 then go to three different analogue video circuits.
One is a set of three buffer/driver transistors (Q4, Q5 and Q6), which drive the RGB outputs on the DIN socket.
The second, is a monochrome composite video circuit.
The third is a PAL colour encoder which then is fed to the modulator to provide the RF UHF TV output signal.
The RGB socket has a combined sync (synchronisation) signal, which comes from /CSYNC, which is itself combined from HS and VS from the 6845.
Combined synchronisation signals are also added to the monochrome composite video and to the signal going to the modulator.

The best video outputs to use are the RGB socket (for colour), or the monochrome composite video output (BNC). If the option to add colour to the composite video output (BNC) has been done, then of course, you will get a colour picture. Note that the clarity of the composite video output picture when the colour signal is included is not as good as the RGB picture. So the best output to test with is the RGB output.

As well as trying different TVs/monitors, can you post up photos of a screen full of text in each MODE (0 to 7).

Do you have any spares?

Mark
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tricky
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Re: Unstable display on issue 7 beeb

Postby tricky » Fri Nov 03, 2017 1:12 pm

I've also never seen anything like that, the closest was a TV trying to do frame rate conversion with movement smoothing badly!

I would recommend RGB to scart on a CRT.
If you make the lead yourself, I would add an ear phone socket to divert the audio to the TV via scart.

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Re: Unstable display on issue 7 beeb

Postby Boydie » Fri Nov 03, 2017 1:28 pm

What happens if you turn the interlace off on the bbc (*tv 0,1 iirc)? The Master's configured to have it turned off by default.
Also, what aspect ratio is the screen set to? Is this an artefact of the tv trying to resize a 4:3 picture to 16:9?

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DutchAcorn
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Re: Unstable display on issue 7 beeb

Postby DutchAcorn » Fri Nov 03, 2017 1:34 pm

Do you have another CRT next to the one you are using that could cause interference? Or other magnetic devices (e.g. speakers)?
Paul

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mlouka
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Re: Unstable display on issue 7 beeb

Postby mlouka » Fri Nov 03, 2017 5:09 pm

Thanks everyone for the input. It does indeed look like the artefacts were being produced by the plasma TV. Using RGB to SCART from the beeb to a 14" CRT TV (4:3 only) then the artefacts seen on the plasma TV are not present. although the Image is rather blurry. Fine for games, not so good for anything requiring mode 0.

Comparing the Master to the Model B, the quality of the image produced by the Master is noticeably better than the beeb on the CRT too. It was excellent on the plasma. On the CRT, it appears slightly sharper, possibly because there is noticeable less flimmer. More correct timing, presumably. The beeb image has a very subtle "vibration", which I presume is what is challenging the signal processing of the plasma tv. Turning off interlacing as suggested by Boydie doesn't seem to make any noticeable difference with the beeb on the CRT.

With the beeb, there is no noticeable difference in image stability between mode 1 and mode 7 or between the UHF and RGB outputs. Image is a bit sharper (no ghosting) with the RGB output, as would be expected.

If this difference between the beeb and the master is to be expected then I guess everything is probably OK, otherwise, looking at 1024MAK's detailed comments, it might be worth swapping out IC6 (and/or IC2) to see if it makes a difference to image stability as the signal/timing appears to be significantly cleaner from the Master. I don't have spares on hand but I have a remote supply as my dad has a stash of four beebs back in the UK, so I can ask him to post me ICs 2 and 6 from one of them for testing if the consensus here is that there shouldn't be any noticeable difference between the Master and the Model B...

Thanks,
Michael
BBC Master 128, BBC Model B i7, Watford Electronics Solderless Sideways ROM board, PMS B2P-6502 2nd proc., PiTubeDirect (internal in Master, external for Beeb), RetroClinic Multi-OS Selector, Sundby 256k RAM/ROM card, MMFS, Cumana disc drives ....

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tricky
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Re: Unstable display on issue 7 beeb

Postby tricky » Fri Nov 03, 2017 11:12 pm

*TV only takes effect after a mode change and I don't think the OS will turn it off for mode 7, so you may not have seen interlace off ;)
I'm fairly sure you should see slight vertical vibration with it on and none with it off.

crj
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Re: Unstable display on issue 7 beeb

Postby crj » Sat Nov 04, 2017 1:25 am

tricky wrote:I don't think the OS will turn it off for mode 7

Indeed it won't.

Back when I first got my shiny new copy of the Advanced User Guide, I was messing around and tried disabling interlace for MODE 7 by direct poking. It soon became apparent why they didn't allow it. (-8

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Re: Unstable display on issue 7 beeb

Postby Boydie » Sat Nov 04, 2017 9:59 am

Turning the interlace off for a CRT display was always fairly subtle, particularly if it's a TV with a scart input. On a proper dedicated monitor, it greatly reduced eyestrain-inducing flicker, but the results for a tv with rgb input (even trinitron) were less impressive.

For a plasma, it may make a lot more difference. Even with optional image processing turned off, you're still asking it to take an analogue horizontal resolution, and 576interlaced vertical, with analogue rgb intensity, and upscale it to fixed digital native panel resolutions of either 1280 or 1920 x 720 (768) or 1080 progressive, with y-bits of colour depth.
That's still a lot of processing, especially if it's also resizing 4:3 to 16:9. And the source was only really designed to work on late-70s vintage small crts.
Perhaps the real surprise is that the video system on the master is do good the tv can do a perfect job. It'd be interesting to see what the result on the plasma is for the master with interlace turned back on.

I find I get similar artefact issues if hooking a VCR or DVDR to a plasma via scart, but you can only tell when the source is displaying a menu because the image problems are usually lost in the overall programme being shown.
A low-resolution system, with little happening on the screen (like a bbc) is likely even more noticeable.

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mlouka
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Re: Unstable display on issue 7 beeb

Postby mlouka » Sat Nov 04, 2017 1:00 pm

Thanks for the input -- I'll do a litte more testing with the plasma TV again this evening with both the beeb and the master to see what effect turning off or on interlacing does for both and report back.
BBC Master 128, BBC Model B i7, Watford Electronics Solderless Sideways ROM board, PMS B2P-6502 2nd proc., PiTubeDirect (internal in Master, external for Beeb), RetroClinic Multi-OS Selector, Sundby 256k RAM/ROM card, MMFS, Cumana disc drives ....

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mlouka
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Re: Unstable display on issue 7 beeb

Postby mlouka » Sat Nov 04, 2017 7:57 pm

Thanks, Boydie -- you were right. The image on the plasma looks great with interlacing turned off. After a bit more fiddling with brightness and contrast on the CRT TV and switching mode after turning interlacing off on the beeb, I've been able to get a good picture on that too, so can declare the beeb healthy. Phew.

I'v attached a photo of the plasma TV similar to the one originally posted to show the improvement after turning off interlacing on the beeb. I don't think there is much difference in quality between the picture from the master and the been now.
Attachments
beeb-interlace_off.jpg
BBC Master 128, BBC Model B i7, Watford Electronics Solderless Sideways ROM board, PMS B2P-6502 2nd proc., PiTubeDirect (internal in Master, external for Beeb), RetroClinic Multi-OS Selector, Sundby 256k RAM/ROM card, MMFS, Cumana disc drives ....

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tricky
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Re: Unstable display on issue 7 beeb

Postby tricky » Sat Nov 04, 2017 8:27 pm

I'd be curious to hear what you think of the competitive quality of the CRT Vs plasma on a 50hz game like my AstroBlaster.
http://bbcmicro.co.uk/game.php?id=1964


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