Having issues fixing comp video on +2 grey.

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HexTank
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Having issues fixing comp video on +2 grey.

Post by HexTank »

I've trying to get a decent image with composite via the RGB connector but failing.

I'm still getting severe ghosting and some weird pixels coming through in the border areas.

I've done a few things since images were taken and the checkerboard I got rid of with removing C3. I've performed the Joules hack with a 15mh choke and a 82pf SMD (although my meter says this is more like 120pf - is this too much to be affective?) and nothing noticeable has changed.

Oddly enough, adjusting L2 made no difference at all, although removing C3 made the colours on the text worse, although crispier. Does anyone know a source for replacement for the TOKO 7KL as I don't have working grey to do side-by-side tests. And the choke mod, only screens online for comparisons show the +2 menu, not the test screen so it's difficult to so what effect it actually has.

The odd pixel stuff in the border is odd, it animates, like every frame you can see a pixel moving along the black lines, and given there's 8 of them, this must be memory accessing which I'm surpised at in the border. I've picked up a new scope so might be able to find what this is as I've never seen images online with the same artefacts. :/

Any ideas where to start?

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RMartins
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Re: Having issues fixing comp video on +2 grey.

Post by RMartins »

Check or re-seat your ULA, or swap for another one.
Check your video generator chip too.
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1024MAK
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Re: Having issues fixing comp video on +2 grey.

Post by 1024MAK »

Which part number / version / issue is this board?

Mark
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HexTank
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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

Iss3. The one that was built by Trotters Independent Traders. :o


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HexTank
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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

I've just been running some more tests, interestingly, the effect is determined upon power on, by actual power to the machine, performing a reset keeps it in whatever state it was in when powered on, so it's random whether it happens or not, but the pattern in consistent.


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HexTank
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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

Interesting developments, have received another TEA2000 to try tonight, but if this makes no difference, I can only assume it's between the TEA and outputs, to which I will remove D3 in case there's some issues that side.

If I short the luminence output and input delay lines, I lose signal on my capture stick and it can fix the issue when I remove the short. Given this and scoping the R digital pin, the ULA seems OK at least.
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1024MAK
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Re: Having issues fixing comp video on +2 grey.

Post by 1024MAK »

The TEA2000 only performs PAL colour encoding, so it’s not likely to be the cause.

The video picture is generated by the ULA. I don’t suppose you have a TV that has a RGB enabled SCART socket?

The ULA in this model generates RGB (one video signal for red, one for green and one for blue) video signals. The TEA2000 converts the RGB video signals as inputs and outputs a PAL composite signal. This is then fed to the DIN socket along with the RGB video signals. The composite video is also fed to the modulator, which then generates a UHF TV signal.

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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

I have a modern TV with scart, I don't have a scart cable as yet, just composite as I want to capture video.

Yes, I realise what the TEA does and the realtions ship with ULA (binary logic for RGB and the clock signals) this is why I've ruled that out.

This is the problem I'm currently trying tyo solve (the ghosting is on the back burner as whatever is causing this could be contributing, but until it's solved I don't know)

Image

If you notice in the bottom left corner, these out of bound white dots line up with with the black pixels in the border and the bounce to be in the normal place, almost animating over several pixels (even though pixels in the memory sense aren't involved here)

Image

And the following picture is when I get the lines and short out the luminence or cause the video out to be out of spec, it's the only way I can get them gone without relying on power cycling and hopeing they don't appear in the first place.

Image

If the ULA was at fault, shorting luminence would have zero effect on what the ULA is spitting out and would solve nothing, but it is, the fact it happens 100% on the same pixel space means something, I'm sure, hell, it could even be the capture card itself and the signal is so borderline and maybe some drift which is causing the receiver to do oddball stuff (which is why I will order a scart cable).

All this coupled with the TEA adjustable coil having zero effect means either the capture card is poop (100% perfect on my 48k+) or one of the passives is pooped (all caps have been replaced but that's not to say other passives aren't out of spec).

I'm in the position of trying the cheapest things first, in an ideal world I'd have a working machine to compare against which would make disgnosing easier. :)
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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

I've just realised, maybe the hsync (the white dots) is the root cause, because the decoding on the backend (TV) would be decoding stuff incorrectly, which perhaps means a clocking issue, although that can't be the Spectrum crystal / ULA as it would never recover with the short.
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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

Verified not the TEA :) although now I have a nice new chip in a socket, so not all lost :)
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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

OK, issue solved. I found a TV with composite input and sure enough, it works (as in no pixels in border), so my theory if not the TEA it would be after it was correct. :lol:

I guess TVs have a better filtering circuits in them and capture devices and are very particular, damnit Amstrad.

FML.

Was a great journey, though.

Thanks.
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Ast A. Moore
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Re: Having issues fixing comp video on +2 grey.

Post by Ast A. Moore »

HexTank wrote: Fri Mar 13, 2020 8:10 pm damnit Amstrad.
Heh. Amstrad has nothing to do with it. No Spectrum ever produced a hundred-percent–compliant video signal, be it RF or composite. It’s because TVs (old TVs in particular) are quite tolerant of the little discrepancies that they can produce decent picture coming from the Speccy. A great deal of modern TVs will have a hard time doing that.

The RGB output of the 128K machines is fine, though, and the picture is excellent.
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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

Ast A. Moore wrote: Fri Mar 13, 2020 8:42 pm
Heh. Amstrad has nothing to do with it. No Spectrum ever produced a hundred-percent–compliant video signal
Citation? As it looks like you're confusing crappy picture with compliance.
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Ast A. Moore
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Re: Having issues fixing comp video on +2 grey.

Post by Ast A. Moore »

HexTank wrote: Fri Mar 13, 2020 10:52 pm Citation?
You can take my word for it. The VSync and HSync timings are off (and the VSync is basically faked), the waveforms are slightly glitchy (depending on a particular ULA revision), and the picture is progressive, instead of being interlaced. This is fine for analog circuitry—old TVs can still maintain sync lock—but digital TVs will trip over the Spectrum’s fake VSync pulse.
HexTank wrote: Fri Mar 13, 2020 10:52 pm As it looks like you're confusing crappy picture with compliance.
Oh, there’s confusion about this, all right, but none of it is on my side. ;)
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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

Wow, don't stroke your ego too hard.

I'd like to think you've not bothered to read the thread and just jumped on board because my Amstrad comment touched a nerve.

I can only say stuff based on facts I see first hand.

I have four other spectrums, two pluses and two rubber keys, only one of these has been recapped, and none of them show symptoms that this Amstrad is showing. The clocks to the TEA chip, on my particular board seem reasonably in spec, and here's the kicker, which had you read the thread , might have have stopped you being obtuse.

I'll group my findings here, to make it easier, in the off chance you might have some musings for experimentation that might locate the issue, as I believe it's fixable.

This isn't 100%, it's determined upon power on.

If I short the luminance delay output and input pins, it fixes without the need to power cycle ( hence this having zero relevance to ULA clocks )

Pressing reset has no effect, but my current theory is some inital capacitor charge which gets the whole show rolling on power up.

The location and the artefact itself when happening is 100% consistent.


Oh, and I was testing on a newfangled digital TV, Logik I think the brand is, a proper dumpster TV, this handles it absolutely spot on.
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RMartins
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Re: Having issues fixing comp video on +2 grey.

Post by RMartins »

Just curious, but does it output the same problem with the capture card and a regular TV, when connected directly ?

I have had lots of problems with capture cards, that do not like the fact that the Spectrum only sends odd PAL frames.
i.e. every single frame is odd, so it does send 50 frames, but all odd. An even frame is never sent.
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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

You mean at the same time? If so I have no means of coupling the signal to test at the moment.

But I'm using the same rgb to composite cable for both tests, direct in to the capture stick and direct to the TV.

I have a few leads to test out, one being the RAMP signal to the TEA, so any naughtiness on the 12v line could be a factor. If it was always screwed I'd probably leave it and move on, but there's clearly something that can be fixed, and it's fun. :lol: but it being inconsistently consistent :?
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1024MAK
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Re: Having issues fixing comp video on +2 grey.

Post by 1024MAK »

The reset button only resets the Z80 processor (plus anything connected to the same reset line, like the memory bank register, the AT sound chip and any expansions if fitted), the ULA and the rest of the video circuitry will be unaffected. Only a power cycle will change the ULAs operation by forcing the circuitry to restart.

Point of order: most 1980s eight bit computers do not produce a fully complaint TV standard video signal.

Some capture cards and some LCD / ‘LED’ TVs definitely don’t like the video output from some 1980s computers. As some of these end up as edge cases (nearly working, or only just working), comparing different models of the same make/type sometimes results with some working and some not working. I have various Atari ST/STFM/STE machines and some VGA monitors. With the Atari’s configured to the high res. mode, one Atari will work with one monitor, but another Atari will not work with the exact same monitor.

Unlike CRT displays, LCD and ‘LED’ monitors sample and capture the incoming video signal. Then try to work out what has been received. Convert it to a format suitable for a LCD, then send it to the LCD.

Also for some reason, some LCD TVs are more tolerant of a sub-standard signal via the UHF TV aerial input compared to the composite video input.

By messing with the 2000 chip, it is possible that the resulting ‘blip’ in the received video signal is causing the video capture card to resync to the video signal, thus ‘clearing’ the fault.

Mark
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Ast A. Moore
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Re: Having issues fixing comp video on +2 grey.

Post by Ast A. Moore »

HexTank wrote: Sat Mar 14, 2020 8:59 am Wow, don't stroke your ego too hard.
I read the thread quite thoroughly, and if someone’s ego was hurt, it certainly wasn’t mine. I’ve a pretty thick skin. I simply pointed out that your supposition of Amstrad’s machines being an edge case was incorrect, and clarified that no Spectrums produced a standards-compliant video signal. You asked me for a “citation” (hello, hurt ego) and suggested I didn’t know what I was talking about (“confusing a crappy picture with compliance”). I then explained—in greater detail—what exactly was the culprit of the out-of-spec behavior, and now it seems to me you got your panties in a bunch.

Trust me, you’re not the first person who’s observed this kind of odd behavior of the Spectrum (or many other micros from the 80s).
Every man should plant a tree, build a house, and write a ZX Spectrum game.

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Re: Having issues fixing comp video on +2 grey.

Post by HexTank »

Thanks Mark, that's actually some useful information that can be actually taken away,

The last paragraph in particular, it might be a case that it's causing a blip forcing a resync making it work, but it's a blip that has a 100% constant. Shame there's not a collection of edge cases to view.
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1024MAK
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Re: Having issues fixing comp video on +2 grey.

Post by 1024MAK »

There has definitely been reports of problems when trying to use some capture cards in the past. But most users only had a single ZX Spectrum to test with.

The normal advice with display problems, is to try and test a ZX Spectrum with a CRT TV or with as many different LCD TVs as they can....

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