ZX Spectrum 48k r2 composite video problems

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paf
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ZX Spectrum 48k r2 composite video problems

Post by paf » Tue Nov 13, 2018 9:18 pm

Hello

I have juste bought again my first computer on ebay... a Speccy 48k. Just had to replace TR4 and it worked.

I have made the composite video modification, simple (wire), with capacitor (100uF), with transistor (BC547), but my TV (Samsung UE40H6400) has difficulties to lock on the video signal:
- about once per second, it loses sync for about half a second
- after some minutes, everything becomes OK.
It does not depend on the type of the modification.

I have no other device with a composite input, so I can't tell if the TV is especially demanding. I guess that the fact that it works after a few minutes is due to the temperature of the components - but I don't know if in the TV or in the Seccy (I speculate it must be in Speccy, because some parts like the ULA get rather hot). I will have to try with my cold spray.

My oscilloscope shows me the following informations when displaying the start screen (c) Sinclair..., i.e. black characters on white (not bright) paper:
  • the video signal looks globally good. Not much high frequency parasites on it
  • the negative horizontal sync pulse (sync to black) is very deep compared with the white level (black to white): with the transistor mod, 0.5V (sync to black) vs. 0.4V (black to white) Image
  • this is proportional to the inverted Y outpout of the ULA (sync 3.3V, black 2.2V, white 1.3V)
    Image
  • adding a 47 ohm resistor in parallel to the TV input (which has 75 ohm) reduces the amplitde: sync to black 0.3V (the "official" value for composite video), but doesn't solve the sync problem
  • the timing of the line sync pulse seems OK (including the color burst, the this one should be irrelevant for sync issues - by the way, the colors are OK), but the field (frame) blanking pulse lasts for about 250us, i.e. about the same as 4 lines and is only one negative pulse (the signal stays low the whole time). I thought the "official" duration would be 19 lines? Image
  • there are color bursts (the same as at the beginning of each line) on the signal during field blanking (i.e. around sync level).
At first, I thought the voltage between sync level and black could be the cause of the problem, but trying several values between 0.2 and 0.5V did not made any difference.

Another reason I see is the field sync pulse, which is much shorter than I thought it should be and include no pulses. But I don't think that a defective ULA would generate a good signal, only with this part of the signal wrong!

The last reason would be the color bursts during the field blanking sync pulse, could they be the reason? Are they normal?

UPDATE:
  • cooling the components on the Spectrum PCB has no effect
  • the settings of the two variable resistors (VR1 and VR2) has an inflluence on sync. When the color is bad (i.e. big chroma signal on the composite signal with black/white display), the TV syncs BETTER!
Any clues? Anybody knows if the above is "normal" for the Speccy?

Thanks and regards
Patrick
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1024MAK
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Re: ZX Spectrum 48k r2 composite video problems

Post by 1024MAK » Wed Nov 14, 2018 1:19 am

Keep in mind that the ZX Spectrum (along with various other home computers of the time) does not generate an interlaced video signal. It just repeats the same field over and over again. And as you have noticed, some of the timing is not quite to the TV broadcast specifications.

All the above does not matter to most CRT displays. But LCD TVs sample the signal. Then digitally process the signal. They are designed to look for signals complying to the broadcast standards... Hence some LCD TVs cannot be persuaded to show the video from some 1980s computers. Note that a so called ‘LED’ TV is just a LCD TV that uses white LEDs for the LCD backlight instead of cold cathode fluorescent tubes.

Mark
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paf
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Re: ZX Spectrum 48k r2 composite video problems

Post by paf » Wed Nov 14, 2018 9:25 pm

Thank you 1024 MAK. Yes, I'm aware of these facts but actually, digital processing could allow a modern TV to be much more tolerant to a bad composite signal with almost no costs at all. But it's probably not in the objectives of Samsung's R&D department!

--> Could anybody tell me if what I'm observing (signal shapes and timing) is typical for a ZX Spectrum 48K issue 2, or if something could be defective?

--> Is there a chance that the PNP transistor mod, which allows DC decoupling with a capacitor as well as a stronger signal on 75 ohm, could solve my sync problem?

Thanks, Regards
Patrick
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My computers: Sharp PC1211, PC1500, Sinclair ZX Spectrum 48K, QL, Commodore Amiga 500, PC, PC, PC, PC :-(

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1024MAK
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Re: ZX Spectrum 48k r2 composite video problems

Post by 1024MAK » Thu Nov 15, 2018 1:45 am

I can’t tell you if your signals are typical, ‘cus it’s been a while since I last ‘scoped the video output of a Speccy...
But if you can wait until the weekend, I can grab some ‘scope traces of an issue 1 board. Or maybe of an issue 2 board if I can persuade it to work (I have a poorly issue 2 awaiting attention).

Mark
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Re: ZX Spectrum 48k r2 composite video problems

Post by paf » Thu Nov 15, 2018 6:47 am

It would be great, thank you! After 36 years I should be able to wait until the weekend ;)

What I'm interested in, when displaying the start screen ("(c) Sinclair...") with pale white and some black characters:
- output of the ULA, DC voltage of sync level, black level and white level (if you can practically measure it, on pin 17)
On the composite signal:
- voltage between sync level, black level and white level
- shape and length of the field sync (flat or with impulses)
- are there color bursts during the field sync and if yes, how big?

I'm slowly beginning to think that color subcarrier (burst or modulated signal or both) could be the reason my TV has synchronization problems, as those depend on the color settings (VR1 and VR2). Maybe I should try to "remove" the color subcarrier from the composite signal to check if it is the cause.

By the way, do you who is responsible for the color burst, does the ULA order the LM1889 to generate it or does the LM1889 generate it "spontaneously" based on the line sync level on Y?
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My computers: Sharp PC1211, PC1500, Sinclair ZX Spectrum 48K, QL, Commodore Amiga 500, PC, PC, PC, PC :-(

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Re: ZX Spectrum 48k r2 composite video problems

Post by paf » Thu Nov 15, 2018 8:52 am

... just noticed that the ULA's Y signal (luminance) does not go into the LM1889 at all... only the chrominance signals U and V. How does the LM1889 when to generate the chrominance subcarrier and when not?
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My computers: Sharp PC1211, PC1500, Sinclair ZX Spectrum 48K, QL, Commodore Amiga 500, PC, PC, PC, PC :-(

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Re: ZX Spectrum 48k r2 composite video problems

Post by paf » Thu Nov 15, 2018 8:06 pm

... and that the burst is commanded by the U or V output of the ULA, which is output once all ca. 60us even during the field blanking.
Seems to be the normal way of working of the Speccy... and maybe not much appreciated by modern digital TV sets...
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My computers: Sharp PC1211, PC1500, Sinclair ZX Spectrum 48K, QL, Commodore Amiga 500, PC, PC, PC, PC :-(

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Re: ZX Spectrum 48k r2 composite video problems

Post by paf » Mon Nov 19, 2018 1:48 pm

OK, guys, after some more measurement and reading Chris Smith's great book "The ZX Spectrum ULA: How to design a microcomputer" (incredible work, I recommend it), I'm a little less stupid. If you are interested in the book, look at http://www.zxdesign.info/book/

Now it's clear that all unmodulated components of the video signal (luminance Y including sync pulses and chrominance U and V including the color bursts) are issued by the ULA. The LM1889 only modulates U and V according to the PAL standard and TR1 and TR2 add them to Y, creating the full composite video signal which goes to the RF modulator.

The explanations in the book about the "castrated" vertical sync signal (much shorter than the norm and without the pulses) lead me to believe that this could be the cause for my LCD TV's synchronisation problems. Maybe combined with the fact that the color burst is also issued during vertical sync. But I'm still surprised by the signal levels I measure and the facts that the color settings have an influence on the synchronisation.

I have noticed that the voltages my 5C112E-2 ULA outputs for Y (sync 3.3V, black 2.2V and white (not bright) 1.3V) are very different from the ones predicted in the book with a 6Cxxx ULA (4.3V. 2.45V, 0.54V). I don't know if this is a normal difference between these two ULA generation, or a problem in mine. Thus I would greatly appreciate if somebody could measure his/her Speccy (if possible with a 5C ULA) in the "post-boot" state ("(c) Sinclair Research 1982") and tell me:
  • the voltages for sync, black and white (not bright) on the Y pin (17) of the ULA
and, if possible:
  • the voltages for sync, black and white (not bright) of the composite video signal (on the big rear connector of on the input of the RF modulator)
  • if there is a color burst all 64us even during vertical sync (inside the ca. 250us long sync pulse every 20ms).

Any help would be appreciated!
Regards
Patrick

PS: if the ULAs were still available at a reasonable price today, I would probably just buy another one to try...
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My computers: Sharp PC1211, PC1500, Sinclair ZX Spectrum 48K, QL, Commodore Amiga 500, PC, PC, PC, PC :-(

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Ast A. Moore
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Re: ZX Spectrum 48k r2 composite video problems

Post by Ast A. Moore » Mon Nov 19, 2018 2:07 pm

paf wrote:
Mon Nov 19, 2018 1:48 pm
PS: if the ULAs were still available at a reasonable price today, I would probably just buy another one to try...
It would be cheaper—at least, easier—to buy another Issue 2 Spectrum.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

paf
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Re: ZX Spectrum 48k r2 composite video problems

Post by paf » Mon Nov 19, 2018 7:49 pm

Yes, but it would be even nicer if a nice person would measure those voltages in an issue 2 Spectrum and tell me the results...
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My computers: Sharp PC1211, PC1500, Sinclair ZX Spectrum 48K, QL, Commodore Amiga 500, PC, PC, PC, PC :-(

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