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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 14th Sep 2011, 6:57 pm   #21
SiriusHardware
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Default Re: ZX81 on modern TV

Quote:
Originally Posted by cmjones01 View Post
I think the ZX81 is unusual amongst home computers in producing such a deficient video signal. Most other machines were either colour, and therefore had to have a back porch for their colour burst, or had a black background to the screen anyway so this problem didn't arise, or were simply more deluxe designs which did a better job of generating the video signal.
Even the ZX81 eventually did it correctly when the late version of the ULA (version 2C210E) was introduced, but sadly most units seem to have gone out with the earlier ULA (version 2C184E) fitted. Getting hold of the late version ULA is the most elegant way to fix the problem, but needless to say all conventional sources for them have already dried up.
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Old 15th Sep 2011, 6:48 am   #22
SteveSmit
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Default Re: ZX81 on modern TV

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It is entirely possible that other computers designed for older televisions will have troubles on modern equipment. I have a C64 but it's not been fired up in a long time, ditto my Atari ST - neither since the mid 1990's. But I have read about people having problems with 64's and Spectrums.
Quote:
I think the ZX81 is unusual amongst home computers in producing such a deficient video signal. Most other machines were either colour, and therefore had to have a back porch for their colour burst, or had a black background to the screen anyway so this problem didn't arise, or were simply more deluxe designs which did a better job of generating the video signal.
I do have an old early 90s Hitachi 15" portable TV in my attic, which as far as I know is still functional. For the minimal hassle it'd take, I think I'll fish that down this weekend just to eliminate that as a potential problem. Or possibly, fix the problem, in which case that'll become my old computer monitor

If that doesn't work, I'll get out my old VC-20 or my C64-C and try both of those, both of which I know work okay.
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Old 17th Sep 2011, 2:52 pm   #23
Gulliver
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Default Re: ZX81 on modern TV

My ZX81's are both early models...indeed one is so early the ULA simply says "8112" on it - meaning 12th week of 1981. It is soldered onto the motherboard and not in a chip socket, no other identifying marks. Given that the ZX81 was launched in February 1981 and mine was manufactured in late March 1981 it is a *very* early example. It even has the klutzed ROM giving the wrong square root for 0.5

So any problems, it's going to have them. I have modded it to give a composite video output but of course it's still missing important signals.
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Old 25th Nov 2011, 6:53 pm   #24
DragonForce
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Default Re: ZX81 on modern TV

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Originally Posted by Amraduk View Post
Is it not possible to get at the video signal itself, after any digital processing by the ZX81, and add the back porch there, although that might blank data that should be visible?

Regards,

Dave.
Yes, the ZX81 outputs composite video to the input of the UM1233 modulator - the other line is a +5v feed. The exact same modification can (and often is nowadays) performed on the whole ZX Spectrum range. By the way, FYI, the Spectrum used the LM1881 chip mentioned previously.

Lloyd

Last edited by DragonForce; 25th Nov 2011 at 6:58 pm. Reason: spelling error
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Old 28th Nov 2011, 1:05 am   #25
Lucien Nunes
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Default Re: ZX81 on modern TV

Perhaps you can get it ready-made for composite signals in the form of those 'video signal conditioner' or 'copy enhancer' devices that used to be marketed as a useful tweak box to correct video deficiencies when dubbing. Their real purpose of course was to strip Macrovision protection from playback of pre-recorded material, which might well have been done by a similar system using an LM1881 and timers to replace the mutilated lines and eliminate the varying back porch level.

Lucien
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Old 28th Nov 2011, 1:20 am   #26
cmjones01
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Default Re: ZX81 on modern TV

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Originally Posted by DragonForce View Post
By the way, FYI, the Spectrum used the LM1881 chip mentioned previously.
I don't think that's true: the LM1881 is a sync separator, for which the Spectrum would have had no need, and I don't think it even existed in 1982 when the Spectrum was launched. It did, however, use the LM1889 PAL encoder chip.

Chris
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Old 28th Nov 2011, 4:01 pm   #27
DragonForce
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Default Re: ZX81 on modern TV

Agreed - apologies.
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