ZX81 (stable?) Video Out

Discussions about Sinclair ZX80 and ZX81 Hardware
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wietze
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Joined: Thu Jun 25, 2020 2:30 pm

ZX81 (stable?) Video Out

Post by wietze »

Hi there guys,

recently Ive set my mind on getting a ZX81 and attaching it to a more modern display (e.g. non 1084 crt display).

However, I notice that my digital devices do not detect a stable signal output, and thus the display remains either a) black, because the modern hardware cant make sense of the unstable input or b) I have a display but it constantly `rolls' over the display vertically.

I am new to this forum, but am wondering if you guys have encountered (and hopefully resolved) this issue.

I have seen the FPGA ULA replacement and ZXVid composite mod; would either of these solve the problem I have? Are there alternatives to get a stable out?

Kind regards,
Wietze

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mrtinb
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Re: ZX81 (stable?) Video Out

Post by mrtinb »

This device will do the trick https://www.sellmyretro.com/offer/detai ... -ZX81-3712

But it has to be turned very precise, and sometimes it changes after a while.

To be honest I have difficulty getting a non rolling/blinking display on my modern VGA screen, even with all the converters I've bought.

Even modern ULAs won't give a signal that is satisfactory for modern displays. Modern displays are simply too picky.

I've finally got a decent display with a CGA-to-VGA-card in-between the ZX81 and the VGA screen, but the software needs to use the ROM rutines for display. If a custom display routine is used, that is a little different, then the picture is lost.

I have an older 9" B/W transportable TV as backup if the signal is not acceptable to the VGA screen.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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1024MAK
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Re: ZX81 (stable?) Video Out

Post by 1024MAK »

Hello Wietze and welcome to the forum :D

There are various factors at play here.

There are three different ULA versions, only the last version that was produced for Sinclair is compatible with colour TVs (see the ZX81 variations link in my signature below for details).

A lot of modern TVs are not happy or don’t produce a good display if the RF VHF/UHF signal is not on a recognised TV channel frequency, but alas, the modulator in a ZX81 is analogue and even if it was tuned to the correct frequency when manufactured, it will have drifted since then.

And lastly, the ZX81 was designed to work with CRT technology, which is very forgiving. Various tricks were used to keep the cost of the computer low, hence a ZX81 does not produce a signal to the broadcast standard. Instead of producing proper interlaced frames, instead the same frame (but with possibly different picture information) is repeated. This is different to what the TV is expecting.

Despite this some LCD TVs will actually work okay with a ZX81. But finding one that does work okay is a bit hit and miss. So please don’t assume because one does not work, that they are all the same.

There are ways and add-ons that can help. The Z8-CCB module and the ZXVid can clean up the signal, add a “back porch” to the video signal and produce a composite video signal, all of which helps.

Or if you have the last ULA version, you can try the composite modification (details linked in my signature below).

An alternative is indeed to use a replacement ULA module.

Good luck.

Please do feel free to ask any questions.

Mark

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