ZX81CCP video output for ZX81

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
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Paul
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Re: ZX81CCP video output for ZX81

Post by Paul » Fri Oct 12, 2012 11:44 am

Thanks for testing.
So it was a false alarm. Better to often than never.
Downloading will then have to wait until I am home.
In theory, there is no difference between theory and practice. But, in practice, there is.

RWAP
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Re: ZX81CCP video output for ZX81

Post by RWAP » Fri Oct 12, 2012 7:40 pm

PokeMon wrote:
RWAP wrote: PokeMon - where you are talking in the manual about SYNC failures - maybe a picture would help illustrate what this means to the non-technical out there
What do you mean exactly ?
The section about adjusting the circuit ?
Yes - that is where I mean - when you talk about turning the PIXEL pot all the way down and then adjusting the SYNC one until you see no failures...

It is not very clear what users need to look for

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PokeMon
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Re: ZX81CCP video output for ZX81

Post by PokeMon » Fri Oct 12, 2012 8:40 pm

Can not specify exactly because it depends on your TV how sync failure is handled.
Have TVs showing a message, have TVs showing soemthing else.
They have to adjust for a valid picture on screen like described.
The hint of sync failures are just for persons who understand what sync is and how sync failures look. :mrgreen:

Rink
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Re: ZX81CCP video output for ZX81

Post by Rink » Sat Oct 13, 2012 11:34 am

One ZX81 modded, one to go.

Firstly, I have to say that the picture quality from the ZX81CCP is unbelievably good on my tv. What was once blurry and dark is now super crisp - the whites are as white as fresh powder on a sunny day in the arctic mountains, while the blacks are as dark as the saddness in a child's heart on the day he learns that Santa Claus does not exist.

I installed mine in the modulator housing - starting with a ZX81 board which has had the modulator completely removed and disassembled (the insulator board is still in the housing though).

Image

And here it is with the mod installed - yes, I left the wires quite long, it does no harm and I found that the wires did such a good job holding the board in place that I didn't need to glue it.

Image

PokeMon, I couldn't be happier mate. Thank you very much.

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PokeMon
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Re: ZX81CCP video output for ZX81

Post by PokeMon » Sat Oct 13, 2012 4:52 pm

Nice to see the old issue one board. Here the video output is a desaster due to the missing backporch.
I have the same. Looks really nice these curved tracks on the board. ;)

And yes, now you will have more fun with your Zeddy as you can really good see what he's printing on the screen and what he want to say to you.
You can see now the soul of the ZX81 through the crisp characters.
So have fun. 8-)

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TMAOne
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Re: ZX81CCP video output for ZX81

Post by TMAOne » Mon Nov 12, 2012 6:52 am

Just discovered this thread. Wow Pokemon. I don't understand all the technical stuff, but see enough to know that I want one, or two,... no, five,... [okay calm down].

Ah but alas. I am on the wrong side of the Atlantic, where we use NTSC not PAL. Is this a show stopper, or do the different systems only pertain to the specs of the RF signal? Are "RCA" composite inputs the same? Would this tiny elegant device work on my side of the world? And, inevitably, that "other" question,... when will you be creating another batch?

I've been playing with a handful of Zeddies and Timex's, and while I do have an old black and white television to use, (two actually), they won't last forever. I have tried the one transistor composite output, and experimented with the "zero solution" (direct feeding from the ULA output), but get wildly different results on each of 3 household televisions of varying vintage I use as guinea pigs. I want a solution that (1) fits inside the standard case, (2) feeds any relatively modern eyeball interface, and (3) doesn't break the bank. (One computer is being prepared as a surprise gift to an old friend from the "Sinclair days".) In short, the ZX81SCP is exactly what I was looking for.

Anyway, at the very least congratulations on your achievement in creating the circuit every ZX81 needs. That output looks stellar.

Cheers,
Ian

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RetroTechie
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Re: ZX81CCP video output for ZX81

Post by RetroTechie » Mon Nov 12, 2012 1:49 pm

For a color signal it's like: Red/Green/Blue (or Y/U/V) + horizontal & vertical sync -> PAL or NTSC encoder chip -> composite video signal -> UHF or VHF modulator -> RF signal.

The ZX81 skips the 1st step and produces a black & white composite video signal directly. That transistor mod takes the RF modulator out of the equation, which usually translates into a better quality picture. But it doesn't affect signal timing / PAL vs. NTSC issues. Also older version ULA's (2C184E) are missing the so called "back porch" in the video signal, which results in a very dark picture on most TV's. At least that problem should be fixed by above circuit, as it's designed to add in a back porch even if the ULA doesn't produce it.

In the ZX81 you can only set the display refresh rate, which is important for TV's but not entirely the same as PAL <-> NTSC. This is done via a resistor on the main board (R30, near the 7805 regulator). If present -> 60 Hz refresh (for NTSC countries), if missing -> 50 Hz (for PAL countries). But setting to 60 Hz does make the ZX81 a lot slower... :o So you might want to try and add R30 back in - best do that on a ZX81 which has the later version ULA (2C210E).

Better not connect ULA video output directly to outside world, it's very easy to kill an ULA that way. One last thing to try is add a capacitor in series with the transistor mod. Something like 100 to 220 uF (rated 6V or higher). + side to transistor output, - side to the cinch connector's center pin.

From that point on it all depends on what TV you're using. Some may happily display NTSC and PAL signals, some may be happy as long as the refresh frequency is 60 Hz (or 50 Hz, for a PAL TV), some may not like ZX81 display timing no matter what circuitry you add. For example my 20" CRT television produces a slightly 'wobbly' image :x (as if the horizontal sync is unstable, which is not the case), and does so with all my ZX81's including my CPLD-based one. With other machines like ZX Spectrum or MSX, it doesn't have that issue. Perhaps it just doesn't like a black & white signal or something... :?:
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PokeMon
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Re: ZX81CCP video output for ZX81

Post by PokeMon » Mon Nov 12, 2012 10:35 pm

TMAOne wrote: Ah but alas. I am on the wrong side of the Atlantic, where we use NTSC not PAL. Is this a show stopper, or do the different systems only pertain to the specs of the RF signal? Are "RCA" composite inputs the same? Would this tiny elegant device work on my side of the world? And, inevitably, that "other" question,... when will you be creating another batch?
I have still one - and yes it does work in USA for NTSC TVs same way. There is no difference for the steady picture, just for the standby sync generator (used with LOAD for example) it's important to adapt to 60 frames per second. That could be easily done by replacing R1 with 68k (instead of 82k). I can do this before shipping - no problem.

I will send you a PM. ;)

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TMAOne
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Re: ZX81CCP video output for ZX81

Post by TMAOne » Tue Nov 13, 2012 5:02 am

Thanks RetroTechie and PokeMon.

Assuming this one works as expected, (I'm sure it will), I am serious about picking up 2 or 3 more whenever they're available. I have a small handful of ZX81's and Timex's,... 1K, 2K, 16K (with pack), 32K internal mod,... and a ZXpand. I've picked some up on eBay with mixed success, but occasionally combined the working parts from two or more duds to come up with a solid unit. They don't take up much room, but multiple TVs is another matter. They're either "rare and valuable monochrome RF monitors" or "hideous old obsolete TVs", depending on whether you ask me or my wife!

For the longest time I wondered if I was the only person still firing up a ZX81 every year or so to reminisce. It's nice to know the Sinclair is alive and well in Europe, though I wish I had more company here in Canada. I made my living programming mainframes, but it was that first air-cooled reduced instruction set monocore computer with unlimited ribbon disk storage that was the bug that bit me. But I digress again,...

Cheers,
Ian

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sirmorris
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Re: ZX81CCP video output for ZX81

Post by sirmorris » Tue Nov 13, 2012 1:18 pm

TMAOne wrote:unlimited ribbon disk storage
:lol: :lol: :lol: :lol:
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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