Image on CRT's in Fast mode?

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RetroTechie
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Image on CRT's in Fast mode?

Post by RetroTechie »

Just a quick question:

If you take an unmodified ZX81 (read: video output via VHF/UHF modulator), a good old CRT, and the ZX81 enters Fast mode, what is seen on the screen?

I keep 'remembering' :? that in this case the CRT would output 'snow' (random pixels), same as between TV stations when searching VHF or UHF bands. But it's been too long to recall for sure.

With LCD screens you usually get some "no signal" message on screen, screen turns blue or something like that. Some will go to standby if this situation remains for a while, some just wait for signal to return. I've seen similar behaviour with more modern CRT's. But anyone remembers how this was on old TV's that were in use in the early 80's?

I'd test myself, but all ZX81's I own are composite-modded & I'm not breaking out the soldering iron to revert any of them back to VHF/UHF output. :) It should be obvious that composite or VHF/UHF could make a difference in what you get on screen during Fast mode. Also I don't have a CRT any more these days, or easy access to one. (for the record: I'd probably keep 1 or 2 CRT's around if room to store them wasn't an issue - but that's not the case for me... :( )
Lardo Boffin
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Re: Image on CRT's in Fast mode?

Post by Lardo Boffin »

RetroTechie wrote: Wed Apr 19, 2017 5:01 am Just a quick question:

If you take an unmodified ZX81 (read: video output via VHF/UHF modulator), a good old CRT, and the ZX81 enters Fast mode, what is seen on the screen?

I keep 'remembering' :? that in this case the CRT would output 'snow' (random pixels), same as between TV stations when searching VHF or UHF bands. But it's been too long to recall for sure.

With LCD screens you usually get some "no signal" message on screen, screen turns blue or something like that. Some will go to standby if this situation remains for a while, some just wait for signal to return. I've seen similar behaviour with more modern CRT's. But anyone remembers how this was on old TV's that were in use in the early 80's?

I'd test myself, but all ZX81's I own are composite-modded & I'm not breaking out the soldering iron to revert any of them back to VHF/UHF output. :) It should be obvious that composite or VHF/UHF could make a difference in what you get on screen during Fast mode. Also I don't have a CRT any more these days, or easy access to one. (for the record: I'd probably keep 1 or 2 CRT's around if room to store them wasn't an issue - but that's not the case for me... :( )
I have a couple of unmodified zeddys (output to VHF is poor but just about legible) so I can test this but probably not until the weekend - busy busy!

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Andy Rea
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Re: Image on CRT's in Fast mode?

Post by Andy Rea »

As i remember it was just a grey screen, sometimes with very slight interference, also sometimes you could turn the sound up and hear the cpu...

Of course if your TV has a no signal blue screen it will vert soon go blue :mrgreen:

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Becky
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Re: Image on CRT's in Fast mode?

Post by Becky »

Andy Rea wrote: Wed Apr 19, 2017 10:23 am As i remember it was just a grey screen, sometimes with very slight interference, also sometimes you could turn the sound up and hear the cpu...

Of course if your TV has a no signal blue screen it will vert soon go blue :mrgreen:

Regards Andy
That is how I remember it too. I can remember in 3D Monster maze the screen going the way Andy describes while the maze was being drawn. There was definitely some kind of signal still being output, it was not the snow you would see on an un-tuned CRT.

Cheers.
Becky.
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1024MAK
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Re: Image on CRT's in Fast mode?

Post by 1024MAK »

As the modulator is still transmitting, you should not get "snow" with random white/grey/black pixels. But what the TV actually displays does depend on the TV circuitry.

The older types will display a grey, dark grey or even a black screen (with no proper sync signals and no proper picture information, the circuits cannot tell where the black level or the white level is). Only the type that has signal detection circuitry will drop back to the "no valid signal screen" (typically a blue screen, sometimes with a message).

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Re: Image on CRT's in Fast mode?

Post by Moggy »

Yup black/grey screen for me and to hear the CPU as Andy says you had to de-tune the TV slightly, I can still hear it now BEEP BEEP DA-DA_DADA, I still have such a set-up so I might just go and relive a few old memories and record it even. :lol:
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Re: Image on CRT's in Fast mode?

Post by 1024MAK »

I've just checked on a CRT TV. This Matsui branded set (sold by Dixons Stores Group in the U.K.), model number 20V1T (20 inch/51cm screen) produces a blue screen when one of my zeddies is connected via UHF.
IMG_4380.JPG
ZX81 via UHF
(422.01 KiB) Downloaded 286 times
No signal - blue screen
No signal - blue screen
IMG_4379.JPG (175.48 KiB) Viewed 4267 times
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Re: Image on CRT's in Fast mode?

Post by mrtinb »

My old small TV just diaplay white screen, as if no text was displayed.
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1024MAK
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Re: Image on CRT's in Fast mode?

Post by 1024MAK »

So now I tried a 14 inch Matsui TV...
IMG_4387.JPG
ZX81 via UHF on 14" TV
(408.19 KiB) Downloaded 281 times
IMG_4386.JPG
FAST mode black screen with sparkles
(184 KiB) Downloaded 281 times
As Moggy says, if you turn up the sound, you get different effects depending on what the ZX81 is doing. I never noticed this before, as I normally have the sound turned all the way down...

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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PokeMon
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Re: Image on CRT's in Fast mode?

Post by PokeMon »

RetroTechie wrote: Wed Apr 19, 2017 5:01 am Just a quick question:

If you take an unmodified ZX81 (read: video output via VHF/UHF modulator), a good old CRT, and the ZX81 enters Fast mode, what is seen on the screen?

I keep 'remembering' :? that in this case the CRT would output 'snow' (random pixels), same as between TV stations when searching VHF or UHF bands. But it's been too long to recall for sure.
Nope - no snow on screen. That comes via RF only if no signal found and "video in over air" toggles indifferent ...
For a ZX81 it depends.
If you are on command line you still have a valid picture which flickers during each keystroke.
It shows the picture by software only if in input mode.

If you run a program it will get into a steady black screen until the program stops and go back to the command line. There might be some side effects when doing weird thinks with IN/OUT instructions which could lead to patterns on screen or audio output or slightly colour in the picture if a colour carrier is faulty detected by the TV.
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