Is there a way to keep the image stable when loading from tape? AKA where are my loading bars!

[gwpt]

I just got my vLA81 and installed it and the picture looks crystal clear.
I'm running it into an old school Samsung TV-348 which I assumed would be good for composite.

The issue I have, is when I do LOAD "" the picture instantly disappears and goes back to the TV 'blue screen' which is what it does when it has no input.
During the load process, it might flicker once or twice for a split second but that it's.
As soon as the load finishes the picture reappears

Is there a way around this? (mods to the video out etc?) I would like to see the loading bars!
I have my vLA81 connected directly to the video out via 2 capactors (a 100uf and 100nf in parallel as suggested by 1024MAK)
This was happening with my previous 555 mod with the orginal ULA too, but I was hoping the vLA81 would fix it. no luck
(Anyone know the technical details of why it doesn't work?)

[mrtinb]

No. It's your TV that wants to display blue screen instead of the real signal from your computer. On some TVs you can disable this "more knowledgable" feature, as it's quite disturbing with old computers. On newer TVs you cannot disable this "feature".

[gwpt]

Thanks Martin, drats, that's annoying, I was hoping that TV (mid 90s) would be old enough to behave.

[Moggy]

Yes just to concur with Martin every LCD type TV I have ever used, old and new, did the blue screen dance when loading.

[1024MAK]

The technical reason is complex. But here's a simplified answer.

The ZX81 uses the Z80 processor in combination with the ULA to produce the correct signal for a TV (or monitor) to display. TVs in the 1980s used analogue circuitry. This circuitry would display whatever it could from whatever signal it could find. Including the moving and forever changing "static" when there was no actual TV signal.

Further, most TVs by this time had synchronisation systems that in the absence of receiving the correct synchronisation signal, would 'fall back' to using internally generated synchronisation signals so that at least some image would be displayed without horizontal or vertical rolling or the entire picture flashing.

The designers of the ZX81 took advantage of this. But more modern CRT TVs and all LCD TVs instead switch to displaying an internally generated "no signal" screen.

When the computer is in FAST mode or the Z80 processor is busy and does not have time to help with the generation of the video signal, the computer does not produce a valid video signal. Examples of The Z80 being busy include tape operations and ZX Printer (or Alphacom 32 printer) operations.

During this time, the picture information and the correct synchronisation signals are not produced. Hence because the TV cannot detect a valid signal, it's switching to its "no signal" screen.

I think someone did come up with an additional circuit so that modern TVs would be provided with correct synchronisation signals. But I forget who and where I read about it.

Mark

[mrtinb]

PokeMon made af version of his famous ZX8-CCB circuit, that included a microcontroller on the back side. This version generates a constrant sync signal. However PokeMon does not sell this version of the circuit any more.

[1024MAK]

Ahh, thank you Martin

Mark

[gwpt]

Thanks for the details Mark, makes more sense now.

Martin - do you know if the plans for the microcontrollered ZX8-CCB were ever released? I'd be happy to make one!

[mrtinb]

The firmware for the microcontroller was not released.