Beginner's question about loading programs on ZX spectrum

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Seven.FFF
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Re: Beginner's question about loading programs on ZX spectrum

Post by Seven.FFF »

I think it could be reconstructed, maybe even quite easily*.

Richard T Russell recovered the missing chroma info from the dot crawl on monochrome recordings of unfiltered TV programs, 10 years ago using a program he wrote in BBC BASIC.

Admittedly he did write the spec for BBC BASIC and worked in BBC Engineering R&D half his life, but still, it gives an idea of what is possible :D

*depending on the video source. And how much was cut off in the overscan area.
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Ralf
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Re: Beginner's question about loading programs on ZX spectrum

Post by Ralf »

Reconstruct a program from loading border stripes? I'm afraid it's theoretically possible but wouldn't work correctly in practice.

Any video recording, traditional or digital, consists of frames. There are 30, 40, 50 or whatever any other number of frames recorded per second.
It works perfectly for human eye giving an illusion of movement but I'm afraid it will miss information being displayed between frames. You will have big holes of unknown data in your program.
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4thRock
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Re: Beginner's question about loading programs on ZX spectrum

Post by 4thRock »

From the PAL signal specs (if we trust Wikipedia https://en.wikipedia.org/wiki/PAL) we get:
» Total time for each line 64.000 μs

From somewhere on the Web, I got:
» normal loading pulse 0.244 milliseconds

PAL only has 576 active lines, out of 625.
So you don't record for 3136μs » 3.13600 milliseconds and there's data loss.

Please correct this if I'm wrong :lol:
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ZXDunny
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Re: Beginner's question about loading programs on ZX spectrum

Post by ZXDunny »

Yes, it's not possible. There is a period of "flyback" where the TV's beam resets to the top-left corner of the screen after it has reached the bottom-right - any data that is transmitted during that period is lost. Also there are no stripes drawn to the paper section of the TV screen (pixel/attr data there) so you might not be able to distinguish between 1s and 0s.

Of course, you could analyse the sounds coming from the internal beeper - but then you might as well just play the tape :)
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Seven.FFF
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Re: Beginner's question about loading programs on ZX spectrum

Post by Seven.FFF »

Yes, I concur. I realised this just after I posted, and put in that qualification as an edit. But it's not just overscan, the v.flyback is a showstopper. Oh well.
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