Leander the cave and nostalgia

[NorthWay]

I thought making laser holes (possibly non-visible) was for generating weak bits? Then you don't need to write, just read miltiple times?

[WayneK]

Originally Posted by Galahad/FLT

I'm assuming the same font has made a guest appearance in an intro of yours?

Not yet! :P

I saw you (+ Mr Aplin) giving Jon a hard time on YouTube too, hehe... poor guy, just because his memory is failing :P

He made some nice games, especially on Sega Megadrive!

[Amiga1992]

I didn't know that Puggsy had copy protection on the MEgadrive. Kinda wanna make it trip on my Everdrive, but it works just fine!

I love all these conversations, keep'em coming.

[Rotareneg]

The literal weak bit is just that: flux transitions with an amplitude low enough to not be reliably read, like from physical damage to the disk. Then there is the common one where an out of spec sequence with no flux transitions causes the drive to go out of sync and misread the following bits until it gets synced up again. And finally there's the one where the transitions are present but have weird timing/spacing which confuses the drive as it tries to stay in sync.

[RichAplin]

Originally Posted by AmigaRob

Even if the write tab was not an issue, I would have thought a copy protection system that relied on a hole in the disk causing a faulty track would also have been particularly easy to get around by looking at XCopy to see which track was damaged during a copy attempt, and then forcing the Amiga to repeatedly write to the track while removing the disk. The track would be completely knackered.

You can't physically damage a track by writing garbage to it, it can always be erased and re-written. The concept of this copy protection is that the game writes to the track and whatever it reads back literally has a gap in the data where the hole is. This would work, but it's an impractical idea, the disk duplicator would tell you to F off - unless you want to pay huge pile of extra money in which case obviously anything is possible - and that would be the end of the idea.

The 'weak bit' effect has a couple of causes but the main one is that when you write a long section of disk with no transitions at all, the drive that's reading it back turns up its read amplifier gain (AGC) until it starts picking up noise and crosstalk from adjacent tracks. The effect depends on the AGC circuit in the drive of course; some will do it quicker than others. Normal disk encoding (e.g. MFM) ensures that there are a minimum number of pulses written to the disk regardless of the data you're writing. When reading back these are required both to control the AGC (in the drive) and also to lock the PLL (in the Amiga) that slices the bits you're interested in out of the drive's read output.
You can confuse both systems; for example write a '1' and then ten thousand '0's and read it back. What you're going to see are phantom random bits (which appear only partially random if some of them are crosstalk from an adjacent track), this is AGC noise. If you write a reasonably dense 1/0 pattern but you start putting in too many 1's or 0's, you may confuse the PLL, which would I believe appear as randomly inserted or removed bits. Hours of fun.
With something like a Kryoflux you can jiggle the bit timing very precisely, so a track that confuses the PLL would be easier to write.