The Grind a Day: thousands of Apple II floppy disks archived

http://ascii.textfiles.com/archives/5498

by pabs3 • 3 years ago

175 35 3 years ago

ascii.textfiles.com

ajross4• 3 years ago

Applesauce pushes out three general types of disk images in its work. Fluxes, which are to-the-bit accurate portrayals of the magnetic flux of the floppy disks.

This is a distressingly common misperception. In fact the flux stream that you get out of a reader like a Greaseweazle is the TTL-level signal from the drive. This is the output of a differential amplifier and crossing detector. In theory, the raw drive head will show zero volts when nothing is changing under the read head, a very small positive voltage when the magnetic flux reverses in one direction, and a symmetric negative signal when it shifts the other way.

What you get out of the amplifier is a digital (!) pulse of ~1us when the signal shifts in either direction. And due to the way the amplifier works, when "nothing is changing" it tends to just amplify noise in the environment and invent fictitious transitions. Proper disk encodings limit the distance between transitions (generally to no more than three bit cells) to deal with this.

This behavior was actually a core copy protection trick. The custom drive writing the commercial software would gate the transitions in the middle of a sector, leading to a region that would read different noise every time. Correct consumer hardware isn't able to do that.

And that's why the flux files are so large: they're storing multiple revolutions of every track, precisely so that tools like the WOZ (Apple) or ATX (Atari) converters can intuit the style of copy protection (there seems to be no clean equivalent in the Commodore world, their "archived" copy protected media tends to be reverse engineered the last I checked?)

A fun project I'd love to see someone try some day would be to wire up a proper analog ADC chain (doesn't need to be more than ~8 bit at 2 Mhz or so) to the unamplified drive head output and see if a cleaner representation of the flux can be read.

TacticalCoder• 3 years ago

This behavior was actually a core copy protection trick. The custom drive writing the commercial software would gate the transitions in the middle of a sector, leading to a region that would read different noise every time.

Can you explain that a bit more? On the software side, how could you detect if you had a pirated disk or not? Read a few times the same data and make sure it'd always be different?

Meic• 3 years ago

Exactly that.

EvanAnderson5• 3 years ago

Sometimes called a "weak sector".

TMWNN2• 3 years ago

Read a few times the same data and make sure it'd always be different?

As EvanAnderso and Meic said, yes. Think of it as the Schrodinger's cat of copy protection; as long as the bad sector is in a state of quantum superposition, the game works. It's when the superposition collapses into a definitive state that the game fails to load.

myself248• 3 years ago

I think "gate" here is being used to mean "gate off", or "inhibit".

The writer would simply refrain from writing anything for a moment, leaving the original bulk-erased magnetic nothingness in the middle of a sector.

EvanAnderson5• 3 years ago

There was a talk at VCF[0] a couple years ago discssing exactly what you're describing in the context of tapes. It was very interesting and some of it has good applicability for floppy disk preservation

[0] https://www.youtube.com/watch?v=sKvwjYwvN2U

EvanAnderson5• 3 years ago

Since I can't edit anymore:

This article re: imaging floppies in the analog domain made the rounds on HN a couple years ago:

https://scarybeastsecurity.blogspot.com/2021/05/recovering-l...

The comments have some good links and background:

https://news.ycombinator.com/item?id=27187435

funstuff007• 3 years ago

Yes, these flux files are yuge (given the amount of actual data they contain). It seems to me they should compress well, but I can't see to remember if tried to do so the last time I fiddled with flux files--during the pandemmy when I thought the world was ending and it was important that my 8th grade book reports be preserved.

KerrAvon• 3 years ago

I'm confused. You're saying it's portrayal of the magnetic flux of the floppy disks, but you were distressed when someone else said it? What part of Jason Scott's description did you take exception to?

Here's Applesauce's author's description, FWIW:

At the lowest level, they are all capturing the amount of time between magnetic flux transitions on the media. The real differentiator comes from what you do with all of these flux timings.

https://applesaucefdc.com/what-is-applesauce/

ajross4• 3 years ago

An SCP file is NOT a direct measurement or representative capture of the flux transitions on the media. It's the output of a machine that imperfectly tries to measure them and inserts noise and delays into the data instead.

And not only in a meaningless "not as accurate" sense -- the drive loses exactly the information that's critical to implementing a working copy. That's why you have to sample a track multiple times for the benefit of downstream analysis/RE tools, and why a naive capture can't be written back to a disk to get the same results.

It's not a good situation. A driver reader that *could* measure the actual flux transitions wouldn't have these problems, but we don't have such a device yet.

My point was that a true capture is/should-be possible, but not with the hardware as implemented by Greaseweazel et. al.

EvanAnderson5• 3 years ago

The Applesauce is at the mercy of the analog board in the drive. Artifacts of the signal processing done there are going to be reflected in the data stream the Applesauce receives from the drive.

A more authentic representation of the flux on the disk could be made by amplifying and sampling the analog waveform straight off the drive heads. A variety of signal processing functions could be applied to that waveform vs. what the electronics on the drive will do.

Octokiddie• 3 years ago

Regarding the copy protection process itself, there's an interesting interview at one of the links:

The most common protection schemes were the ones that were productized and resold to hundreds of publishers. This was coordinated through the disk duplication houses, who offered copy protection as a “value add” on top of mastering the disks themselves. Publishers got the benefit of the latest and greatest copy protection without needing to play the cat-and-mouse game themselves.
The E7 bitstream, a.k.a. “generic bit slip protection,” was the most common. It was a sequence of 1s and 0s, specially crafted so the first half could be read “in phase,” then the code would intentionally skip half a byte and read the second half “out of phase.” Bit copiers would drop bits due to hardware limitations, and the out-of-phase values would be wrong. It was brilliant.

https://paleotronic.com/2018/06/15/confessions-of-a-disk-cra...

timmytokyo• 3 years ago

4am (along with qkumba) is also responsible for Total Replay, a single disk image containing hundreds of old Apple ][ arcade-style games you can run from a single, beautiful launcher app.

See:

https://archive.org/details/TotalReplay

https://github.com/a2-4am/4cade

https://www.youtube.com/watch?v=pki7rsGNxXs

FearNotDaniel• 3 years ago

So "4am" is a person? And "they" is the pronoun referring to that person? I couldn't parse the first two paragraphs after reading them three or four times over. Then I started skipping down the page trying to figure out if I (who was around in the eight-bit days, and is interested in hacker-history and nerd-archaeology type stuff, but not personally involved in any related "scene") would be able to make sense of it, or if it was purely written for "insiders". Eventually figured out enough context that, yes, the grammar checked out and I was able to read it and make sense of it. Only then did I come back to "But 4am is an engineer" that strongly implied (but didn't entirely confirm), we are talking about a person here and not a time of day or an abstract concept.

I'm not knocking people for choosing unusual names/handles, after all it's always been a part of hacker culture, or for choosing "they" as a pronoun, which is somewhat inevitable these days... but a little more careful writing of the article intro would make it soooo much easier for the casual reader to just cosy up and enjoy the story. Which was great, by the way. As hoped for - nothing I can act upon, but it tickles my nerd-gills and makes me feel enlightened.

TedDoesntTalk• 3 years ago

I had the same experience. Read the first part more than once. Confused for a while until I had enough context and understanding that there were two developers. Then I went back and re-read the beginning. Native English speaker here.

tecleandor• 3 years ago

Took me a while, and maybe it's implied by the kinda niche theme of the post, but 4am is an Apple II floppy hacker/cracker/tinkerer

https://mastodon.social/@a2_4am

textfiles3• 3 years ago

"Ah," I said, writing about my friend 4am and their Apple II cracking towards discussing project management. "It's going to be a little confusing when this gets to the front page of Hackernews, with all those busy people skimming the post, but I hope they'll not be scared by a modern pronoun usage and a link set to the projects to determine what I'm talking about."

themadturk• 3 years ago

I was having a similar problem. Glad I wasn't alone!

schwartzworld2• 3 years ago

I had an apple II as a kid. By the time I got it, it was an old computer, but the person who sold it to us included hundreds of floppy disks. Some were games, some utilities, some I never understood what they were supposed to do. It was an amazing learning device could be booted to a Basic prompt for easy programming.

Considering how shitty most software targeted towards kids is today, I am nostalgic for my old apple. I don't have any devices that fit the bill. Basically these requirements:

- easy selection of programs: for a 7 year old, putting a disk in and hitting the power button is as easy as it gets. Diskettes are more easily discoverable than things embedded in a launcher or cli. All I had to do was flip through the physical disks.

- no spyware / in app purchases / monetization attempts. You either had a full version of a program or shareware, but you knew what you were getting

- no internet. I'd be comfortable with something connected but lacking a web browser. Browsers have distractions, and it's too easy to accidentally navigate away from the page.

We got our daughters CD players because they fit the same bill better than mp3s. A non-literate kid can easily pick out their favorite physical media and start playing. Media can be easily shared or swapped.

Is there a way to boot directly to an apple emulator and just select from a list of roms? So it feels like the experience I had as a kid?

StrictDabbler• 3 years ago

In the article, the archive.org page is listed.

https://archive.org/details/wozaday?&sort=-week&page=16

About 1,500 titles. Click and play in the browser. There are no ads or spyware on archive.org. Thumbnails and proper labelling.

Point a browser at this site, put it in kiosk mode so they can't go anywhere else, and you're golden.

I think you will find all dedicated emulators for the Apple II involve more fussing about with poorly-labeled disk images than a kid will want to do.

"ramaadv1a.img? Sure, that sounds like fun! Oh, I have to swap the disk... I'll press F6, scan down the list and figure out that 'flip the disk' in this case means I want ramadva1b.img, even though on other titles the convention was b1a. Oh, I guess this is just a text-mode cooking tutorial program from 1985."

schwartzworld2• 3 years ago

Thank you, but doing it within a browser is not what I've been trying to do.

EvanAnderson5• 3 years ago

My daughter used an Apple IIgs with a FloppyEmu[0] when she was younger (and still uses it some). Getting a real Apple II may be beyond your means (they are getting ridiculously expensive for as many as were made) but, if you do, the Floppy Emu is a real improvement to quality of life.

[0] https://www.bigmessowires.com/floppy-emu/

nicolas_t• 3 years ago

I'm doing something like that with a misterfpga. I set it up with an amiga core and an atari st core and a list of disk images my son can use.

It's then setup as a desktop for him to use as his personal computer.

The Grind a Day: thousands of Apple II floppy disks archived

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