It certainly seems that it would be THEORETICALLY POSSIBLE, with an extreme budget, to build a high resolution device similar to the 3M Magnetic Tape viewer, . . . https://blog.adafruit.com/2020/03/01/the-magnetic-tape-viewer-see-the-sound-on-a-tape/

On Thu, 14 Apr 2022, John Foust via cctalk wrote:
And how often do those antique viewers come up on eBay and at what price?
Modern ones are for sale for about $120:
https://store.arnoldmagnetics.com/product/284/magnetic-viewer-b-1022

Alas, although still a lot of it around, magnetic recording is not as common as it once was, and increasingly, new technology is solid state. So, there isn't any where near as much incentive to build devices other than as novelties.

But you can't really use an optical method, right?  You need to scan
the field another way.

If the entire artifact were drenched with magnetic disclosing fluid, then it could be a simple optical capture. But the 3M device is truly non-destructive. It is only a tiny window. I am not aware of any 8" diameter versions of it.

And if you set a limit on disk platter tech to anything made before 1985,
what's the magnetic resolution you'd need and what would you need to detect?
1985 would include 96tpi "1.2M" "high density", 720K 3.5" (135tpi), and hard disks with at least 600 cylinders.

Could the reconstruction be software-based, reconstructing from data
gathered while scanning X-Y across surfaces using a mechanism not
unlike a scanner or 3D printer?  Or would you need to scan like
the flying head that wrote the data back then?

Yes, if there is a good clean opticl image, then the software would have to clean it up, including filling in by context for any uncertain spots, and convert the concentric circles in the image into an array of lines (tracks), decode the flux transition data into bit patterns (with a basic understanding of FM, MFM, GCR, and a few flavors of RLL). At that point, you would have a disk image comparable to what could be obtained by examining the disk in a drive.


RAMAC platter:
So, I am making a 24" patio table out of it (under 3/8" tempered glass).
http://www.ed-thelen.org/RAMAC/RAMAC_Plaque_v40.pdf

A rare item.  How many others have you seen in the wild?

This is the only one that I have seen. I'm sure most decent sized museums have one, or even an assembled multi-platter (FIFTY-TWO!) system.
I didn't even know that it was RAMAC until Mike Albaugh told me.
Wil Price had it at Merritt College in the late 1960s, in the 1401 room, and brought it along when the college moved up onto the hill. I used it, core planes, cards, etc. in small history component of some of the classes that I taught. By the time that I was ready to retire (constructive discharge), the college shifted its focus from programming to "Remedial Computer Literacy For The Digital SweatShop", so I was the only one who could appreciate it.

--
Grumpy Ol' Fred                 ci...@xenosoft.com

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