12" Floppy Disks

tony duell ard at p850ug1.demon.co.uk
Tue Sep 15 11:17:46 CDT 2015

> The PLATO IV terminals (the hardwired Magnavox ones, not the later microprocessor based 
> ones) had an optional  "Audio player".  That used a floppy disk of about that size, storing
> analog audio snippets (in analog form, not digitized -- remember, this was around 1972). 
>  Seek was done by a pneumatic D/A converter, essentially.  There were 128 tracks, each 
> with 32 sectors.
> Those disks had no sleeve -- you'd just slide the bare magnetic disk into the player mechanism.

I have an office dictating machine here, I think it is badged 'Olympia' which used magnetic disks. It
appears that the disk had a spiral groove cut in it, like a gramophone record, the magnetic head ran
along this [1] and could be lifted and repositioned (like the tone arm of a gramophone) to skip long
distances forwards or backwards.

[1] I have heard of a similar machine with a sprial 'cam' as part of the machine to guide the head, this
does not have that or any evidence of it. The magnetic head does the guiding.

I do not have any disks for it, but from the user guide (which I do not know what happeend to) the normal
disk was 'rigid' (I assume hard plastic), there was also a flexible version for sending by mail. From the size
of the slot and turntable, I think the disk is about 7" diameter,

I think with this machine you just inserted the 'naked floppy'. No sleeve.

I don't know the exact date, but from the technology used (a pair of B9A based valves and 
contact-cooled metel rectifiers) I would guess early-mid 1960s.


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