Re: Did Commodore cheat with the quad density floppies?

----- Original Message ----- 
From: "Mia Magnusson" <mia@plea.se>
To: <cbm-hackers@musoftware.de>
Sent: Sunday, January 06, 2019 9:17 AM
Subject: Re: Did Commodore cheat with the quad density floppies?

> ... Maybe the 8050 were one of the first drives to have a smaller head gap than most DD drives did?
 
There were a number of other systems around since the late 70s that managed to get close to 500KB/side formatted capacities out of DD disks using various techniques including GCR, more bytes/sector etc.; I'm not aware of anyone else other than Apple using zone recording but these were the wild west days of incompatible disk formats, so anything is likely...

Micropolis, who supplied many of the drives in the 8050, was one of the first to go to a higher TPI (100) than the then standard 48 TPI; Tandon and MPI also made 100TPI drives, but in the end 80 tracks at 96TPI became the 'QD' (and HD) 'standard' since that also made reading and writing 48TPI diskettes possible by double-stepping.

Interesting side note: rarely seen outside of Japan, but some manufacturers used exactly the same format in all diskette sizes (8", 5.25QD/HD and 3.5") which is why you'll find options on some 5.25" and 3.5" drives to select the 8" standard 360 RPM instead of 300. 

5.25" HD drives normally run at 360RPM; many have an option to select 300 RPM, but compatibility with DD and QD disks is usually achieved by changing the transfer rate instead.