Re: Did Commodore cheat with the quad density floppies?

----- Original Message ----- 
From: "smf" <smf@null.net>
To: <cbm-hackers@musoftware.de>
Sent: Tuesday, January 08, 2019 10:44 PM
Subject: Re: Did Commodore cheat with the quad density floppies?


> On 09/01/2019 02:33, Mike Stein wrote:
>> And why would you adjust the transfer rate with a DD drive?.
> 
> If the previous access was to a HD disk in a HD drive.

I think we're really talking about different things; if you're just talking about switching between a DD drive and an HD drive then obviously the controller would select the appropriate transfer rate for that drive & diskette.

A DD drive will always spin at 300 RPM with a transfer rate of 250Kbps, so there's no 'switching'.

But an HD drive normally spins at 360RPM at 500Kbps when using an HD disk; in order to use a DD disk which normally spins at 300RPM it would have to switch to a lower drive speed, and some drives and controllers did indeed provide that option.

What I'm talking about is another clever hack, (err, solution) adopted by IBM, again presumably to be able to use *standard* single-speed HD drives with DD disks. Instead of changing the speed they spin the drive at the usual 360RPM but simply use a transfer rate of 300Kbps instead of 250 (300/250 = 360/300).