The 720k DSDD in PC HD Drives problem explained

Why doesn't your perfectly good floppy disc work? A guide to what is happening - and why things don't seem to work as you expect:

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The 720k DSDD in PC HD Drives problem explained

Unread post by parametric » Tue Feb 05, 2019 12:15 pm

Hi Folks,

In an attempt to clarify this problem, which bamboozles many users, I should like to offer this to help explain WHAT is happening - and why things don't seem to work as you expect:

These pics show a small section of the Disk track. The first thing to note is that the 720k DD Drive, which is fitted to many boards in addition to the SY85 was the FIRST 3.5" drive produced, which explains their presence on many Synths of this era.

When the HD 1.44MB Drive was developed later on, the increased capacity was achieved by higher coercivity media AND by doubling the track count - which was in turn achieved by HALVING the head height (So the disk size remained the same at 3.5").

This head height difference between the two Drive types is the source of the problems that we see . . . .

Take a 720k disk and FORMAT it in your Synth.

Save a Bank/Sequence/whatever from the Synth to that disk, and you get:


The FULL track is taken up by the Data. This Disk can be put away and reloaded to the Synth at any time.
This disk can also be read by the PC and the data copied to the HD and archived there.
It can be copied back to another disk for loading into the Synth at any Time - SUBJECT TO CERTAIN PRECAUTIONS.

Now take this same disk to the PC. Format it as 720k and copy a Bank/whatever to it from the HD.
Due to the small head size of the HD Drive, the track on the floppy will now look like this . . . . .


When you attempt to load the Disk, the head in the Drive in your Synth will try to interpret BOTH data streams with its wider head - and you'll get a <BAD DISK> or similar error in the display.

This disk is no longer any good for the Synth until you
Take the reformatted disk to the PC and copy your Bank/whatever to it.
The track on the Disk now looks like THIS:


You can take this disk to your Synth and it will read it just fine. The PC can of course read it too.

The CERTAIN PRECAUTIONS I spoke of above is merely this.

When you want to LOAD stuff stored on the PC, to your Synth . . . . .

FORMAT the media first IN THE SYNTH
Copy the data from the PC to the Floppy
LOAD the data in to the Synth.

I hope that explains it better and clears up the confusion . . .

Best Regards

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