Single-Density vs Double-Density disks

All questions regarding the dumping of media go here.
Post Reply
Posts: 112
Joined: Mon Nov 21, 2011 5:21 pm

Single-Density vs Double-Density disks

Post by rcade » Fri Aug 31, 2018 8:27 pm

This is just for my information. I know there is a difference in coercivity for DD vs HD 5.25" media, but is there any difference between an SD and a DD disk? Is it just a certification rating only (3000bpi vs 6000bpi) or nothing at all?

I have some disks that are labeled as SD, and they can be formatted as DD, but they have errors. Is this just a coincidence because they are old/used?

I did Google it, and most people say the media is the same, but if that is so, why would they have sold disks as SD instead of SD/DD?
Pete Rittwage
C64 Preservation Project

Posts: 178
Joined: Wed Feb 18, 2015 8:18 pm

Re: Single-Density vs Double-Density disks

Post by SomeGuy » Fri Aug 31, 2018 10:48 pm

Single Density and Double Density disks have the same coercivity. The difference is the encoding scheme. Single Density uses FM encoding while Double Density uses MFM encoding. FM encoding is a bit more resistant to errors.

In some cases a manufacturer may have performed some surface testing at the factory or pre-formatted the disk for a specific system. Commonly, lower quality material was designated as "single density", but otherwise there was no technical difference. The primary reason for marking disks as such was to give consumers some confidence that the media matched their system, and a reason to charge more for the "better" ones :P

The same coercivity material is also used for so-called "Quad Density" disks (Double Density at 96tpi/80 tracks), and 48TPI media will usally work fine at 96tpi. Additionally, most "Single Sided" disks actually had magnetic media on both sides. It is interesting to think that a "Single Sided/Single Density 48TPI" disk intended to store around 96K can often be formatted fine as Quad Density at around 800k depending on the specific geometry.

Also any 5.25" disk marked as Double Sided/Double Density (or quad density) can be used fine on any system that needs single density or single sided disks.

Post Reply