Single density

Lore & Background

Single density was the initial capacity designation for 8-inch floppy disks, employing an iron oxide coating and 300-oersted write heads. The modulation method used was FM encoding, with a track width of 0.330 mm, yielding a density of 48 tracks per inch (tpi) and 5,876 bits per inch (bpi).

For 5¼-inch media, the SD (1D) designation was generally identical to that of 8-inch disks, carrying over the same characteristics. The term 'single density' contrasts with later designations such as double density (DD), which doubled capacity by replacing FM encoding with improved line codes like MFM or GCR.

Disk density designations describe a set of characteristics that affect areal density or data encoding efficiency, including modulation method, track width, coercivity, and magnetic field direction. Single density represents the baseline from which these parameters were refined in subsequent formats.

Reader's Guide

Single density's significance lies in establishing the foundational parameters for floppy disk storage. As the first generation of magnetic floppy media, it set the track width of 0.330 mm, 48 tpi, and 5,876 bpi that later designations would modify to increase capacity. The use of iron oxide coating and 300-oersted write heads became the reference point for double density and quad density improvements.

Its legacy is primarily as a baseline: double density doubled capacity by switching from FM encoding to more efficient line codes such as MFM, while quad density further increased capacity by narrowing track width. The single density designation persisted into 5¼-inch media with identical characteristics, showing the standard's longevity across form factors.

While single density itself was eventually superseded by higher-capacity formats, its role in defining the initial technical specifications for floppy disks—coercivity, encoding method, track geometry—made it the foundation upon which the entire floppy disk ecosystem was built. Without the single density standard, the subsequent evolution through double, quad, high, and extra-high densities would not have had a common starting point.

Did You Know?

• Single density floppy disks use an iron oxide coating and 300-oersted write heads.
• FM encoding is used for single density, yielding 5,876 bits per inch.
• The track width for single density 8-inch media is 0.330 mm, giving 48 tracks per inch.
• 5¼-inch single density disks were generally identical in characteristics to 8-inch single density disks.