Disk density
Capacity designation on magnetic storage, usually floppy disks.
Disk density is a label used for magnetic storage, particularly floppy disks, to indicate storage capacity. Each label refers to a combination of factors that influence how tightly data can be packed on the disk or how efficiently it is encoded. These factors include the modulation method, the width of the tracks, the coercivity of the magnetic material, and the direction of the magnetic field.
For 8-inch disks, the first generation used an iron oxide coating and was called single density (SD or 1D). These drives had 300-oersted write heads, used FM encoding, and had a track width of 0.330 mm, resulting in 48 tracks per inch (tpi) and 5,876 bits per inch (bpi). Double density (DD or 2D) doubled the capacity by replacing FM encoding with more efficient line codes like MFM, M²FM, FM/MFM, or GCR.
For 5¼-inch disks, the SD and DD labels were essentially the same as those for 8-inch disks. Quad density (QD or 4D) doubled capacity again by narrowing the track width to 0.160 mm, achieving 96 tpi. Some manufacturers, including Micropolis, Tandon, Micro Peripherals (MPI), and Teac, used 100 tpi for quad-density drives, which were not compatible with 96 tpi models. The Commodore 8050 and 8250 were rare drives that used a 375 kbit/s GCR code instead of the usual 250 kbit/s double-density format, allowing roughly 500 kilobytes per side. High density (HD) improved capacity by combining 96 tpi track density with a cobalt disk coating and stronger 600-oersted write heads, enabling 9,646 bpi.
For 3½-inch disks, double density (DD) used an iron oxide coating like 5¼-inch DD and QD disks, but with stronger 670-oersted write heads and a narrower track width of 0.115 mm, giving 135 tpi and 8,717 bpi. High density (HD) switched to a cobalt coating, similar to 5¼-inch HD disks, and used 700-oersted write heads to achieve 17,434 bpi. Extra-high density (ED) doubled capacity over HD by using a barium ferrite coating and a special write head that enabled perpendicular recording. Triple density (TD) tripled the capacity over ED by tripling the track density and improving other parameters, while still using longitudinal recording.
- 8-inch sd
- 48 tpi, 5,876 bpi, 300-oersted write heads, FM encoding, 0.330 mm track width
- 8-inch dd
- Doubles capacity over SD via improved line code (MFM, M²FM, FM/MFM, or GCR)
- 5¼-inch qd
- 96 tpi (some manufacturers used 100 tpi, incompatible with 96 tpi), 0.160 mm track width
- 5¼-inch hd
- 96 tpi, 9,646 bpi, 600-oersted write heads, cobalt coating
- 3½-inch dd
- 135 tpi, 8,717 bpi, 670-oersted write heads, 0.115 mm track width, iron oxide coating
- 3½-inch hd
- 17,434 bpi, 700-oersted write heads, cobalt coating
- 3½-inch ed
- Barium ferrite coating, perpendicular recording
Lore & Background
Disk density designations originated with 8-inch media, where single density (SD or 1D) described the first generation of floppy disks using an iron oxide coating, 300-oersted write heads, FM encoding, and a track width of 0.330 mm for 48 tracks-per-inch (tpi) and 5,876 bits-per-inch (bpi). Double density (DD or 2D) doubled capacity by replacing FM encoding with improved line codes such as modified frequency modulation (MFM), modified modified frequency modulation (M²FM), FM/MFM, or group coded recording (GCR).
For 5¼-inch media, SD and DD designations were generally identical to those of 8-inch disks. Quad density (QD or 4D) doubled capacity over DD by narrowing track width to 0.160 mm for 96 tpi, though some manufacturers (Micropolis, Tandon, Micro Peripherals (MPI), Teac) used 100 tpi, which were incompatible with 96 tpi models. The Commodore 8050 and 8250 were rare instances of drives that used 375 kbit/s GCR code instead of the usual 250 kbit/s double-density format, storing roughly 500 kilobytes on one side of a disk. High density (HD) improved capacity using 96 tpi with cobalt coating and 600-oersted write heads, allowing 9,646 bpi.
3½-inch media introduced double density (DD) using iron oxide coating, 670-oersted write heads, 0.115 mm track width, 135 tpi, and 8,717 bpi. High density (HD) switched to cobalt coating with 700-oersted write heads for 17,434 bpi. Extra-high density (ED) doubled capacity over HD using barium ferrite coating and perpendicular recording. Triple density (TD) tripled capacity over ED by tripling track density and improving other parameters, using longitudinal recording.
Reader's Guide
Disk density designations provided a standardized way to describe the storage capacity and technical characteristics of floppy disks across different form factors. The progression from single density to double, quad, high, extra-high, and triple density reflected incremental improvements in recording technology, including changes in modulation methods, track widths, coercivity, and magnetic coatings. These designations helped users and manufacturers identify compatible media and drives, though occasional incompatibilities arose, such as the 96 tpi versus 100 tpi quad-density drives. The Commodore 8050 and 8250 drives demonstrated that alternative encoding schemes (375 kbit/s GCR) could achieve higher storage within the same density class. The evolution from iron oxide to cobalt to barium ferrite coatings, and from longitudinal to perpendicular recording, marked key technological shifts that increased areal density without changing the physical disk size. Triple density represented the highest capacity designation, tripling the capacity of extra-high density through increased track density and other parameter improvements while retaining longitudinal recording.
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