Disk density

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5¼-inch DD-Disk

Disk density is a capacity designation on magnetic storage, usually floppy disks. Each designation describes a set of characteristics that can affect the areal density of a disk or the efficiency of the encoded data. Such characteristics include modulation method, track width, coercivity, and magnetic field direction.

8-inch media [edit]

Single density (SD or 1D) describes the first generation of floppy disks that use an iron oxide coating. Floppy drives utilize 300-oersted write heads, FM encoding, and a track width of 0.330 mm (0.0130 in) for a density of 48 tracks-per-inch (tpi) and 5,876 bits-per-inch (bpi).

Double density (DD or 2D) doubles capacity over SD by replacing FM encoding with an improved line code, such as modified frequency modulation (MFM), modified modified frequency modulation (M²FM), FM/MFM or group coded recording (GCR).

5¼-inch media [edit]

SD (1D) and DD (2D) designations were generally identical to those of 8-inch disks.

Quad density (QD or 4D) doubles capacity over DD by narrowing the width of tracks to 0.160 mm (0.0063 in) for a density of 96 tpi. Some manufacturers (Micropolis, Tandon, Micro Peripherals (MPI), Teac) used a track density of 100 tpi for quad-density drives, which were incompatible with 96 tpi models.

The Commodore 8050 and 8250 are rare instances of drives that used 375 kbit/s GCR code instead of the usual 250 kbit/s double-density format and they could store roughly 500 kilobytes on one side of a disk.

High density (HD) improves capacity by utilizing a 96 tpi track density in conjunction with improved cobalt disk coating and stronger 600-oersted write heads, allowing 9,646 bpi to be written.

3½-inch media [edit]

Double density (DD) 3½-inch disks use an iron oxide coating, just as with 5¼-inch DD/QD disks. However, drives utilize stronger 670-oersted write heads and a narrower track width of 0.115 mm (0.0045 in) for a density of 135 tpi and 8,717 bpi.

High density (HD) 3½-inch disks switch to a cobalt disk coating, just as with 5¼-inch HD disks. Drives use 700-oersted write heads for a density of 17,434 bpi.

Extra-high density (ED) doubles the capacity over HD by using a barium ferrite coating and a special write head that allows the use of perpendicular recording.[1][2]

Triple density (TD) triples the capacity over ED by tripling the track density and improving other parameters.[3][4][5] The drives used longitudinal recording.[2]


Size 8-inch 5¼-inch 3½-inch
Density SD DD SD DD QD HD DD HD ED[1] TD[4]
Disk coating Iron oxide[1][6] Cobalt[1][6] ? Iron oxide, Cobalt?[1] Cobalt[1][6] Barium ferrite[1] ?
Coercivity [Oe] 290,[7] 300[8][1][6] 600,[1][6] 660,[7] 670[8] ca. 600 600?,[1][6] 660,[7] 670[8] 700,[8] 720[7][1] 750?,[1] 1060,[7] 1200 ?
Coating thickness [μm] ? ? ? 100,[8] 2.5,[7] 2.0-3.0[6] ? 55,[8] 1.3,[7] 1.0-1.5[6] ? 65,[8] 1.9,[7] 2.0-3.0[6] 55,[8] 0.9,[7] 1.0-1.5[6] ? ?
Track width [mm] 0.330[1] 0.160[1] ? 0.115[1] ?
Track density [tpi] 48[1] 96[1] or 100 96[1] 67.5[9] 135[1] 406.5[4]
Bit density [bpi] 5,876,[2][1] 5,922[10] 9,646,[1] 9,870[2] 8,650[9] 8,717[2][1] 17,432,[2] 17,434[1] 34,868[2][1] 36,700[4]
Recording direction longitudinal / horizontal[1] perpendicular / vertical[1] longitudinal[2] / horizontal[1]

See also[edit]


  1. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa Mueller, Scott (1994). Hardware-Praxis - PCs warten reparieren, aufrüsten und konfigurieren (in German) (3rd ed.). Addison-Wesley Publishing Company. ISBN 3-89319-705-2.
  2. ^ a b c d e f g h Shah, Katen A. (1996) [September 1992, April 1992]. Intel 82077SL for Super-Dense Floppies (PDF) (Application Note) (2 ed.). Intel Corporation, IMD Marketing. AP-358, 292093-002. Archived (PDF) from the original on 2017-06-19. Retrieved 2017-06-19.
  3. ^ IDG (1988-07-29). "Hitachi-Maxell bietet NEC neue 12,5-MB-Floppy an". Computerwoche (in German). Tokio, Japan. Archived from the original on 2017-06-19. Retrieved 2017-06-19. […] Hitachi-Maxell hat ein 3½-Zoll-Diskettenlaufwerk mit einer Speicherkapazität von 12.5 MB entwickelt. Nach eigenen Angaben will das Unternehmen auf OEM-Basis den Hersteller NEC mit den Geräten versorgen. Dort sollen sie als externer Speicher für die neue PC-Serie PC88VA3 eingesetzt werden. Die Laufwerke mit der Bezeichnung PC FD810.1 sind voraussichtlich ab dem vierten Quartal dieses Jahres lieferbar. Bis Ende 1989 will NEC die ersten 150000 Geräte verkauft haben. […]
  4. ^ a b c d NEC μPD72070 - Floppy Disk Controller Specification Version 2.0 (PDF). 2.0 preliminary. NEC Corporation. October 1991. Archived (PDF) from the original on 2017-03-20. Retrieved 2017-03-20.
  5. ^ "PC-88VA <ハードウエア>" (in Japanese). 1995-06-24. Archived from the original on 2017-06-18. Retrieved 2017-06-18.
  6. ^ a b c d e f g h i j Yaskawa, Seiichi; Heath, John (1988). "3. Data Storage on Flexible Disks". In Mee, C. Denis; Daniel, Eric D. (eds.). Magnetic Recording. Vol. II: Computer Data Storage (1st ed.). McGraw-Hill Book Company. pp. 130–169. ISBN 0-07-041272-3.
  7. ^ a b c d e f g h i Guzis, Charles P. (2011-08-12) [July 2009]. Johnson, Herbert R. (ed.). "Use of HD and DD drives, diskettes". Archived from the original on 2017-06-19. Retrieved 2017-06-19. (NB. Based on specifications by the National Media Laboratory, associated with 3M/Imation.)
  8. ^ a b c d e f g h Scott, Greg (1990-07-16). "More on Floppies". U-M Computing News. 5 (12). University of Michigan, Information Technology Division: 10–11. Retrieved 2017-06-19.
  9. ^ a b c Epson PF-10 Operating Manual (PDF). Nagano, Japan: Epson Corporation. 1984. Archived (PDF) from the original on 2017-06-19. Retrieved 2017-06-19. (NB. The Epson PF-10 is an external diskette drive for the Epson PX-8 (Geneva) CP/M laptop.)
  10. ^ Williams, John J. "FM vs. MFM encoding". Disk Service Manual III - Unleash the Power of Your System! (PDF) (III ed.). Alamogordo, New Mexico, USA: Consumertronics Co. Retrieved 2021-11-02.

External links[edit]