Double Density

Double Density abbreviated DD and 2D, is a potential of a plurality of recording densities, on floppy disks. It is for 3.5 "- 5.25" - and 8 "floppy disks used with a physical storage capacity of about 100 KB to 1 MB.

The IBM - PC double-sided 5.25 " DD disks from MS- DOS version 2.0 with 360 KB formatted ( the rarer one-sided, 180K ) had, in MS -DOS 1.0 with 320 or 160 KB. Others Computer systems have slightly different capacities such as 140 KB on the Apple II and Commodore 170 KB to 8 -bit systems, 130 KB CP/M-8-Bit-Systemen at each side.

The 3.5 " DD disks are formatted with 720 KB contrast, both the IBM PC and Atari ST. During Apple Macintosh, there are first 400 KB ( on one side), later 800 KB ( two-sided) and the Amiga 880 KB. for the PC and the Amiga there are alternative formats for 3.5 "disks, which can store up to 950 KB on a floppy disk. With the exception of the early Mac and Atari ST formats all of the usual 3.5 " DD formats are two-sided.

A 3.5 " DD floppy disk has only one characteristic hole with a slider for the write protection. In contrast, 3.5 " floppy disks with high density, a second characteristic hole that marks them as such. DD and HD disks are identical except for the quality of the data layer and the price. Therefore, " often old DD floppy disks in the early days of HD disks just " were at 3.5 reamed "to save money: the disk of a suitable drive was by attaching a second characteristic hole in the right place as HD detected disk and could take up twice as much data. However, not all DD disks had the necessary quality, so that this approach could lead to data loss.

A 5.25 " DD floppy disk has nominally 48 tracks per inch ( tpi = tracks per inch) and a 3.5" DD floppy disk 135 tracks per inch. On the PC, a total of 40 (5.25 " ) or 80 (3.5") tracks per disk side used. The number of sectors per track and bytes per sector is on the PC with two floppy sizes (5.25 "and 3.5" ) in Rule 9 sectors per track and 512 bytes per sector. Other computer systems are often used sectors only 256 bytes in size, of which she brought, depending on the coding efficiency 10 to 21 per track.

DD disks are now rarely used.

The DD technique is applied to the CD media, however, can only be written by special burners. These media are called DDCD simple.