Double density

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Double density, often shortened DD, is a capacity designation on magnetic storage, usually floppy disks. It describes the use of an encoding (or modulation) of information, which can encode on average twice as many bits per time unit compared to single density. This is achieved by using MFM instead of FM.

Floppy disk drives have been using this method since 1978 with the introduction of the 360 KiB (MS-DOS FAT size) drives.

The early hard disks also used this encoding, but was replaced by the more efficient RLL method.