Double density, often shortened DD, is a capacity designation on magnetic storage

Magnetic storage

Magnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using...

, usually floppy disk

Floppy disk

A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

s. It describes the use of an encoding (or modulation

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

) of information

Information

Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...

, which can encode

Encode

Encode may refer to:* Can be related to "Code"* Encode ApS, a Danish software company* Encode SA, a Greek information security company* ENCODE, the ENCyclopedia Of DNA Elements...

 on average twice as many bits per time unit compared to single density

Single density

Single density, often shortened SD, is a capacity designation on magnetic storage, usually floppy disks. It describes the use of an encoding of information using Frequency modulation, also known as biphase mark code....

. This is achieved by using MFM

Modified Frequency Modulation

Modified Frequency Modulation, commonly MFM, is a line coding scheme used to encode the actual data-bits on most floppy disk formats, hardware examples include Amiga, most CP/M machines as well as IBM PC compatibles. Early hard disk drives also used this coding.MFM is a modification to the original...

 instead of FM

Frequency modulation

In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

.

Floppy disk

Floppy disk

A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

 drives have been using this method since 1978 with the introduction of the 360 KiB (MS-DOS

MS-DOS

MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...

 FAT

File Allocation Table

File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...

 size) drives.

The early hard disk

Hard disk

A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

s also used this encoding, but was replaced by the more efficient RLL

Run Length Limited

Run length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. This is used in both telecommunication and storage systems which move a medium past a fixed head. Specifically, RLL bounds the length of stretches ...

method.