History
In August 1999, Panasonic, SanDisk, and Toshiba first agreed to develop and market the SD (Secure Digital) Memory Card, which was a development of the MMC. With a physical profile of 24 mm × 32 mm × 2.1 mm, the new card provided both DRM up to the SDMI standard, and a high memory density for the time.
The new format was designed to compete with Sony's Memory Stick format that was released the previous year, which featured MagicGate DRM, and was physically larger. It was thought that DRM features would be widely used due to pressure from music and other media suppliers to prevent piracy. The features are largely unused.
At the 2000 CES trade show Matsushita, SanDisk and Toshiba Corporation announced the creation of the SD Card Association, to promote SD cards. It is headquartered in California and its executive membership includes some 30 world-leading high-tech companies and major content companies. Early samples of the SD Card were available in the first quarter of 2000, with production quantities of 32 and 64 megabytes available 3 months later.
In April 2006, the SDA released a detailed specification for the non-security related portions of the SD Memory Card standard. In addition, they released specifications for the SDIO (Secure Digital Input Output) cards and the standard SD host controller. During the same year, specifications were finalised for the small form-factor microSD (formerly known as TransFlash) and SDHC, with capacities in excess of 2 GB and a minimum sustained read/write speed of 2.2 MB/s.
Design and implementation
An SD card, mini SD card, and micro SD card from top to bottom.
SD cards are based on the older MultiMediaCard (MMC) format, but have a number of differences:
The SD card is asymmetrically shaped in order not to be inserted upside down, while an MMC would go in most of the way but not make contact if inverted.
Most SD cards are physically thicker than MMCs. SD cards generally measure 32 mm × 24 mm × 2.1 mm, but can be as thin as 1.4 mm, just like MMCs (see below).
The contacts are recessed beneath the surface of the card, protecting the contacts from contact with the fingers.
SD cards typically have transfer rates in the range of 10-20 MBytes/s, but this is always changing, particularly in light of recent improvements to the MMC standard.
Devices with SD slots can use the thinner MMCs, but the standard SD cards will not fit into the thinner MMC slots. miniSD and microSD cards can be used directly in SD slots with a simple passive adapter, since they differ in size and shape but not electrical interface. With an active electronic adapter, SD cards can be used in CompactFlash or PC card slots. Some SD cards include a USB connector for compatibility with desktop and laptop computers, and card readers allow SD cards to be accessed via connectivity ports such as USB, FireWire, and the parallel printer port. SD cards can also be accessed via a floppy disk drive with a FlashPath adapter.
Optional write-protect tab
When looking at the card from the top (see pictures) there is one required notch on the right side (the side with the diagonal notched corner).
On the left side may be a write-protection notch. If this is present, the card cannot be written to. If the notch is covered by a slide-able write protection tab, or absent, then the card is writeable.
Not all devices support write protection, which is an optional feature of the SD standard.
Some SD cards have no write-protection notch, and it is absent completely in the MicroSD and MiniSD formats.
Some music and film media companies (e.g. Disney) have released limited catalogs of records and/or videos on SD. These usually contain DRM-encoded Windows Media files, making use of the SD format's DRM capabilities. Such media is usually permanently marked read-only, by adding the notch with no tab.
File system
Like other flash card technologies, most SD cards ship preformatted with the FAT or FAT 32 file system. The ubiquity of this file system allows the card to be accessed on virtually any host device with an SD reader. Also, standard FAT maintenance utilities (e.g. ScanDisk) can be used to repair or retrieve corrupted data. However, because the card appears as a removable hard drive to the host system, the card can be reformatted to any file system supported by the operating system.
Defragmentation tools are used on hard disks to optimise the file system access speed. On an SD card, this is pointless, as any block can be accessed as fast as any other. Doing this will wear the card out slightly, as only so many writes can be made before failure.
However, note that any file recovery tool will struggle to recover files from highly fragmented data if the File Allocation Table becomes highly corrupted.
Speeds
There are different speed grades available which are measured with the same system as CD-ROMs, in multiples of 150 kB/s (1x = 150 kB/s). Basic cards transfer data up to six times (6x) the data rate of the standard CD-ROM speed (900 kB/s vs. 150 kB/s). High-speed cards are made with higher data transfer rates like 66x (10 MB/s), and high-end cards have speeds of 200x or higher. Note that maximum read speed and maximum write speed may be different, with maximum write speed typically lower than maximum read speed. Some digital cameras require high-speed cards (write speed) to record video smoothly or capture multiple still photographs in rapid succession. The SD card specification 1.01 allows for a maximum speed of 66x. Higher speeds of up to 200x are defined by specification 2.0.
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