Secure Digital
Secure Digital ( SD ) is a non-volatile memory card format developed by Panasonic, SanDisk, and Toshiba for use in portable devices. Currently it is widely used in digital cameras, digital camcorders, handheld computers, netbook computers, PDAs, media players, mobile phones, GPS receivers, and video games. Standard SD card capacities have a maximum of 2 GB . The capacity range for high-capacity SDHC cards overlap, beginning at 4 GB, but reaching as high as 32 GB as of mid-2009. SDXC (eXtended Capacity), a new specification announced at the 2009 Consumer Electronics Show, allows for up to 2-TB capacity cards.
The format has proven very popular. Changes to the interface of the established format have made some older devices designed for standard SD cards (≤4GB) unable to handle newer formats such as SDHC (≥4GB). All SD-cards have the same physical shape and form factor however, which causes confusion for many consumers.
History
In August 1999, SanDisk, Matsushita 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, which was released the previous year, featured MagicGate DRM, and was physically larger. It was mistakenly predicted that DRM features would be widely used due to pressure from music and other media suppliers to prevent piracy.
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 parts of the SD Memory Card standard. The organization also released specifications for the SDIO (Secure Digital Input Output) cards and the standard SD host controller. During the same year, specifications were finalized 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 MByte/s .
Design and implementation
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 as with MMCs can be as thin as 1.4 mm if they lack a write-protect switch; such cards, called "Thin SD", are described in the SD specification, but they are non-existent or rare in the market as devices that would require a thinner card are usually utilising the smaller (and thinner) versions of SD: miniSD or microSD.
- The card's electrical contacts are recessed beneath the surface of the card, protecting them from contact with a user's fingers.
- SD cards typically have transfer rates in the range of 10-20 MB/s, but this number is subject to change, due to recent improvements to the MMC standard.
Devices with SD slots can use the thinner MMCs, but standard SD cards will not fit into the thinner MMC slots. miniSD cards can be used directly in SD slots with a simple passive adapter, since the cards 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 sliding write protection tab, or absent, then the card is writeable. Because the notch is detected only by the reader, the protection can be overridden if desired (and supported by the reader).
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 are usually permanently marked read-only by adding the notch with no tabs.
File system
Like other flash card technologies, most SD cards ship preformatted with the FAT or FAT 32 file system on top of an MBR partition scheme. 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.
SD cards with 4 GB and smaller capacities can be used with many systems by being formatted with FAT16 ( 4 GB only possible by using 64 kByte clusters, and not widely supported) or FAT32 file system (common for file systems 4 GB and larger). Cards 4 GB and larger can only be formatted with a file system that can handle these larger storage sizes, such as FAT32.
SD cards are plain block devices and do not in any way imply any specific partition layout or file system thus other partition schemes than MBR partitioning and the FAT file systems can be used. Under Unix-like operating systems such as Linux or FreeBSD, SD cards can be formatted using, for example, the UFS, EXT3 or the ReiserFS file systems; under Mac OS X, SD cards can be partitioned as GUID devices and formatted with the HFS+ file system. Under MS-Windows and some unix systems, SD cards can be formatted using the NTFS and on later versions exFAT file system. However most consumer products will expect MBR partitioning and FAT16/FAT32 filesystem.
Fragmentation may slow down the effective write speed:. Defragmentation tools may be used. However, it is unnecessary to use any disk optimization tool because on an SD card the time required to access any block is the same. Defragmenting an SD card will wear the card out slightly, as the number of writes are limited before failure occurs (usually 100 000 times ).
Speeds
There are different speed grades available, measured the same 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).
Note that maximum read speed and maximum write speed may be different. Maximum write speed typically is 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. This requires a certain sustained speed, or the video stops recording. For recording, a high maximum speed with a low sustained speed is no better than a low speed card. The 2.0 specification defines speeds up to 200x.
Some manufacturers use the read speed in their X-ratings, while others (Kingston, for example) use write speed.
This table lists common ratings and minimum transfer rates.
SD Speed Class Ratings
SD Cards and SDHC Cards have Speed Class Ratings defined by the SD Association. The SD Speed Class Ratings specify the following minimum write speeds based on "the best fragmented state where no memory unit is occupied":
- Class 2: 16 MBits/s (2 MByte/s)
- Class 4: 32 MBits/s (4 MByte/s)
- Class 6: 48 MBits/s (6 MByte/s)
SD and SDHC cards will often also advertise a maximum speed (such as 133x or 150x) in addition to this minimum Speed Class Rating. Important differences between the Speed Class and the traditional "X" speed ratings are:
- the ability of the host device to query the SD card for the speed class and determine the best location to store data that meets the performance required
- class speed defines the minimum transfer speed.
Even though the class ratings are defined by a governing body, like "X" speed ratings, class speed ratings are quoted by the manufacturers but unverified by any independent evaluation process.
On 21 May 2009, Panasonic announced new "class
Secure Digital - Wikipedia, the free encyclopedia
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