|
-
History
-
Mathematics
-
Science
|
-
Society
-
Technology
-
All portals
|
|
|
Other areas of Wikipedia
-
Help desk
— Ask questions about using Wikipedia.
-
Reference desk
— Serving as virtual librarians, Wikipedia volunteers tackle your questions on a wide range of subjects.
-
Village pump
— For discussions about Wikipedia itself, including areas for technical issues and policies.
-
Community portal
— Bulletin board, projects, resources and activities covering a wide range of Wikipedia areas.
-
Site news
— Announcements, updates, articles and press releases on Wikipedia and the Wikimedia Foundation.
-
Local embassy
— For Wikipedia-related communication in languages other than English.
Wikipedia's sister projects
Wikipedia is hosted by the Wikimedia Foundation, a non-profit organization that also hosts a range of other projects:
Wikipedia languages
This Wikipedia is written in English. Started in 2001, it currently contains 2,919,858 articles. Many other Wikipedias are available; some of the largest are listed below.
-
More than 250,000 articles:
Deutsch
·
Español
·
Français
·
Italiano
·
Nederlands
·
日本語
·
Polski
·
Português
·
Русский
·
Svenska
·
中文
-
More than 100,000 articles:
العربية
·
Bahasa Indonesia
·
Català
·
Čeština
·
Dansk
·
Esperanto
·
한국어
·
Magyar
·
Norsk (bokmål)
·
Română
·
Slovenčina
·
Suomi
·
Türkçe
·
Українська
-
More than 40,000 articles:
Bahasa Melayu
·
Български
·
Eesti
·
Ελληνικά
·
English (simple)
·
فارسی
·
Galego
·
עברית
·
Hrvatski
·
Lietuvių
·
Norsk (nynorsk)
·
Slovenščina
·
Српски / Srpski
·
ไทย
·
Tiếng Việt
-
More than 20,000 articles:
Azərbaycan
·
ইমার ঠার/বিষ্ণুপ্রিয়া মণিপুরী
·
Bosanski
·
Brezhoneg
·
Cymraeg
·
Euskara
·
हिन्दी
·
Íslenska
·
ქართული
·
Latina
·
Latviešu
·
Lëtzebuergesch
·
Македонски
·
Shqip
·
Srpskohrvatski / Српскохрватски
·
Tagalog
Secure Digital
(
SD
) is a non-volatile memory card format developed by Matsushita, SanDisk, and Toshiba for use in portable devices. Today it is widely used in digital cameras, handheld computers, PDAs, Media Players, mobile phones, GPS receivers, and video game consoles. Standard SD card capacities range from 4 MB to 4 GB, and for high capacity SDHC cards from 4 GB to 32 GB as of 2008. The SDXC (eXtended Capacity), a new specification announced at the 2009 Consumer Electronics Show, will allow for up to 2 TB capacity cards.
The format has proven to be very popular. A change in the established format, however, while allowing capacities greater than 4 GB (SDHC), has created compatibility issues with older devices that cannot read the new format. The fact that SDHC format cards have the same physical shape and form factor as the older format has caused considerable confusion for consumers. SDHC cards require SDHC-capable device firmware generally not found with older devices.
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 MB/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 and microSD 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 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.
SD cards with 4 GB and smaller capacities can be formatted to either FAT16 (4 GB card can be formatted to FAT16 only with 64k clusters) or FAT32 file systems. Cards 8 GB and larger can only be formatted with a file system that can handle these larger storage sizes, such as FAT32.
Defragmentation tools are used on hard disks to optimize the file system access speed. On an SD card, this is unnecessary, as any block can be accessed as fast as any other. Doing this will wear the card out slightly, as a limited number of writes can be made before failure.
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. SanDisk classify their cards thus: Ultra II denotes a minimum read speed of 15 MB/s (100x), Extreme III denotes a maximum speed of 30 MB/s (200x) and Extreme IV which delivers up to 45 MB/s (300x). 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. Note that this requires a certain sustained speed, or else the video will stop recording. A high maximum speed with a low sustained speed will be no better than a low speed card in terms of recording. Higher speeds of up to 200x are defined by specification 2.0.
Some manufacturers use the read speed in their X-ratings, while others use the write speed. One company that uses the write speed in its X-rating is Kingston.
The following table lists some common ratings and their respective
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: 2 MB/s - 13x
-
Class 4: 4 MB/s - 26x
-
Class 6: 6 MB/s - 40x
SD and SDHC cards will often also advertise a maximum speed (such as 133x or 150x) in addition to this minimum Speed Class Rating. One critical difference between the Speed Class and the maximum speed ratings is 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. "Maximum speed" ratings are quoted by the manufacturers but unverified by any independent evaluation process.
On 21 May 2009, Panasonic announced new "class 10" SDHC cards, claiming that this new class is "part of SD Card Specification Ver.3.0". As of that date, the SD Association's Web site did not include information on this new class or new specification.
Openness of standards
