Secure Digital (SD) is a non-volatile memory card format developed by Panasonic, SanDisk, and Toshiba for use in portable devices. 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 cards have a maximum of 4 GB capacity, even though the official standard allows for 2 GB. The capacity range for standard- and high-capacity (SDHC) cards overlap, beginning at 4 GB, but SDHC's upper limit had reached 32 GB as of mid-2009. SDXC (eXtended Capacity), a specification announced at the 2009 Consumer Electronics Show, allows for up to 2 TiB 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 (4 GB) unable to handle newer formats such as SDHC (4 GB). All SD cards have the same physical shape, which causes confusion for many consumers.
Design and implementation
Capacity Standard SD: 1 MB to 4 GBSDHC: 4 GB to 32 GB
SDXC: 32 GB to 2 TiB
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 × 24 × 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 most devices requiring a thinner card use 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 80–160 Mbit/s, but this number is subject to change, due to recent improvements to the MMC standard.
vices 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.
Speed Class Rating
The Speed Class Rating is the official unit of speed measurement for SD Cards, defined by the SD Association. It is equal to 8 Mbit/s, and it measures the minimum write speeds based on "the best fragmented state where no memory unit is occupied":
The following are the ratings of some currently available cards:
* Class 2: 16 Mbit/s (2 MB/s)
* Class 4: 32 Mbit/s (4 MB/s)
* Class 6: 48 Mbit/s (6 MB/s)
* Class 10: 80 Mbit/s (10 MB/s)
Even though the class ratings are defined by a governing body, like × speed ratings, class speed ratings are quoted by the manufacturers but unverified by any independent evaluation process. In applications that require sustained write throughput, such as video recording, the device may not perform satisfactorily if the SD card's class rating falls below a particular speed. For example, a camcorder that is designed to record to class 6 media may suffer dropouts or corrupted video on slower media.
Important differences between the Speed Class and the traditional CD-ROM drive speed measurement ("×" speed ratings) are that speed class:
1. may be queried by the host device
2. defines the minimum transfer speed.
Since the class rating is readable by devices, they can issue a warning to the user if the inserted card's reported rating falls below the application's minimum requirement.
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".[10] Toshiba also announced cards based on the new 3.0 spec[11] As of December 2009[update], the SD Association's Web site does not include information on this new class or new specification.
The × rating is a unit of measurement equal to 1.2 Mbit/s. It is derived from the standard CD-ROM drive speed of 1.2 Mbit/s. Basic cards transfer data up to six times (6×) the data rate of the standard CD-ROM speed (7.2 Mbit/s vs 1.2 Mbit/s). The 2.0 specification defines speeds up to 200×, but unlike the class rating system, does not mandate that x-ratings measure the card's sustained write-speed. For most cards, the maximum read speed is typically faster than its maximum write speed, leading some manufacturers to use read-speed as the ×-rating measurement. Other vendors, such as Kingston, use write-speed.
SD and SDHC compatibility issues
The SDHC specification was completed in Ju
ne 2006,but by that time, non-standard high-capacity (>1GB) SD cards (based on the older 1.x specification) were already on the market. The two types of storage cards were not interchangeable, creating some confusion among customers. SD and SDHC cards and devices have these compatibility issues :* Devices that do not specifically support SDHC do not recognize SDHC memory cards. Some devices can support SDHC through a firmware upgrade.
* SDHC devices are backward compatible with SD memory cards.
* Some manufacturers have produced 4 GB SD cards that conform to neither the SD2.0/SDHC spec nor existing SD devices.
* File System: SD cards are typically formatted with the FAT16 file system, while SDHC cards are typically formatted as FAT32. However, both types of cards can support other general-purpose file systems, such as UFS2, ext2 or the proprietary exFAT for example.
* Microsoft Windows may need a hotfix to support accessing SDHC cards
SD and SDXC compatibility issues
SD and SDXC cards and devices have these compatibility issues:
* SDHC devices will only support the SDXC cards which use UHS104 speeds; SDHC devices will not recognize the SDXC cards which use the faster (SD 4.0), final specification of SDXC.
* SDXC devices are backward compatible with SD and SDHC memory cards.
* Linux (with a proprietary driver), Microsoft Windows 7/Vista SP1+, and Mac OS X Snow Leopard (on the Mac Mini released in June 2010) are the only operating systems to currently support SDXC
(From Wikipedia, the free encyclopedia)
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