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SD Speed Class
Source: chinaflashmarket   Editor: Helan   Update Time :2010/1/7 15:41:21

More Speed Creates More Options
The SD Association is constantly innovating to ensure SD technology stays current with the needs of consumers and product manufacturers everywhere. All SD memory cards, in any size or memory capacity, come with different speed varieties designed to suit a myriad of uses.  This flexibility allows developers to select just the right speed for the right device.  
These new speeds are available in a new specification defining the minimum data transfer speeds for both SD/SDHC memory cards and SD/SDHC host products. This enhancement is easily recognized by three levels of Speed Class, class 2, 4 and 6.  Speed Class 2 guarantees a minimum transfer speed of 2 mega bytes per second (MB/s), Speed Class 4 guarantees a minimum transfer speed of 4 MB/s and Speed Class 6 guarantees a minimum transfer speed of 6 MB/s.

With the new Speed Classes, the host product has the option to check the fragmented state of the card and immediately calculate the write speed in each part of the card. The host can then determine where best to write the data according to its speed requirement.
Fragmentation and Speed
The memory of a card is divided into minimum memory units. The host writes data onto memory units where no data is already stored. As available memory becomes divided into smaller units through normal use, this leads to an increase in non-linear, or fragmented storage. The amount of fragmentation can reduce write speeds so higher SD card speeds help compensate for fragmentation.
The SD Association has specified operating conditions for SDHC memory cards to guarantee the minimum speed according to the card's speed class. Class logos are provided to help identify cards that comply with these specifications.


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