The SD Association is the body that sets the standards to which SDHC, or secure digital high capacity cards, must adhere. The organization has set forth two different types of classes: speed class and UHS speed class. These two formats are not compatible with one another. Additionally, while SDHC-compatible devices can also use SD cards, SD-compatible devices cannot use SDHC cards -- unless their manufacturers issue firmware upgrades to allow it.
Class 2
Class 2 cards are part of the SDHC speed class designation, and can be used for standard definition video recording. They feature a minimum data transfer rate of 2MB per second, although they may be faster. Class 2 cards use a normal bus interface.
Classes 4 and 6
Speed classes 4 and 6 can both be used for high-definition video recording. Class 4 cards feature a minimum data transfer rate of 4MB per second, while class 6 cards will do a minimum of 6MB per second. These cards, like the class 2, also use a normal bus interface.
Class 10
Speed class 10 can do high-definition video recording, in addition to supporting high definition still consecutive recording. These cards feature a minimum data transfer rate of 10MB per second, and use a high speed bus interface.
UHS-1
UHS stands for "ultra high speed," and is a speed class of its own. These cards can handle raw high definition video -- both in large file sizes and recording in real time. Their data transfer rate is a minimum of 45MB per second, and can only use a UHS bus interface. The UHS-2 class was announced by SDHC card manufacturer SanDisk in January 2011, but as of May 2011, no UHS-2 cards exist yet.
Considerations
These classes don't just apply to SDHC cards. Speed classes 2 through 10 also apply to SD, miniSD, microSD, miniSDHC, microSDHC, SDXC and microSDXC cards. The UHS speed class applies to SDHC UHS-1 and SDXC UHS-1 cards. These memory cards, like other types of memory, become fragmented over time as you store more and more data. Each time the device you're using your SDHC card in tries to save a file, it first looks for a large, uninterrupted space on the card so that it can store all the data together. When fragmentation occurs, cards with slower write speeds take longer to write to, because the data has to fill all the fragmented pieces of free space on the card. Faster write speeds, conversely, help combat the inherent slowness of fragmentation.
Tags: data transfer, data transfer rate, transfer rate, Class cards, feature minimum