Classes of SD cards

在数码相机和多媒体手机普及化的时代，消费者对记忆卡的容量需求日益剧增。当您选购记忆卡的时候，您是否对于卡上面的Class 2、Class 4、Class 6之类的字眼感到好奇？我来帮大家揭开疑惑吧！
-------------------------------------------------------------------------------------------------------

SD Speed Class/UHS Speed Class

Greater Performance Choices

There are two kinds of Speed Class, "Speed Class" and "UHS Speed Class."

As a characteristic of flash memory, actual transfer speed varies. Variable speeds are difficult to reliably record streaming content such as video because it requires a constant writing speed. Speed Class and UHS Speed Class provide the constant speed necessary for video recording by designating a minimum writing performance so that minimum and constant speed is guaranteed for camcorders, video recorders and other devices with video recording capabilities under the conditional write operation specified in the specification.

Speed Class, designated as Class 2, 4, 6 and 10, is designed for normal and high speed bus interface (mode) and UHS Speed Class 1 is designed for UHS-I bus interface*. (Speed Class and the UHS Speed Class are not compatible.)

*UHS (Ultra High Speed), the fastest performance category available today, defines bus-interface speeds up to 104 Megabytes per second for greater device performance in SDXC and SDHC host products.

	Marks	Operable Under...	Applications	SD Memory Card
Speed Class		Normal Bus I/F	SD video recording	SD, miniSD, microSD SDHC, miniSDHC, microSDHC SDXC, microSDXC
			HD ~ Full HD video recording
		High Speed Bus I/F	Full HD video recording HD still consecutive recording
UHS Speed Class		UHS-I Bus I/F	Full higher potential of recording real-time broadcasts and capturing large-size HD videos	SDHC UHS-Iand SDXC UHS-I

UHS Speed Class Compatibility

Speed Class Compatibility

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, SDXC, SDHC UHS-I and SDXC UHS-I memory cards to guarantee the minimum speed according to the card's speed class. Speed logos are provided to help identify cards that comply with these specifications.