Memory cards Speed Class and Benefit

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　could be achieved for camcorders, video recorders and other devices with video recording capabilities under the conditional write operation specified in the standard.

Speed Class, designated as Class 2, 4, 6 and 10, is designed for normal and high speed bus interface (mode) and UHS Speed Classes 1 and 3 are designed for UHS 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 312 Megabytes.

	Mark	Minimum Serial Data Writing Speed	SD Bus Mode	Application
UHS Speed Class		30MB/s	UHS-II UHS-I	4K2K Video Recording
		10MB/s		Full HD Video Recording HD Still Image Continuous Shooting
Speed Class		10MB/s	High Speed
		6MB/s	Normal Speed	HD and Full HD Video Recording
		4MB/s
		2MB/s		Standard Video Recording

Because of the variety of high-performing and feature-rich devices in the market, there is a demand for higher performance SD memory cards. The SDA has introduced a higher bus interface with higher theoretical read and writing speed specifications for the SDHC standard and SDXC standard.

Bus Interface	Card Type	Bus Mark	Bus Speed	Spec Version
Normal Speed	SD, SDHC and SDXC	---	12.5MB/s	1.01
High Speed	SD, SDHC and SDXC	---	25MB/s	2.00
UHS-I	SDHC and SDXC		50MB/s (SDR50, DDR50) 104MB/s (SDR104)	3.01
UHS-II	SDHC and SDXC		156MB/s 312MB/s	4.00

Maximum speed differs from the bus I/F speed. It varies depending upon the card performance. The average speed that a device writes to an SD memory card may vary depending upon the device and the operation it is performing. The speed may also depend on how other data is stored on the SD memory card.

Although normal-speed cards and high-speed cards can be used in UHS-I and UHS-II host devices, the conventional performance levels of UHS-I supported host devices can only be attained with UHS-I memory cards, and the conventional performance levels of UHS-II supported host devices can only be attained with UHS-II memory cards.

via sdcard