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.
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
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