SD card are not the fastest storage devices, but they are usually good enough for the job. One of the main use case is storage for video cameras, but with devices supporting 4K2K (2160p) recording they are close to their minimum limits. That’s why the SD association announced Ultra High Speed (UHS) Speed Class 3 (U3), which provides at least 30 MB/s write speed.
The updated SD card speed classes are shown in the table below.
|Marks||Operable Under||Applications||SD Memory Card|
|Speed Class||High Speed Bus I/F||Full HD video recording
HD still consecutive recording
|SD, miniSD, microSD
SDHC, miniSDHC, microSDHC
|Normal Bus I/F||HD ~ Full HD video recording|
|SD video recording|
|UHS Speed Class||UHS-I Bus I/F
UHS-II Bus I/F
|Full higher potential of recording real-time broadcasts and capturing large-size HD videos
(UHS Speed Class1 denotes a 10 MB/s minimum write speed)
|SDHC UHS-I and UHS-II, SDXC UHS-I and UHS-II|
|Capable of recording 4K2K video
(UHS Speed Class 3 denotes 30 MB/s minimum write speed)
The new standard will support SDXC UHS-I and UHS-II, and SDHC (Secure Digital High Capacity) UHS-I and UHS-II memory cards and devices. SDHC is limited to 32GB, whereas SDXC is limited to 2TB in theory, but currently the largest you can buy is Lexar Professional 400x SDXC UHS-I Card that sells for an astonishing $900.
UHS-I cards, specified in SD Version 3.01, support clock frequency of 100 MHz, providing up to 50 MB/s, but UHS-I cards declared as UHS104 also support a clock frequency of 208 MHz, which could transfer 104 MB/s. UHS-II cards ,specified in SD version 4.0, can raise the maximum transfer rate to 312 MB/s. One of the fastest, if not the fastest, SD card available currently is the Toshiba EXCERIA UHS-II providing 120MB/s write speed, and 240MB/s read speed. This card is specified as U1 speed, and would certainly match the new UHS U3 speed class.