Technical Terms Explained
 
UDMA
  
The support for UDMA (Ultra Direct Memory Access) was implemented by the CF Association back in CF 3.0 using controllers to store data in flash modules. The biggest advantage of using UDMA mode is to boost transfer speed. Under CF 2.0 specifications, it uses programmed I/O mode which featured a maximum transfer rate of 16.6MB/sec. Under CF 4.1 specifications, using UDMA mode 6 that boasts a maximum transfer speed of 133MB/sec. In addition to the significant speed increases, to maximize this advantage would require both the CF card and the device to support the same UDMA mode to work at its maximum potential.
 
Class 2, 4, 6
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 standard.
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 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
 
 
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
SDXC, microSDXC
U Normal Bus I/F HD ~ Full HD video recording
    SD video 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-I and SDXC UHS-I
 
The card itself must have the SDHC logo and Class rating:
  
SDHC still support Content Protection for Recordable Media (CPRM). With the same dimensions as SD cards, the fastest SDHC Class 6 cards features only a minimum of 6MB/sec write / read when compared to SD’s 120x or 150x high speed which translates into 20MB/sec, why is that? The fact is that many memory card labels state their read speeds! The write speeds are usually significantly lower. Whereas SDHC Class ratings states that the card’s read / write speeds must both meet the stated Class rating. According to the table above, Class 6 cards must be able to operate in 40MHz, store minimum 64KByte block size of data packet s and can read / write at a minimum speed of 6MB/sec.
 

About Compatibility with Host Devices

Before choosing your card, it is important that you understand how to use your memory card to its greatest ability. SDXC memory cards and devices (digital cameras, video cameras, laptops, etc.) are now available. Learn how to use your new SDXC memory card to optimize its performance.

Confirm Compatibility
Before inserting your SDXC memory card, confirm the device is compatible with the SDXC standard by locating an SDXC logo on the device or referring to the device's user manual or manufacturer information.

Understanding Backwards Compatibility
* SDXC memory cards must only be used with SDXC devices.
* SDXC devices can use SD memory cards, SDHC memory cards and SDXC memory cards.
* SDHC memory cards can be used with SDHC devices and SDXC devices.
* SDHC devices can use both SD memory cards and SDHC memory cards.
* SD devices can only use SD memory cards

UHS Speed Class Compatibilit

Speed Class Compatibility