<P> The higher speed rates are achieved by using a two - lane low voltage (0.4 V pp) differential interface . Each lane is capable of transferring up to 156 MB / s . In full duplex mode, one lane is used for Transmit while the other is used for Receive . In half duplex mode both lanes are used for the same direction of data transfer allowing a double data rate at the same clock speed . In addition to enabling higher data rates, the UHS - II interface allows for lower interface power consumption, lower I / O voltage and lower electromagnetic interference (EMI). </P> <P> SD card speed is customarily rated by its sequential read or write speed . The sequential performance aspect is the most relevant for storing and retrieving large files (relative to block sizes internal to the flash memory), such as images and multimedia . Small data (such as file names, sizes and timestamps) falls under the much lower speed limit of random access, which can be the limiting factor in some use cases . </P> <P> With early SD cards, a few card manufacturers specified the speed as a "times" ("×") rating, which compared the average speed of reading data to that of the original CD - ROM drive . This was superseded by the Speed Class Rating, which guarantees a minimum rate at which data can be written to the card . </P> <P> The newer families of SD card improve card speed by increasing the bus rate (the frequency of the clock signal that strobes information into and out of the card). Whatever the bus rate, the card can signal to the host that it is "busy" until a read or a write operation is complete . Compliance with a higher speed rating is a guarantee that the card limits its use of the "busy" indication . </P>

What does the class on a sd card mean